cycling_injuries

The Biomechanics, Etiology, and Treatment of Cycling Injuries William H. Sanner, DPM* William D. O’Halloran, DPM†

The authors review the biomechanics of cycling and discuss the ideal cyclist’s morphology. Examination of the cyclist when resting and when cycling is described. A variety of overuse injuries commonly sustained by cyclists are reviewed, and strategies for altering the cyclist’s mechanics to relieve the pain are described. Because the bicycle and the cyclist must be considered as a unit, this article offers instruction for adjusting the bicycle as well as the cyclist. (J Am Podiatr Med Assoc 90(7): 354-376, 2000)

The bicycle invented in the early 1800s was merely one wheel set in front of another with the two wheels connected by a piece of wood. The cyclist sat on the wood and propelled the bicycle forward by pushing on the ground. In the late 1800s, the Penny Farthing bicycle was invented. The Penny Farthing bicycle was much like today’s tricycle, with the pedals on the front wheel directly turning the front wheel, propelling the cyclist forward. The height of the front wheel varied considerably. Around the turn of the nineteenth century, the chain-driven bicycle became popular. It has matured over the last 100 years, but the bicycle of today is basically the same as the bicycle of 80 years ago (Fig. 1).

During the last 60 years, the injuries sustained by cyclists have also remained much the same. Knee problems are the most common and most serious of cycling-related overuse injuries. Because the incidence of overuse injuries in cycling is low compared with those from running or soccer, they have generally been overlooked in the past. In the last 30 years, cycling has enjoyed increased popularity and interest. Research has concentrated on how to make a cyclist go faster, but little attention has been directed toward the care and prevention of overuse injuries in cycling.

This article provides basic information about the biomechanics and etiology of overuse cycling in*Chairman, Department of Podiatry, Ochsner Clinic of Baton Rouge, 9001 Summa Ave, Baton Rouge, LA 70809. †Private practice, Fort Collins, CO.

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juries so that the sports specialist can increase the enjoyment of cyclists and prolong their participation in the sport. The information is specifically related to cyclists who experience pain as a result of their cycling. The improvement of cycling efficiency or speed is not the primary concern here, but those benefits may follow.

Biomechanics of Cycling Pedal Cycle While seated or standing, the cyclist produces the power to move the bicycle forward by pushing its pedals. The pedals move in a circle as they rotate around the bottom bracket. One complete circular motion is called a pedal cycle and is divided into two phases. In the power phase, the cyclist pushes down on the pedal and makes the greatest contribution toward moving the bicycle and the cyclist forward. The power phase begins at top dead center and ends at bottom dead center. While seated, the cyclist can apply the force of approximately half of his or her body weight to the pedal during the power phase.

When standing, the cyclist can apply the force of up to three times body weight to the pedal because the cyclist pulls up on the handlebars while pushing down on the pedal (Fig. 2).1 The power phase of the pedal cycle is followed by the recovery phase, which progresses from bottom dead center to top dead center. Some cyclists actively

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Handlebar stem Seat tube Top tube

Rear sprocket Handlebar

Chain ring Bottom bracket Crank arm Pedal spindle

Figure 1. The multigear bicycle and its components.

pull up during the recovery phase so that the powerphase limb need not waste effort lifting the recoveryphase limb (Fig. 2). Accomplished cyclists complete 60 to 120 pedal revolutions per minute and thus place each limb under power-phase stress between 3,600 and 7,200 times per hour.

Power Phase Gregor 2 and Okajima3 studied the magnitude and direction of force applied to the pedal. The force applied to the crank is most effective in converting to a rotational force (torque) when the force is perpendicular to the crank. Figure 2 demonstrates the vector of force as it is generally applied to the pedal. As expected, the forces are greatest and closest to being perpendicular to the crank during the middle half of the power phase. During the first and last quadrants of the power phase, the forces are a little less than during the middle half, but because the direction is more parallel than perpendicular to the crank, little force of the first and last quadrants is translated into

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rotational force. For maximum efficiency, the bicycle should be adjusted so that the cyclist can take advantage of the magnitude and direction of force.

The bicycle can be adjusted by, for example, changing the seat height or adjusting the design of the bicycle equipment (as explained later). Several drive-train designs have been created in an attempt to improve efficiency. The elliptical chain ring and cam drives are based on the maximum rotational force taking place in the middle half of the power phase. The Biopace system (Shimano, Irvine, California) is based on the speed of muscle contraction and is designed to be nearly the reverse of the elliptical and cam drive systems. To date, variations on the round chain ring have not been proven to be more efficient3, 4 or to relieve stress on the legs while cycling.

These variations have been widely accepted in the cycling community. In general, the round chain ring is recommended because of the lack of improved efficiency of alternative chain rings and because of the smoother motion with smaller peak stress points around the pedal cycle.

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TDC

BDC

Figure 2. The pedal cycle power phase goes from top dead center (TDC) to bottom dead center (BDC). The recovery phase is the remainder of the pedal cycle.

The dashed lines are vectors representing the approximate direction and relative magnitude of the force applied to the pedal by the lower extremity.

To propel a bicycle, muscles move the major lower-extremity joints: the hip, knee, and ankle. This section describes the general joint motion, the muscle phasic activity, and the torque that occur during cycling. Alteration of the joint mechanics is discussed in a subsequent section. The cyclist’s position on the bicycle affects the joint range of motion used during cycling.

The hip extends approximately 55° during the power phase.5 The thigh begins the power phase at 10° to 20° below a horizontal position. With hip extension, the thigh moves to a position between 50° and 75° from the horizontal by the end of the power phase; this is approximately perpendicular to the torso of the rider whose hands are on the top of the handlebars (Fig. 3).

During the first two-thirds of the power phase, the gluteal muscles assist in extending the hip joint; the more the hip is flexed, the more effective these muscles are. As the gluteal muscles extend the hip, which pushes the leg down on the pedal, the paraspinal muscles stabilize the pelvis and lower back to provide a stable origin for the gluteal and hamstring muscles.

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The hamstring muscles also extend the hip and are active during the last three-quarters of the power phase and the early recovery phase.6 The torque at the hip increases during the first quarter of the power phase (gluteal muscles), peaks during the middle half of the power phase (gluteal and hamstring muscles), and gradually decreases in the early recovery phase (hamstring muscles).3 Therefore, throughout the power phase, either the gluteal or the hamstring muscles extend the hip and create a downward force on the pedal, particularly during the critical middle half of the power phase.

Of the lower-extremity joints, the knee goes through the greatest range of motion, approximately 75°. The knee begins the power phase flexed approximately 110° and extends to a position flexed approximately 35°.5 The extension torque at the knee occurs predominantly in the late recovery phase and the first half of the power phase, corresponding to the phasic activity of the quadriceps muscle. The quadriceps muscle is active in the late recovery phase and the first two-thirds of the power phase. The quadriceps muscle is responsible for pushing the pedal forward and over top dead center and for providing early-power-phase propulsion for the cyclist. The contralateral hamstring muscles assist the quadriceps muscle at top dead center by pulling the opposite pedal through bottom dead center. Because of the action of the quadriceps muscles, the force vector in the early power phase is directed downward and slightly forward.6, 7 The quadriceps muscle is generally given credit for knee extension during the power phase, but the hamstring and gluteal muscles are also important.

The quadriceps muscle is active primarily during the first half of the power phase while the hamstring muscles are active through most of the power phase.6 When the quadriceps muscle contracts, it extends the knee and creates a downward force on the pedal instead of a kicking-forward motion, since the gluteal muscles are pulling the entire limb down on the pedal. The friction of the pedal keeps the foot from slipping off. In fact, if the foot is attached to the pedal with a cleat, the knee would extend even if the cyclist did not have quadriceps muscles, because hip extension would push the pedal down and thus extend the knee.

The hamstring muscles have the longest activity period of any lower-extremity muscles during cycling. When the hamstring muscles pull posteriorly on the knee, the knee does not flex because the foot is held on the pedal by friction or by a cleat. Instead, the posterior pull on the knee extends the knee and creates downward pressure on the pedal. If the

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Figure 3. In the early power phase, the thigh is 10° to 20° below the horizontal plane; the knee is flexed approximately 110°; and the ankle is approximately at a right angle. The power in the early power phase is supplied primarily by the quadriceps and gluteal muscles and in the late power phase by the hamstring and calf muscles.

During the power phase, the vertical reaction forces from the pedal pronate the foot. In the early recovery phase, the flexed thigh is 50° to 75° below the horizontal plane; the knee is flexed approximately 35°; and the ankle is moderately plantarflexed. The hamstring muscles flex the knee and bring the leg through bottom dead center. The iliopsoas and rectus femoris muscles begin hip flexion well into the recovery phase. The tibialis anterior muscle attempts to dorsiflex the ankle and supinates the foot.

cyclist’s foot is held on the pedal by a toe clip, a cycling shoe, or both, the downward movement of hip extension translates into knee extension because the limb is forced to travel with the pedal.

During the early power phase, the ankle is as dorsiflexed as it will become (or at its point of least plantarflexion). During the middle half of the power phase, the calf muscles move the ankle to a plantarflexed position. The ankle range of motion is approximately 25°.5 Cyclists frequently plantarflex their ankles throughout the entire pedaling cycle. When this is the case, the ankle is least plantarflexed during the early power phase and reaches its greatest

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plantarflexion during the second half of the power phase.

The soleus muscle is active during the middle half of the power phase, and the gastrocnemius muscle becomes active at the same time but continues its activity well into the recovery phase.5 The calf muscle activity serves three purposes: 1) It enables the hip and knee downward forces to be resisted at the ankle; otherwise, a soft transfer of force to the pedal would occur. The peak plantarflexory force at the ankle corresponds to the peak downward force on the pedal.6 2) As the ankle planterflexes, the calf muscles pro-

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vide a significant propulsive force in the second half of the power phase, thus contributing to turning the crank and propelling the cyclist.

3) The activity of the calf muscle places the foot in a plantarflexed position during the early recovery phase, which greatly improves the ability of the hamstring muscles to contribute to turning the crank in the early recovery phase.8 The cyclist may continue to apply power to the pedal into the early recovery phase by contracting the hamstring muscles to flex the knee, but only if the foot is secured to the pedal; otherwise, the foot will pull off posteriorly.

Gregor et al6 determined the torque patterns at the hip, knee, and ankle and the resultant moment at the crank. During the first half of the power phase, the torque at the knee increases from the late-recoveryphase levels, peaks at about 25% of the power phase, and declines to less than half the peak by 50% of the power phase. In contrast, the torques at the ankle and hip gradually increase and peak during the third quarter of the power phase. The peak torques at the knee and ankle are roughly equal to each other and are approximately half that of the hip. The resultant moment of the crank gradually increases throughout the first half of the power phase, peaks during the third quarter of the power phase, and then declines in the early recovery phase. This means that the quadriceps muscle and other knee extensors provide most of the power during the first half of the power phase and that the ankle plantar flexors and hip extensors provide the power in the second half of the power phase.

While the knee extends, it also adducts owing to the normal valgus angulation of the distal femoral condyles relative to the femoral shaft and to the foot motion during the power phase. The distal femur appears to have greater disparity in its radius of curvature distally than posteriorly, leading to a greater disparity in distal condylar size distally than posteriorly.

This disparity results in the knee’s moving medially as the tibia moves from the posterior to the distal femoral condylar surface while the knee extends.

The foot pronates during the power phase because a dorsiflexory force equal to the force pushing down on the pedal pushes up on the forefoot. This dorsiflexory force causes the midtarsal and subtalar joints to pronate and the medial column of the foot to invert and dorsiflex. As the foot pronates, the leg rotates internally. Dorsiflexion of the medial column of the foot may invert the relationship of the forefoot to the leg, which then tilts the entire limb closer to the midline of the body (Fig. 4).

As the knee moves closer to the bicycle, the Q angle increases because the hip and feet are at fixed

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Figure 4. The flexed knee at top dead center moves away from the bicycle. With medial column dorsiflexion, hamstring activity, and knee extension during the power phase, the knee at bottom dead center moves closer to the bicycle, which increases the functional Q angle.

distances from the bicycle. An increase in the Q angle can disturb the medial and lateral tension balance at the patella (Fig. 4). Additionally, the vastus lateralis and rectus femoris muscles have a more oblique pull on the patella, which can dominate the vastus medialis muscle and result in tracking problems.

Recovery Phase The recovery phase realigns the foot and leg for the next power phase, thereby creating a smooth transition from one power phase to the next and providing a rest period for the power-phase muscles. During the recovery phase, the weight of the recovering limb always applies some downward force on the pedal.

The weight produces a negative torque at the crank,

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which reduces the effectiveness of the opposite limb’s power phase.8 Some cyclists try to reduce recovery-phase negative pedal force by actively flexing the recovery-phase hip with the iliopsoas and rectus femoris muscles and by flexing the knee with the hamstring muscles. For the hip flexors to help lift the recovery-phase limb, there must be toe clips on the feet. Cleated shoes are necessary for the hamstring muscles to contribute to propulsion by flexing the knee in the early recovery phase, thereby reducing negative pedal weight.

As the pedal moves up toward top dead center, the hip and knee flex. As the knee flexes, it moves laterally because the shape of the posterior surface of the femoral condyles is more nearly symmetrical than the distal surface and because subtalar and midtarsal joint resupination externally rotates the leg and tilts the leg away from the bicycle top tube. Almost all subtalar and midtarsal supination occurs in the first half of the recovery phase as the limb is being pulled upward and the dorsiflexory force on the forefoot is reduced.

If the cyclist is wearing cleated cycling shoes, the hamstring muscles will flex the knee through bottom dead center and provide a positive propulsive moment at the crank through most of the first third of the recovery phase.5, 9, 10 The gastrocnemius muscle may also contribute to knee flexion in the early recovery phase. Hamstring and gastrocnemius muscle activity of the recovery-phase limb helps the powerphase limb, whose quadriceps muscle is working to extend the knee through top dead center and the early power phase. Without cleats, the hamstring muscles cannot contribute in the early recovery phase because the foot will be pulled off of the pedal.

During the second half of the recovery phase, the anterior tibial muscle begins dorsiflexing the ankle, and the quadriceps muscle begins knee extension and hip flexion.5, 9, 11 During the recovery phase, the ankle may be actively dorsiflexed by the anterior leg muscles until the foot is nearly horizontal and possibly dorsiflexed relative to the leg. Some cyclists dorsiflex the ankle and drop the heel below the pedal level in the late recovery phase.

Ideal Morphology of a Cyclist Ideally, the cyclist’s legs should function like pistons as they pump up and down over the pedals. For straight piston-like action, the legs should be straight and the hips the same width as the pedals; this allows the cyclist to sit atop the bicycle and push down with a knee that has almost no valgus angulation. The knee and ankle axes would parallel the

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pedal spindle, and the muscles would apply force roughly perpendicular to the pedal axis of rotation.

For the ankle to be oriented in approximately the same direction as the pedal spindle, malleolar (tibial) torsion must be low. This will also allow the feet to sit straight on the pedals.

With the straight limb, the Q angle would be relatively low, meaning that the muscle pull on the patella, particularly that of the vastus lateralis and vastus medialis muscles, would be straighter and the quadriceps muscle could apply a purer extension vector of force at the knee. Additionally, the straight leg would have little valgus angulation of the distal femoral condyles relative to the shaft, and thus the posterior and distal femoral condyles would be nearly the same dimensions, allowing the knee to flex and extend during cycling without a lot of medial and lateral knee rocking.

An important characteristic of the ideal cyclist’s foot is a rearfoot that sits directly under the ankle, making forces applied from the leg down on the foot more centered on the foot. The medial column of the foot should have limited dorsiflexion and inversion range of motion for stability, and the plantar plane of the forefoot should not have an inverted position relative to the leg when the foot is loaded plantarly. A medial column that dorsiflexes and inverts enough to create an inverted forefoot or a structurally inverted forefoot will cause the leg to rotate medially, which disturbs knee alignment by increasing the Q angle.

It is less important for the ideal cyclist to have bone lengths and muscle morphology conducive to cycling. The primary bone-length advantage for cycling is a longer-than-average femur. The longer femur allows a longer lever arm for the gluteal muscles to extend the hip and pull the leg down, and it allows a greater mass of thigh muscles (quadriceps muscle) to have a direct line to the center of the knee (patella).

Muscles play an extremely important role in cycling and are generally neglected when bicycles are adjusted to fit the cyclist. The cyclist’s quadriceps, hamstring, and gluteal muscles should have normal flexibility. Tight quadriceps muscles can create increased patellar retinaculum friction on the femoral condyles during the first half of the power phase.

As bottom dead center is approached, knee extension is maximal. The higher the seat, the further the knee will extend. Because the hip is flexed, knee extension can create significant tension in the hamstring muscles (Fig. 5). Tight hamstring muscles may limit how high the cyclist can raise the seat and extend the knee before injuring the hamstring muscles or creating dramatic side-to-side knee deflections.

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Figure 5. As the knee and hip extend, the insertion of the hamstring muscles moves farther from its origin.

This is because of the large range of motion through which the knee moves and the distance of the hamstring muscles from the knee axis. The result is increased tension on the hamstring muscles if the seat is raised too high.

Gluteal muscles that are too short can create unnecessary tension in several places. The gluteus maximus muscle inserts into the iliotibial band. A short gluteus maximus muscle places extra tension on the iliotibial band, creating increased friction over the femoral epicondyle, femoral condyle, and greater trochanter. The paraspinal muscles stabilize the lower back and pelvis for the gluteal muscles. Tight gluteal muscles can create hyperextension of the back and corresponding low-back pain.

Basic Cycling Technique Even if all of the above factors are optimized, proper technique is required for total efficiency. Technique involves controlling the speed traveled by varying the cadence and the gear selection. Cadence refers to the number of times the pedal makes a complete pedal cycle each minute. Gear selection determines how many times the rear wheel makes a complete rotation for each complete pedal cycle. The higher the gear, the more rotations of the wheel for each pedal cycle and the more force the cyclist must apply to the pedal to move the longer distance. A basic rule of thumb in gear selection is that the cyclist should be

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able to comfortably maintain a cadence of at least 70 rpm, and preferably more than 80 rpm, for a prolonged period. If the cyclist cannot maintain a cadence in the neighborhood of 80 rpm, the gear selection is too high and too much pedal pressure is being applied for the condition of the limbs and cardiovascular system.

A cadence of less than 70 rpm places too much strain on the leg muscles and knee. When the gear selection is too high, the cyclist is forced to “muscle� the pedal through the power phase. A higher cadence and lower gear allows the cyclist to go farther with less muscle fatigue, soreness, and opportunity for injury.

Most bicycles today have at least ten gears created by combinations of five to eight rear sprockets and two to three front chain rings. As many as 24 gear combinations are possible. Gears are described in gear inches corresponding to the height of the front wheel of a Penny Farthing bicycle and range from 20 to 150 inches. The distance traveled by one complete pedal cycle of a 40-inch gear equals one complete rotation of a 40-inch-tall Penny Farthing bicycle wheel.

Most bicycles have 10 or 12 distinct gears ranging from 40 to 100 inches. The gears are divided into low (20 to 60 inches), intermediate (60 to 80 inches), and high (greater than 80 inches) ranges. Low gears require relatively little force on the pedal to propel the bicycle because the rear wheel may go around only once or twice for each complete pedal revolution.

Low gears are used primarily for going uphill or riding into strong winds; intermediate gears are used by most cyclists when the terrain is relatively flat; and high gears are used when the bicycle is going fast, such as when the cyclist is going downhill or riding with the wind. High gears require more force on the pedal during the power phase because the wheels complete several more revolutions than in low or intermediate gears. The use of high gears for prolonged periods places considerable stress on the lower extremity and may result in injury.

Small bicycle computers can provide information that helps in selecting the proper gear. These computers inform the cyclist about cadence, speed, heart rate, and many other performance details. Heart rate is used as an objective measure of effort. For example, if the heart rate begins to rise above a certain predetermined maximum limit, the gear selection and cadence can be altered to lower the heart rate and often still allow the cyclist to maintain the same speed.

Knowledge of the cadence can be used to reduce the likelihood of overstraining the legs. If the cadence drops too low, the cyclist should shift into a lower gear to reduce strain. Thus the small bicycle comput-

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er provides valuable information that allows the cyclist to ride with optimal efficiency and technique.

Bicycle Equipment and Adjustment Options The bicycle can be adjusted so that the cyclist can ride in the most efficient position, or in a position less likely to result in injury. Although these two concepts are not always mutually exclusive, this article will emphasize injury prevention rather than speed.

To fit the bicycle to the cyclist, the sports specialist can make five bicycle adjustments: seat height, forward or backward seat position, height of handlebars and their distance from the cyclist, length of the crank, and foot position. Any further adjustment is available only by purchasing a new frame.

Bicycle seat height and forward or backward seat position are the most important adjustments for fitting the bicycle to the cyclist. The seat height is measured as the distance from the seat to the pedal spindle when the pedal is at the bottom of the pedal cycle and the crank is in line with the seat tube (Fig. 6).

A lower seat height will maintain the hip and knee in a more flexed position. The higher the seat, the less flexed the hip and knee throughout the entire pedal cycle. Because body morphology varies so much from cyclist to cyclist, no one seat-height formula is appropriate for all cyclists. With the lower

A

seat, the knees tend to track better and stress on the hamstring muscles is less, but oxygen uptake efficiency may be decreased.

The most traditional seat height, and the lowest sensible seat height, equals the cyclist’s inseam length (ground to symphysis pubis). This lower limit of seat height allows the cyclist to easily reach the pedal with his or her heel while seated when the pedal is at bottom dead center. Lower seat heights are reserved for people with specific knee problems that are aggravated by knee extension or as a method of diminishing ankle motion for people with ankle problems. The highest sensible seat height (usually between 105% and 115% of inseam length) is determined by raising the seat until just before the cyclist’s pelvis begins to rock back and forth. The rocking pelvis is a sign that the cyclist is reaching for the pedal at bottom dead center. Assuming the cyclist has ideal morphology, the hamstring muscles restricting knee extension are the limiting anatomical factor that determines the highest seat height. More frequently, the sports specialist will find that the limiting factor for seat height is how well the knees track in the higher seat position. Disparity in femoral condyle size will cause the knee to go through a lot of side-to-side motion with each pedal cycle when the seat is too high and the knee is forced to function in a more extended position. Cyclists tend to like the higher seat position because it is more like a walking

B

Figure 6. A, Seat height is measured in line with the seat tube from the top of the seat to the pedal positioned at bottom dead center. B, Leg length is the distance from the greater trochanter to the floor, and inseam length is the distance from the symphysis pubis to the floor while the cyclist is wearing shoes.

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or running posture, a higher cadence is easier to maintain, and, for some cyclists, the oxygen uptake efficiency may improve significantly. The sports specialist should assess several different seat heights for their effect on the cyclist’s biomechanics.

Avid cyclists usually disapprove of altering seat height by more than a half-inch, particularly if the suggestion is to reduce the seat height. If the seat height contributes to the cyclist’s injury, adjustments of less than an inch have almost no effect on the muscle phasic activity and tension or on joint position and motion during cycling. Although altering seat height is frequently very important to resolve a cyclist’s problem, other treatment options are usually also needed.

How far forward or backward the seat is adjusted can also affect the leg mechanics. The distance from the cyclist’s hip to the pedal decreases as the seat is moved forward and increases as the seat is moved backward. Spinning (high cadence) is facilitated by moving the seat forward, and increased power is associated with a seat in a rear position. The traditional method of adjusting the fore and aft seat position is to drop a plumb line from the posterior surface of the patella when the crank is in a horizontal position.

The plumb line should point directly to the pedal axle.12 This so-called KOPS method (knee over the pedal spindle) is very empirical and has relatively little to do with the cyclist’s individual morphology. Cyclists who need reduced hip flexion and greater knee flexion because of their particular problems should have their seat moved forward.

The crank arms connect the pedals to the bicycle.

The crank-arm length determines the size of the circle made by the feet, the amount of hip and knee flexion and extension through one pedal cycle, and the amount of up-and-down motion necessary for one complete pedal cycle. Crank-arm length should be correct before seat height is adjusted. Longer crank arms (170 mm to 180 mm) have traditionally been used for greater leverage for climbing, and shorter crank arms (160 mm to 170 mm) for higher cadence and quick acceleration.

The most important factor the sports specialist should consider when evaluating crank-arm length is the cyclist’s leg length. Although the difference between a 165-mm crank arm and a 175-mm crank arm may seem small (merely 1 cm, or three-eighths of an inch), an excessively long crank arm can create considerable problems. As a general rule, from an injuryprevention standpoint, cyclists will do better with a slightly shorter crank arm than one that is slightly too long.

The biggest problem with using crank arms that

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are too long is that the knee must go through a greater excursion and the tibia must travel a greater distance around the femoral condyles. The knee will move farther from the bicycle at top dead center and closer to the bicycle at bottom dead center because of the disparity in size of the medial and lateral condyles. A longer crank arm also means that the patellar retinaculum must travel greater distances around the femoral condyles with each pedal cycle, which provides greater opportunity for friction problems.

Buttars 13 described a formula for determining proper crank-arm length: multiply the distance in millimeters from the ground to the greater trochanter by 0.185. Crank arms are commonly sold in lengths of 160, 165, 170, 172, 175, and 180 mm. For example, for a cyclist who measures 900 mm from the floor to the greater trochanter, a 166.5-mm crank (900 mm × 0.185) would be ideal. If the number resulting from the formula falls between the available crank-arm lengths, the sports specialist should recommend the shorter crank. In this case, a 165-mm crank arm would be appropriate. Most cyclists who need a crank-arm change have been using cranks that are too long. With the reduction in crank-arm length, the cyclist should use slightly lower gears and a higher cadence to make the adjustment comfortable. No research has demonstrated enough benefit from the leverage provided by the longer crank arm to compensate for the energy expended by the greater upand-down leg motion.1 Torso and upper-body adjustments are performed by adjusting the handlebars up and down or by moving them farther away or closer. By lowering the handlebars or moving them farther away, the cyclist will bend over further. In this position, the pelvis is rotated and tension is placed on the hamstring muscles; more weight is transferred onto the hands and arms; and wind resistance is reduced. The back may become hyperextended and the back (paraspinal) muscles must work harder the further the cyclist bends over. This is because the gluteal muscles are under more tension with greater hip flexion and can be more effective. This greater gluteal muscle tension also creates increased iliotibial band tension.

The traditional method of adjusting the handlebars is to position them so that the seated cyclist’s upper body is angled approximately 45° with respect to the horizontal plane with the arms slightly flexed at the elbows.12 Racers generally prefer to have the handlebars a little lower, allowing them to ride in a more aerodynamic position. The up-and-down adjustment can easily be made on the bicycle, but the stem length is usually changed by buying a new stem (adjustable stems are available).

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Foot position can be changed with toe clips, cleats, and wedging the cycling shoe. Toe clips are metal or plastic devices that protrude from the front of the pedal and come up over the top of the foot to secure the foot to the pedal. When attaching a toe clip to the pedal, one should place it to allow the shoe to sit as close as possible to the cyclist’s angle of gait (Fig. 7). A more sophisticated method is to align the knee-joint axis perpendicular to the line of progression with the cyclist seated on the bicycle.

The lower leg is allowed to hang free, and the foot is placed in the subtalar joint neutral position. The toe clip is then positioned to allow the foot on the pedal to remain in this position. An example of improper toe-clip placement is a toe clip that sits too far medially. The toe clip will force the foot to adduct relative to the correct transverse-plane foot placement on the pedal. Inappropriate transverse-plane foot placement can create severe knee problems. The toe clip should be long enough for the ball of the foot to sit directly over or slightly anterior to the pedal spindle and not posterior to it.

Toe clips significantly improve the cyclist’s comfort by reducing the peak activity of the quadriceps muscles because more muscles contribute to propulsion.10 Oxygen uptake is unchanged when cyclists use toe clips, but most cyclists can approximately double the distances they can ride without toe clips. This occurs because more muscles become active, keeping the oxygen demand constant. However, because more muscles share the work, less work is required of each muscle group, which prolongs endurance.

Similarly, the use of cleated cycling shoes to secure the foot to the pedal increases the number of muscles used for propelling the bicycle and deter-

mines the transverse-plane angle of the foot on the pedal. The frontal-plane position of the foot can be altered by wedging the shoe or placing foot orthoses within the shoe. Altering the frontal-plane position of the foot can change the distance the knee tracks from the top tube.

Although it is not possible to alter the size of the bicycle, it is occasionally necessary to recommend a different bicycle size. A crude method of choosing the proper bicycle size is for the cyclist to straddle the top tube of the bicycle. When the feet are flat on the ground, the crotch should clear the top tube by about an inch. Cyclists do not need to be able to touch the ground while seated because they should dismount the bicycle seat when the bicycle comes to a complete stop. The correct bicycle size should be determined with the rider seated on the bicycle to allow assessment of seat tube, top tube, and stem length. Bicycle racers usually ride bicycles sized slightly small so that there is less bicycle to flex when they exert a firm push on the pedal and because they generally ride in a more bent-over position.

Cycling clothes tend to be formfitting so as to reduce wind resistance. Once the cyclist reaches speeds of 12 mph, wind resistance becomes the greatest impediment to speed. Cyclists in regular clothing have twice the wind resistance as those who use cycling clothing. The crotch of cycling shorts is usually padded to reduce chafing, and cyclists frequently wear padded gloves to reduce focal pressure on their hands.

The most important piece of equipment is the helmet. Over 1,000 cycling-related deaths occur each year in the United States; almost all are the result of head injuries. One-third of all head injuries in cycling

Figure 7. The toe-clip position can influence the angle of the foot on the pedal. Left to right: adducted, straight, abducted.

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are fatal. Cyclists rarely die from head injuries sustained while wearing a helmet.

Cycling Shoes Novice cyclists usually wear recreational shoes when riding a bicycle. Cyclists who are a bit more serious about the sport will frequently buy bicycle-touring shoes. The touring shoe’s stiff sole protects the foot from the pedal and is somewhat more efficient in transmitting force to the pedal,10 yet can also be used for walking. The touring-shoe sole frequently has one or more grooves that grip the pedal and allow the cyclist to pull back and up slightly during the recovery phase. The problem is that the grooves on the bottom of the cycling shoe are not angled appropriately in the transverse plane for every cyclist. A touring shoe thus may cause severe knee symptoms in certain individuals.

Cleated cycling shoes are made specifically for bicycle riding. The cleat on the bottom of a cycling shoe secures the foot to the pedal, allowing the cyclist to forcefully pull the foot back and up during the recovery phase as well as push down during the power phase without fear that the foot will come off of the pedal. Cleated cycling shoes result in a greater portion (300° versus 200°) of the pedal cycle in which the cyclist contributes to forward progression and creates a smoother transmission of force to the bicycle.10 Reducing the workload of the quadriceps muscle during the late recovery and early power phases reduces knee strain and fatigue because more muscles share the responsibility of propulsion and there is less negative force on the recovery-phase pedal for the power-phase limb to overcome. The traditional cleat has a groove on its underside in which a portion of the pedal sits. The strap of the toe clip holds the groove of the cleat on the pedal. The strapless cleats and pedal systems secure the foot to the pedal similarly to ski boot binding systems. Some strapless systems allow transverse-plane motion.

A cleated cycling shoe has a rigid plastic, wood, or leather sole to prevent the cleat from being torn away from the sole and to protect the cyclist’s foot from the pedal. Another advantage is that a rigid-soled cycling shoe will not twist as much as a recreational shoe. If the foot has a tendency to roll inward with pronation, the sole of the firmer shoe will not twist with the foot and thus can contribute to foot stability.

Cleated cycling shoes are made over an adducted or inflare curved last that is shaped to be compatible with a 2- to 4-inch heel (forefoot plantarflexed on the rearfoot; toes dorsiflexed on the metatarsals). The upper is made of leather or synthetic materials.

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Strapless cleats require reinforced uppers because the cyclist uses the shoe and not the toe strap when pulling up.

One shoe modification is wedging to realign the lower extremity during the pedal cycle. The wedges are generally varus in nature and can be applied to the sole or inside of the front of the shoe (Fig. 8).

Some cleated cycling shoes have a forefoot valgus wedge built into the sole that will tilt the lower extremity closer to the bicycle. Having the knee closer to the top is a more aerodynamic position but can create biomechanical problems for many cyclists, as has already been discussed. When examining the shoe, the sports clinician should always set the shoe down on the counter edge as if the cleat were on the pedal and check that the bottom of the front of the shoe is parallel to the edge of the table.

Many shoes are made by European companies over lasts designed for the narrower European foot.

When cyclists have complaints that may be caused by excessive compression, the sports specialist should check the cyclist’s shoes first.

Pedal and Cleat Systems The most common pedal and cleat systems have a pedal designed to accept a toe clip and strap. The toe clip and strap, as previously discussed, allow the cyclist to pull up on the pedal during the recovery phase to maintain the cleat on the pedal. The following section on cleat adjustment will explain how this is accomplished.

The cleat must be adjusted so that the foot is angled in the transverse plane on the pedal on the basis of the cyclist’s anatomy. The amount of malleolar torsion has the greatest influence in determining whether the foot should sit on the pedal in-toed, straight, or out-toed. The cleat must be adjusted properly to avoid serious knee injuries, such as meniscal tears and patellofemoral misalignment disorders created by transverse-plane torsional forces.

In the last 20 years, strapless cleats have become very popular. Strapless cleats secure the foot to the pedal with a locking mechanism on the underside of the shoe, making straps unnecessary. The advantage of strapless pedals is that no strap puts pressure on the top of the foot, and the foot is slightly easier to release from the pedal. Reducing pressure on the top and sides of the foot reduces pressure on the cutaneous nerves and dorsalis pedis artery, alleviating the numb sensation common to cycling and allowing the feet to stay warmer in cold weather. The first strapless pedal systems were also unique because they allowed a little transverse-plane motion of the

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B

Figure 8. A, Anterior view of cleated cycling shoe sitting on a pedal. B, Varus wedge is placed inside of cycling cleat, canting the shoe.

foot on the pedal. This can be very helpful for some cyclists but injurious for others.

As the foot pronates and supinates during the pedal cycle, closed-kinetic-chain mechanics dictate that there will be some leg rotation. When a cyclist stands to apply greater force on the pedal, the leg and foot orientation relative to the pedal changes slightly. Cyclists who have femoral condyles with large size differences (eg, genu valgum) have more leg rotation with knee flexion and extension. For these cyclists, a cleated system that does not allow transverse rotation can be very restrictive.

Cleat Adjustment. No matter what style of cleat the cyclist uses, the cleat must be aligned properly to place the foot and leg in an orientation that does not cause excessive abnormal twist, torsion, or binding at the various lower-extremity joints. The foot position on the pedal is closely related to malleolar torsion and is maintained in this position by the cleat.

The position of the cleat on the shoe must be adjusted every time an adjustment is made to the bicycle or the shoe or if foot orthoses are added to the shoes.

The cleat can be adjusted in three ways. The best method is to use a Rotational Adjustment Device (RAD) pedal (Fit Kit Systems, Inc, Billings, Montana) while the cyclist is riding the bicycle on a wind-load simulator. The RAD pedal, developed by William Ferral of the New England Cycling Academy, sets the cleat in a position with no abnormal rotational force on the lower extremity. The RAD pedal is sensitive to transverse-plane force and can indicate to the cyclist when the cleat position minimizes rotational forces.

When watching the RAD device in use, one can observe the amount of “normal” lower-extremity rotation that occurs with each pedal cycle.

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In the second method of cleat adjustment, the cyclist sits on a wind-load simulator and pedals at a regular cadence in moderate gear. When the cleats are loose, the foot naturally rotates to its proper position unless the toe clip interferes. A second person is necessary to tighten the cleat before the cyclist removes the foot from the pedal. If the foot is removed from the pedal before the cleat is tightened, the cleat will rotate. While this angle is maintained, the shoe is removed from the pedal and the cleat is loosened slightly and slid forward so that the ball of the foot is over or anterior to the pedal spindle, and then the cleat is tightened again.

The third method is to align the knee joint axes so that they are perpendicular to the line of progression.

The foot is placed in the subtalar joint neutral position, and the cleat is placed to maintain the orientation of the foot. However, to avoid frequent alignment mismatch of the knee and ankle joint axes and to accommodate the variation in foot morphology, some experimentation with cleat position may be necessary.

One problem during cleat adjustment is that the toe clip can direct the foot to an incorrect position. It is best to make cleat adjustments with the toe clip very loose or completely off of the pedal.

Pedals. Cyclists commonly use two kinds of pedals. The most common type, called a quill pedal, has a spindle around which the pedal rotates and arms going out to two vertical pieces (the quill). The traditional cycling cleat attaches to the posterior portion of this pedal. This type of pedal creates no problems if the cyclist has a firm-soled shoe. A soft-soled shoe allows the quill to put irritating pressure on the foot in a very small area. A modification of the standard quill pedal is the Biopedal (Biosports, Inc, Mill Valley,

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California). The Biopedal can be adjusted so that the quills are inverted, thus allowing for accommodation of a forefoot varus deformity or a mobile medial column, and it can be adjusted up and down to accommodate limb-length inequalities.

Platform (touring) pedals are flat on the top surface to reduce pressure on any particular foot area.

This pedal is popular with some cyclists because a regular soft-soled walking shoe can be used without the disadvantages of excessive quill pressure.

Because of its flat surface, the platform pedal can be adjusted somewhat with a varus wedge. The quill type of pedal is very difficult to adjust. To create a frontal-plane cant, the cyclist must take the quill pedal to a machine shop to have a portion of the pedal removed. Because the Biopedal allows for correction of improper adjustments, it could have advantages over the standard quill pedal. Before a quill pedal is sent to a machine shop, it is usually best to put a wedge inside the slot on the cleat or on the bottom of the shoe to tilt the shoe in the desired direction. The wedge can be removed later if the adjustment is unsatisfactory.

Bending the spindle is generally unsatisfactory because the axle maintains its angulation relative to the crank. Bending the spindle works if the spindle rotates independently of its threaded base.

Another variation on the quill pedal is the Shimano AX drop pedal (Fig. 9). The top of the drop pedal is below the point of pedal rotation. This places the ball of the foot at the same level as the point of rotation of the pedal and is thought to provide for a more natural rotation motion. What actually happens is that the foot remains relatively horizontal throughout the pedal cycle. Through the years, a horizontal foot throughout the pedal cycle has been perceived as being a good cycling style. Some cyclists have learned to enjoy this type of pedal, but in general, it did not prove very popular because most cyclists use a plantarflexed foot position. The advantage of the drop pedal for the sports specialist is in treating limb-length inequalities. The drop pedal can be placed on the long-leg side and a standard pedal on the opposite side to balance the limb-length inequality. This can accommodate a quarter-inch to a half-inch of the limb-length inequality. If a thickersoled shoe is used on the standard pedal, short-leg side, another quarter-inch to half-inch can be accommodated. The Shimano AX pedal is no longer manufactured and is difficult to find.

Foot Orthoses for Cycling During cycling, the shoe holds the foot orthosis to the foot. The more rigid the shoe, the more the ortho-

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sis will dictate the desired motion to the foot. In the authors’ opinion, the ideal foot orthosis for cycling is as rigid as possible so that it has the greatest influence on the foot. Foot orthoses for the cyclist help to reduce misalignment of the overly pronated foot.

Foot orthoses may also act as a substitute medial column in the foot whose medial column is too inverted or that dorsiflexes too much when loaded.

The forefoot should be posted with an intrinsic forefoot post because it takes up less room in the shoe.

The forefoot post should accommodate the forefoot and rearfoot (subtalar and tibia) varum components observed when the cyclist is in the angle and base of cycling. If rearfoot varus is extensive, the shoe should be wedged to accommodate the tibial component and the foot orthosis used to accommodate the subtalar component.

A rearfoot post can be applied to the orthosis, but the post may create a shoe-fitting problem. It makes very little difference if the rearfoot post is flat or has a biplanar grind. The orthosis will sit on that portion of the biplanar grind that is parallel to the front edge of the orthosis. Touring shoes often accept an orthosis designed for general use. However, a cleated cycling racing shoe generally requires an orthosis made specifically for cycling, in part because of the curved shank of the shoe. It is best to use shoes that have a minimal sagittal-plane curve in the shank and thus are designed with a lower heel in mind.

Some orthotic laboratories and their consultants advocate placing multiple holes in the cycling orthosis to facilitate airflow. It is the authors’ opinion that the drill holes do little to make the foot feel cooler and tend to weaken the orthosis.

In many instances, adding a forefoot extension to the orthosis is advisable. The forefoot extension can be of a soft material to provide cushioning if it does not make the shoes too tight. This can be helpful when the cyclist complains of numb feet. Cyclists frequently have the misconception that soft material in their shoe reduces their efficiency.

The forefoot extension can also have a wedge shape. The varus wedge is almost always the appropriate choice. Besides helping to align the leg by bringing the shoe up to the foot, the varus wedge can distribute the force more evenly across the forefoot.

Distributing force over a larger surface area of the foot reduces focal compression.

An additional benefit of foot orthoses is an increase in submaximal cycling efficiency. Hice et al14 demonstrated that with the use of functional foot orthoses, oxygen consumption and heart rate were reduced while cyclists performed submaximal constant cycling work and that the cyclists felt less

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Figure 9. A, Conventional pedal with the pedal spindle (axle) below the axis of the metatarsophalangeal joints. B, The drop pedal can be used in cases of limb-length inequality but places the pedal spindle very close to the axis of the metatarsophalangeal joints.

fatigue. Their results suggest that foot orthoses can improve the cyclist’s mechanical efficiency.

Cadence To prevent injury, maintaining a cadence of at least 70 rpm and preferably over 80 rpm reduces the pressure on the limb if the cyclist chooses a lower gear.

Once injured, cyclists should definitely stay over 80 rpm and use low gears exclusively. If pain persists, the cyclist should stop riding.

Terrain Injuries associated with pushing too hard may also be due to climbing too many hills. When a cyclist climbs a hill while sitting or standing, the force applied to the pedal (up to three times body weight) is considerably greater than during travel on a flat surface (approximately half of body weight). Thus, if pushing too hard on the pedals contributes to the injury, an adjustment in terrain may help relieve some lower-extremity stress.

Strengthening Cyclists generally have very strong leg and lowerback muscles and are more likely to develop musclestrength imbalances as opposed to having frank weakness. The most common imbalance occurs between the vastus lateralis and vastus medialis muscles. The vastus lateralis muscle has a tendency to become overdeveloped relative to the vastus medialis muscle. When the vastus lateralis muscle can dominate the vastus medialis muscle because of overdevelopment, patellar alignment problems can arise. Overdevelopment of the vastus lateralis muscle relative to the vastus medialis muscle is frequently associated with a situation similar to a functional in-

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crease in the Q angle. To resolve this problem, the strength disparity and Q angle must be addressed. It is not easy to adequately strengthen the vastus medialis muscle, but such muscle-strengthening programs are available. An adjunct to the strengthening program is stretching to decrease tension and reduce friction. Functional increases in the Q angle are generally created by excessive adduction of the proximal leg and internal rotation secondary to subtalar and midtarsal joint motion. Addressing the cause of the foot motion will often lower the Q angle, restoring a balanced line of pull of the vastus lateralis and vastus medialis muscles.

Cyclists who like to “ankle” (forcefully dorsiflex their foot during the recovery phase) have a tendency to overdevelop the anterior leg muscles. Those experiencing discomfort in their anterior leg owing to hypertrophy should be advised not to do anterior legstrengthening exercises because of the danger of developing an ischemic-compartment syndrome.

A few years ago, the standard weight-lifting regimen for a cyclist always included squats. Because squats place tremendous stress on the knees, they are not recommended. Instead, to strengthen the gluteal, hamstring, quadriceps, and calf muscles, equipment that is specific for the joints and muscles to be strengthened over the same range of motion as when cycling should be used. The hamstring, gluteal, and calf muscles have traditionally been neglected because of the emphasis on the quadriceps muscles.

On the basis of research reviewed in the biomechanics of cycling section of this article, it is now known that the quadriceps muscles are not as important as was once believed.

Strengthening exercises for cyclists should be part of a well-balanced routine. If only the quadriceps muscle is strengthened, the strain around the patella will be increased because most of the power-phase work will be done through the patella. The goal

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should be a balanced power system for knee and hip extension during the power phase.

Stretching Stretching is probably the most neglected activity in cycling training. Very few cyclists stretch. In fact, many athletes who routinely stretch before and after other activities do not stretch before or after cycling.

Stretching can be an important part of the regimen for resolving many cyclists’ complaints. The muscles that tend to become short are those that are used the most: the lower paraspinal, gluteal, tensor fascia lata, iliopsoas, quadriceps, hamstring, and calf muscles.

The following stretches will stretch several of these muscles at the same time. The cyclist should stand with the feet together and bend over from the waist, resting the hands on a chair. The chair prevents excessive strain on the lower back. As the cyclist bends over further, the lower back, gluteal, and hamstring muscles are stretched. If there is a need to stretch the calf muscles, the cyclist can walk the hands forward on the chair while maintaining hip flexion. This same exercise can be done with the legs crossed to stretch the tensor fascia lata. The quadriceps muscles are stretched by standing on one foot with the knees close together. The nonweightbearing leg is pulled backward as the foot is pulled toward the buttock.

Emphasizing pulling the leg backward before pulling the foot toward the buttock maximizes the stretch while minimizing strain on the patella. Grabbing the leg with the opposite hand internally rotates the leg, creating greater stretch of the vastus lateralis muscle.

Performing the stretch with the hand on the same side externally rotates the leg and provides greater stretch of the vastus medialis muscle.

Examination of the Cyclist Physical Examination Most cycling injuries result from overuse; therefore, a thorough history must be obtained and a thorough examination performed to determine the anatomical source of pain and the extent and cause of the injury.

If the injury is traumatic, the evaluation and treatment should be approached in the same way as for any other injured person. With the exception of abrasions, traumatic lower-extremity injuries will not be covered in this article. The reader can consult a standard trauma text and other articles for information on traumatic injuries.

Except for questions specific to cycling, the history required of the cyclist is similar to that required of athletes in other pursuits. The history specific to cy-

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cling should include the point during the pedal cycle at which the pain is experienced, the type of terrain on which the cyclist rides, the gears used when riding on flat terrain, and how the cyclist was fitted for his or her bicycle. Once the sports specialist has reviewed the cyclist’s equipment and understands the intensity of participation and training habits, the injured body part can be examined.

The examination of cycling overuse injuries should include examination of the injured body part, gait analysis, general orthopedic examination, morphologic examination, inspection of the bicycle, and observation of the cyclist riding his or her own bicycle. The physical examination is performed to determine the anatomical source of pain and to provide the sports specialist with the opportunity to become familiar with the patient’s anatomy. This is important when the cyclist is examined while riding the bicycle.

When performing gait analysis, the sports clinician should pay particular attention to the position of the cyclist’s foot relative to the knee during the propulsive phase of gait. This approximates the preferred position of the foot on the pedal relative to the knee during the power phase. The propulsive period also provides information as to how the foot reacts to increased load. The foot on the bicycle pedal will react in a similar manner.

The general orthopedic and morphologic (biomechanical) examinations are basically the same for cyclists and other patients. However, some aspects of the examination are peculiar to or more important for the cyclist. While performing the orthopedic or morphologic examination, the sports specialist should keep in mind the morphology of the ideal cyclist versus that of the patient being examined. An example of this is the frontal-plane relationship of the plantar surface of the forefoot relative to the leg with the foot maximally pronated and the medial column of the foot loaded plantarly (Fig. 10). This will simulate the position of the foot relative to the leg during the power phase. An inverted loaded forefoot–to–leg relationship will tilt the leg toward the top tube and may create knee misalignment.

Other measurements of particular interest are hipflexed-neutral transverse-plane position, Q angle, malleolar torsion, frontal-plane ankle position relative to the rearfoot in stance, hamstring length, and inseam length.

Even though the hip itself can function well in any position within its normal range of motion, the softtissue structures around the cyclist’s hip can be affected if the hip is not functioning close to its transverse-plane hip-flexed-neutral position. An example is the gluteal muscles. If the knee moves medially,

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Figure 10. The functional forefoot-to-rearfoot relationship is determined by simultaneously dorsiflexing the medial and lateral columns of the foot and pronating the rearfoot and assessing the frontal-plane relationship of the plantar plane of the forefoot relative to the longitudinal axis leg.

forcing the hip to adduct and rotate internally, the gluteal muscles will be placed under greater tension and an iliotibial band problem could develop.

The high Q angle can contribute to patellofemoral misalignment in the cyclist, just as it does for other athletes. Knee extension by means of the quadriceps muscle activity can lead to patellar tracking problems and muscle-strength imbalances, as already discussed. The key for the cyclist is whether or not the Q angle changes significantly as the knee flexes because of the side-to-side motion created by the disparity in femoral condyle size.

Malleolar torsion is the prime determinant of transverse-plane foot position on the pedal. This, of course, can be affected by abduction and adduction of the foot with pronation and supination.

The ankle may be medial to, in line with, or, occasionally, lateral to the rearfoot. It is important when examining the cyclist to recognize when the ankle is medial to the rearfoot. This situation occurs as a result of the midfoot adduction associated with midtarsal and subtalar pronation. The medial ankle position (a very inefficient method of transmitting power to the drive train) places the ankle at risk for frequent contusions as a result of hitting the crank and may be a sign that the foot is contributing to knee misalignment.

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The hamstring muscles are the limiting factor controlling how high a seat may be raised and how far the knee will extend. If the hamstring muscles are short, the cyclist is forced to ride with a low seat position.

Riding in a bent-over, aerodynamic position places a lot of tension on short or normal hamstring muscles and may lead to injury. Excessive hamstring tension can create low-back strain by causing hyperextension of the spine and can cause knee problems by pulling the knee closer to or farther from the bicycle.

The cyclist’s inseam length is important primarily for determining the high and low extremes of the seat-height position, and leg length is important for assessing the appropriateness of the crank length.

The inseam length is measured from the floor to the top of the crotch or symphysis pubis. The leg length is measured from the ground to the greater trochanter.

The bicycle inspection gives the sports specialist a chance to assess the suitability of the bicycle and its parts relative to the cyclist’s musculoskeletal morphology. The length of the crank arms and seat height are the most important parts of a bicycle inspection. The crank-arm length determines the size of the circle that the feet will travel with each pedal revolution and helps determine the amount of flexion and extension at the hip, knee, and ankle. Seat height is also important because it determines what portion of each joint’s range of motion is used with each pedal revolution, which muscles function, and how efficiently they do their job. Guidelines for correlating seat height to knee function and correlating crank length to leg length are provided elsewhere in this article.

Cycling Analysis Once the sports specialist has a basic understanding of the cyclist’s historical and morphologic profile and has inspected the bicycle, the bicycle should be securely fastened to a wind-load simulator and the cyclist observed riding the bicycle. Initially, the bicycle should be in a medium gear, and the cyclist should be allowed a couple of minutes to warm up and relax before the examination begins. While the cyclist is riding, the clinician should view the cyclist from the front, side, and rear. If the clinician suspects that seat height contributes to the cyclist’s complaint, the seat height should be raised or lowered at least 1 inch and the observation repeated. When viewing the cyclist from the front, the clinician should view the pattern of motion of the knee as well as the foot position and motion.

Ideally, the knee should describe a very narrow oval or figure-eight-shaped excursion with the long

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axis oriented vertically. Should the knee describe a wide oval or figure-eight path that is tilted from the vertical orientation, the cyclist is at risk for problems. Exaggerated foot pronation may tilt the knee medially throughout the power phase or create a motion similar to that created by genu valgum. Genu valgum will cause the knee to move laterally in the late recovery phase and medially in the second half of the power phase because knee extension accentuates the differences in femoral condyle size (Fig. 11).

When viewing the foot, the clinician should watch the position of the ankle relative to the foot. If the foot is pronating, the ankle will move medially relative to the foot, and when supinating, the ankle will move laterally. Almost all feet will pronate as they are loaded during the power phase. This can be exaggerated by having the cyclist shift into a higher gear while riding on the wind-load simulator. The position of the ankle relative to the crank is important for those who occasionally hit the medial malleolus on the proximal crank. The transverse-plane position of the foot on the pedal should be roughly the same as the angle made by the foot relative to the knee in stance.

When viewing the cyclist from the side, the clinician should watch the amount of hip flexion and extension relative to the torso, particularly at the extremes of this motion. The amount of up-and-down thigh motion should be observed relative to the cy-

clist’s size. If the thighs appear to have exaggerated motion, the cranks are too long. The amount of flexion and extension occurring in the knee and ankle is also important.

Because of morphologic variations, there is no set formula, applicable to all cyclists, to describe how much joint motion should occur at each joint through the pedal cycle. The general rule is that if the motion appears excessive, it is probably too much.

Traditionally, cyclists try to keep their feet relatively horizontal throughout the pedal cycle, an impractical method of cycling because most cyclists tend to have slightly short calf muscles, which makes cycling with the foot in a horizontal position nearly impossible. Therefore, most cyclists tend to cycle with their feet slightly plantarflexed throughout the entire pedal cycle. For this reason, cycling shoes are made as if they had a 2- to 4-inch heel. The plantarflexed position of the ankle is generally not a problem even if the plantarflexion is quite extreme. An extremely plantarflexed ankle position throughout the pedaling cycle should alert the clinician to a severe equinus deformity or to excessive seat height. When the seat is too high, the cyclist will reach for the pedal at bottom dead center by plantarflexing the ankle.

When viewing the cyclist from behind, the clinician should observe whether the pelvis is rocking and watch the knee and foot motions. The pelvis

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Figure 11. A, As the knee goes up and down, a normally functioning knee will trace a narrow, vertical figure-eight pattern. B, A poorly functioning knee will trace a wide, slanting figure-eight pattern.

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should be relatively stationary. If side-to-side rocking pelvic motion is considerable, the seat is too high and the rider is reaching for bottom dead center. The knee and ankle motions observed from the front should be confirmed from behind.

Once the cyclist is riding comfortably, the clinician should close his or her eyes and listen to the sound made by the wind-load simulator. Ideally, the cyclist should make equal noise during the power phase of the left and right limbs, and the sound should fluctuate relatively little when the pedals are at top and bottom dead center. Cyclists commonly have very poor pedaling style: they will push very hard and abruptly during the power phase but slowly at top and bottom dead center. This rough style of pedaling places more stress on the power-phase limb because the recovery-phase limb is probably contributing little to propulsion and the power-phase limb is functioning in an explosive manner.

In recent years, the wind-load simulator has become a popular indoor training aid, but unfortunately it contributes to the nonsmooth pedaling style. A wind-load simulator does not provide much momentum to help the cyclist through top and bottom dead center. In fact, the resistance of the fan or magnet slows the pedal speed considerably, and the cyclist must push explosively during the power phase to maintain the desired cadence. Before the advent of the wind-load simulator, rollers were a popular indoor training aid. Wind-load simulators became more popular than rollers because they are easier to ride.

On rollers, the rider had to pedal very smoothly or fall off. Riding on rollers is a lot like riding on a slick, ice-covered lake, forcing the cyclist to have a smooth pedaling style. Before wind-load simulators became popular, such explosive pedaling styles were much less common. This was not because many cyclists had rollers, but because no cyclist had a training device that encouraged an uneven application of power to the pedals.

Cyclists frequently use one leg more than the other. In fact, the dominant leg may vary from one day to the next depending on how each leg feels, injury status, and so on.8 An audible difference between the left and right legs may indicate that one limb is stronger and doing most of the work or that the uninjured limb is providing most of the propulsive power.

Once the clinician has observed the cyclist riding the bicycle, it is important to palpate several areas while the cyclist is riding, even if there are no problems in any of these areas. Most bicycle adjustments made by the clinician have the potential to create problems, and palpation can detect areas at risk for

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future injury. If friction has been determined to be the cause of the injury, the treatment goal is to reduce the amount of friction at the injury site. If no friction is detected during palpation, the body part is probably functioning normally. The clinician should palpate the anterior surface of the femoral condyles to assess the amount of retinacular friction in the vastus medialis and vastus lateralis muscles. The patella and the knee joint line should be palpated for tenderness or friction from the pes anserinus as well as the iliotibial band over the femoral epicondyle and greater trochanter. Palpation while the cyclist is riding is particularly important when that is the only time pain is present.

Palpation of the lower-extremity muscles is a very crude method of determining muscle phasic activity and comparing relative muscular effort. The paraspinal and quadriceps muscles are the easiest to palpate. This may be useful, for example, for the cyclist with low-back pain while cycling. It is not uncommon to find morphologic asymmetry of the paraspinal muscles secondary to asymmetry in their phasic activity. The hypertrophied paraspinal muscles generally contract longer and harder than their contralateral counterparts. The clinician must then determine if the asymmetrical loading of the lower back is created by a limb-length inequality, asymmetry of gluteal or hamstring muscles, injury, or other factors. Once the cause is determined, the clinician must make the appropriate adjustments to remedy the situation. Another example of asymmetrical muscle phasic activity can occur following knee surgery.

The cyclist may not contract the operated limb quadriceps muscle as forcefully or as long as the quadriceps muscle of the unoperated limb.

Etiology of Cycling Injuries and Their Treatment Patellofemoral Dysfunction (Cyclist’s Knee) The injured area for which cyclists most commonly seek treatment is the knee. Knee complaints usually involve pain either deep to or adjacent to the patella.

Anterior knee complaints usually involve the patella retinaculum and the patellofemoral joint. The retinaculum may become inflamed medially, superiorly, or laterally because of excessive friction over the femoral condyles.

Pain emanating from the patellofemoral joint results from either poor tracking or excessive intra-articular compression that has damaged the cartilage.

Cyclists’ patellofemoral-joint problems frequently differ from those of runners in that the cyclist will point

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to the center of the patella and describe the pain as being directly under the patella rather than on the medial or lateral side. Unlike the situation in runners, this is frequently a true chondromalacia.

Retinaculitis and patellofemoral problems are usually due to either excessive extension force across the patellofemoral joint, poor frontal-plane tibiofemoral alignment, or both. Excessive force may result from riding in gears that are too high or from doing excessive hill climbing. With too much force, the retinaculum may be compressed onto the femoral condyles or the patella may be compressed onto the anterior surface of the femur, creating excessive wear and tear. When the seat is rather low and the knee functions in a very flexed position, the vectorial forces compressing the patella on the femur are greater. When the knee is more extended, more extension force across the knee is directed parallel to the articular cartilage, rather than perpendicular to it.

Poor alignment of the tibial and femoral segments results in poor alignment of the patellofemoral joint.

Patellofemoral misalignment may lead to unequal pressure across the patellar articular surface, which creates small areas of excessive pressure on the cartilage and excessive friction of the retinaculum on the femoral condyles owing to increased tension. Patellofemoral misalignment may be created by intrinsic knee angulation that is reflected in the Q angle or by a functional increase of the Q angle created by foot mechanics. The intrinsic knee-alignment problem is due to the natural valgus angulation of the knee, which places the quadriceps muscle lateral to the knee, thus creating an abduction force on the patella as well as an extension force. The abduction force distributes compressive forces unequally across the anterior aspect of the knee. As the seat is raised, allowing greater extension of the knee through each pedal cycle, the disparity of size of the femoral condyles increases, accentuating the valgus relationship.

Normal or excessive knee valgum frequently contributes to knee problems. The normal or excessive disparity in size of the medial and lateral femoral condyles associated with knee valgum means that the knee cannot flex and extend in a straight manner.

Instead, the tibia will trace the shape of a cone with the apex lateral (because the medial condyle has a larger radius), much like the conical motion that occurs at the ankle. During cycling, the foot and hip are fixed in position. Therefore, the knee must move from side to side to compensate for the disparity of condylar size.

The foot can contribute greatly to knee adduction.

The result is a functional increase in the Q angle as the foot tilts medially owing to supination of the mid-

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tarsal joint, medial column dorsiflexion, forefoot varus, or pronation of the rearfoot. Adduction and internal rotation of the knee increase the abduction force by the quadriceps muscle, which results in an imbalance between the medial and lateral forces at the patella.

Most cyclists with lower-extremity injuries secondary to a functional increase in the Q angle have a forefoot varus or develop an inverted forefoot relative to the leg when the forefoot is loaded. To bring the medial side of the foot down to the ground (pedal), the leg is tilted closer to the top tube of the bicycle, which creates a functional increase in the Q angle and the associated muscle imbalance. Forcing the knee closer to the top tube also means that the knee will flex and extend in a direction that is angled relative to the circular motion of the crank. Ideally, the knee axis should be perpendicular to the crank circle and line of progression and parallel to the bottom bracket spindle.

Whenever the knee is forced medially, the lateral thigh muscles develop a mechanical advantage at the patella compared with the medial quadriceps muscle, and tension increases in the vastus lateralis muscle as it is stretched. The vastus medialis muscle may develop retinacular friction problems over the medial femoral condyle as it tries to keep the patella centered in the femoral groove, and the vastus lateralis muscle may develop retinacular friction problems because the tension increases over the lateral femoral condyle.

The functional increase in the Q angle occurs when the knee is forced to function in an adducted and internally rotated position. As the Q angle increases, the associated muscle imbalance develops.

A functional increase in the Q angle can be reduced by treating the cause of the medial shift of the knee.

Cyclists with a true anatomical increase in their Q angle plus a functional increase are less responsive to treatment because knee realignment still leaves a true anatomical increase in the Q angle.

On the basis of watching many cyclists and varying their seat height, the authors have observed that the disparity in the femoral condyle size is greatest at the distal end and least on the posterior surface.

Thus, if the seat is quite high and the tibia is functioning primarily at the distal end of the femur, the sideto-side motion of the knee will be increased as compared with cycling with the seat in a lower position where the tibia is moving on the more equal posterior circumference of the femoral condyles. The lower seat position generally allows the cyclist to function with the knee in a more piston-like fashion with less side-to-side motion. The greater seat heights used by

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cyclists appear to create knee problems because the tibia is forced to move on the more valgus-angled distal femoral surface.

Anterior knee injuries commonly result from a combination of excessive compression force and misalignment. To resolve the problem, the friction of the retinaculum and patella on the femur must be reduced. As the clinician is adjusting the bicycle seat height and making improvements in foot function, there should be a palpable improvement in the anterior knee friction and improved knee motion while the cyclist is riding. If the problem is strictly the result of compression, using lower gears and raising the seat will reduce the compression of the patella and retinaculum on the femur.

Patellar misalignment and uneven distribution of force across the anterior aspect of the knee may be improved by lowering the seat if the cause is intrinsic knee angulation. If the problem is due to foot mechanics, wedging the shoe and using foot orthoses may improve the alignment.

Cyclists generally have well-developed quadriceps muscles, and therefore it is difficult to make a case that weak quadriceps muscles contribute to anterior knee problems. Cyclists’ muscular problems are more often related to a dynamic imbalance than to a frank weakness of the thigh muscles. The vastus medialis muscle, though well developed, is frequently not strong enough to balance the mechanical advantage and larger size of the vastus lateralis muscle.

Strengthening the vastus medialis muscle to improve the balance of these muscles is helpful but is generally not enough to resolve the problem. In fact, some of the best-developed vastus medialis muscles are seen on cyclists who have the worst misalignment problems, because of the increased workload given to this muscle to maintain patellar alignment. Improving the knee motion so that it is closer to the ideal goes a long way toward correcting the quadriceps muscle imbalance.

Short quadricep muscles may also contribute to increased friction of the patellar retinaculum. Stretching quadriceps muscles before and, more importantly, after riding will help the retinaculum glide over the femoral condyles with reduced friction.

Meniscal Injuries Aggravation of preexisting meniscal injuries or injuring the meniscus while cycling is not uncommon.

The cyclist will complain of pain that can be localized to the joint line, and there may be some swelling of the knee. The pain is usually greatest at the bottom of the power phase.

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The meniscus is injured in cycling from extension of the knee and the twisting motion that occurs in the second half of the power phase. Treatment of this injury usually requires an arthroscopic surgical procedure. The cyclist may be made asymptomatic by lowering the seat, which reduces knee extension.

Reducing knee extension reduces the associated twisting motion and the uneven compressive forces between the tibia and femur, but does not heal the pathology. Allowing continued riding may place the cyclist in danger of further meniscal damage.

Iliotibial Band and Greater Trochanter Bursitis As the knee and hip flex and extend, a tight iliotibial band may rub the femoral epicondyle, the greater trochanter, or both. The rubbing on the epicondyle is more common in males and is the most common triathlete injury. Because of their wider hips, female cyclists, runners, and triathletes experience the iliotibial band problem more frequently at the greater trochanter. The iliotibial band may be tight or the gluteus maximus muscle may make it tight. Adduction of the knee in the hip-flexed position makes the iliotibial band tighter.

The gluteus maximus muscle inserts primarily on the iliotibial band; thus the more tension there is in the gluteus maximus muscle, the greater the chance for greater trochanteric bursitis and iliotibial tendinitis or bursitis over the femoral epicondyle. The gluteus maximus muscle is stretched when the knee adducts, thus adducting the hip, or when the cyclist is too bent over (hip flexion).

Stretching the iliotibial band and gluteus maximus muscles is an important part of resolving iliotibial problems. In addition, the mechanical factors that are placing tension on the iliotibial band must be corrected. Reducing adduction and internal rotation in the knee may involve lowering the seat to allow for better knee tracking or treating the foot for those problems that cause knee adduction and internal rotation. Lowering the seat height also reduces iliotibial band tension by reducing knee extension. When knee adduction is not the primary cause or has been corrected by addressing the foot contribution to knee adduction, placing the cyclist in a more upright position may be helpful.

Creating a more upright position for the cyclist will reduce hip flexion and decrease the tension of the gluteus maximus muscle. The upright position is created primarily by raising the handlebars and secondarily by moving the seat forward or the handlebars backward.

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Low-Back Strain Cycling generally creates relatively little stress on the back. Back problems among cyclists are infrequent and usually not severe, but when they do occur they tend to be rather irritating. This article reviews only low-back muscle strain. For back injuries, the reader should consult an orthopedic text.

Low-back strain is usually described as an aching sensation during cycling that may persist for a while after cycling and may occur on one or both sides of the lower back. Examination will usually reveal no specific area of discomfort, and the only remarkable finding may be that one group of the paraspinal muscles is more developed than on the contralateral side.

Complaints of back pain on both sides usually result from riding bent over too far or with too much knee extension during the pedal cycle for the length of the cyclist’s hamstring muscles. The bent-over position creates an advantage for the gluteal muscles to hyperextend the back, and knee extension (because the hip is flexed) creates a similar hamstring pull on the pelvis, resulting in hyperextension of the back.

Treatment may involve adjusting the bicycle so that the cyclist rides in a more upright position, advising the cyclist to ride in lower gears, having the cyclist travel over fewer hills to decrease gluteal and hamstring muscle exertional force, and moving the seat forward to decrease the pull of the gluteal muscles. If the seat is raised to treat another condition, the increase in back flexion may create a problem.

One can minimize the increased pull of the hamstring and gluteal muscles on the pelvis by raising the handlebars at least as much as the seat was raised or by decreasing the stem length to reduce the bent-over position.

Discomfort on one side of the lower back is frequently related to lower-extremity asymmetry. Examination may reveal a limb-length inequality and a secondary overdevelopment of the muscles on one side of the back. When the cyclist is cycling, the clinician may feel that the muscles on the underdeveloped side of the back are barely active. The treatment goal in these instances is to balance the activity of the paraspinal muscles by making the left and right limbs function in a similar fashion. Adjustments for limbs of different lengths can be made through shorter crank-arm length, special raised pedals, and thicker shoe soles for the short limb.

Other problems that can cause asymmetrical lowback discomfort are twisted pelvis and asymmetrical leg function and strength. The noise from the wind-

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load simulator can provide clues as to the asymmetry of leg function. When the cyclist has a scoliosis deformity or a twisted pelvis, altering the thickness of one side of the bicycle seat may go a long way toward balancing the paraspinal muscle function. If the asymmetrical leg function or strength has caused the back strain and fatigue, this must be corrected.

Hamstring Strain and Tendinitis Cyclists with a hamstring injury usually complain of pain in the tendon or insertion of the lateral hamstring muscles. Medial hamstring problems do occur but are less common. Examination may reveal any combination of short hamstring, overextension of the knee at bottom dead center, and excessive bent-over position.

Treatment of excessive tension on the hamstring muscle and tendons usually involves lowering the seat to decrease knee extension and stretching the affected muscles. On a few occasions, adduction of the knee places additional tension on the hamstring muscles, which must also be treated at the same time.

Calf Cramping and Achilles Tendinitis Occasionally, a cyclist may complain of cramping in the calf muscles and pain in the Achilles tendon when cycling. Both problems are created by overuse secondary to excessive ankle dorsiflexion during the power phase. Calf and Achilles tendon problems tend to be associated with a seat that is too low, forcing the heel to drop during the late recovery and early power phases, which dorsiflexes the ankle. A low seat increases the tension on the Achilles tendon and reduces the ability of the gastrocnemius muscle to assist the soleus muscle in resisting dorsiflexion forces from the pedal. The result is more stress on the soleus muscle and on that portion of the Achilles tendon associated with the soleus muscle.

Calf cramping and Achilles tendinitis can be resolved by posterior ankle rest strapping and raising the bicycle seat. Raising the bicycle seat will force the cyclist to reach more for the pedal, thus allowing the ankle to work in a more plantarflexed position.

The plantarflexed position reduces stress on the Achilles tendon, and greater knee extension allows the gastrocnemius muscle to assist the soleus muscle in resisting the dorsiflexion forces. In addition, stretching exercises for both the gastrocnemius and soleus muscles should be part of the program. Occasionally, shorter cyclists need shorter cranks so that the foot is not forcefully dorsiflexed at top dead center and yet the cyclist can reach the pedal at bottom dead center.

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Numb Feet A numbing sensation of the foot during cycling is very common, but a problem for which cyclists infrequently seek medical care. Generally, a numb sensation in the foot is a secondary complaint. Two exceptions to the numbing complaint for which the cyclist may seek medical advice are the numb sensations between the third and fourth toes secondary to Morton’s neuroma and a loss of sensation on the dorsum of the foot due to injury of the superficial dorsal cutaneous nerves of the foot.

Compression of nerves is the common etiology for all of these complaints. To address the cause of the numbing foot sensation, the sports specialist needs to inspect the pedal, sole of the shoe, width of the shoe, and toe straps. The quill pedal, if used with a soft-soled shoe, can place highly concentrated pressure across the plantar surface of the forefoot.

Firmer-soled shoes may help resolve the problem.

Cycling shoes are frequently narrower than the foot.

Thus tight shoes commonly cause compression complaints. Cyclists who use toe straps frequently complain of generalized forefoot numbing or have a complaint that can be related to the position of the toe strap on the foot. If the toe strap contributes to the problem, use of a strapless cleat and pedal system may completely alleviate the complaint.

In addition to changing the cycling equipment, the sports specialist may recommend a soft material to cushion the inside of the shoe, if the shoe has adequate room. Often, the firm-soled cycling shoe protects the foot from the pedal but contributes to the forefoot discomfort because it is so firm. Cyclists who have predominantly plantar forefoot numbing may benefit from actively pulling up during the recovery phase to decrease the pressure. A functional foot orthosis within the shoe will also reduce the force on the ball of the foot by distributing the pressure over a larger surface area of the foot. In a similar fashion, a metatarsal “cookie” or bar can be applied to the inside of the shoe to partially distribute the force to the metatarsal shafts and to spread the metatarsals so that there is less nerve compression.

Medial Malleolar Contusions A frequent primary complaint is that the cyclist’s medial malleolus strikes the base of the cranks during the second half of the power phase. This can be a very painful injury and has a tendency to make the cyclist cautious about pushing hard on the pedal.

The cause of this complaint is a high (oblique) midtarsal joint axis that, when pronated, moves the

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rearfoot medially and closer to the crank. If the high oblique axis of the midtarsal joint combines with high malleolar (tibial) torsion, the abducted foot position on the pedal brings the rearfoot even closer to the crank, thus increasing the probability that the medial malleolus will strike the crank.

Deciding to reduce only the abducted angle of the foot on the pedal may result in severe knee injury. Instead, it is best to first use a foot orthosis 15 and wedge the shoe to reduce the midtarsal pronation.

When the midtarsal pronation is reduced, the ankle will move slightly away from the crank. Because the foot is less pronated with the foot orthosis, the angle of the foot on the pedal must now be changed to a less abducted position to avoid severe knee problems. Thus, to properly resolve the problem of the medial malleolus striking the crank during the power phase, the midtarsal joint and the angle of the foot on the pedal must be addressed together.

Abrasions One of the most common traumatic experiences for the cyclist is falling off of the bicycle. This may happen as a result of hitting a hole, slipping on ice or gravel, negotiating a curve too fast, mechanical failure of the bike, or trying to avoid a car or other object. Besides head injuries and those from falling on the outstretched arm, a cyclist usually receives several abrasions. For racers, this is just part of the sport.

Cyclists falling off of their bicycles have a tendency to fall on their sides. The areas that receive the most abrasions are the forearm, the area superficial to the greater trochanter, the lateral side of the knee, the lateral side of the thigh and leg, the lateral malleolus, and occasionally the side of the torso or back. Abrasions that occur superficial to an osseous prominence tend to be deeper and more severe. The abrasion can expose the underlying bone. Abrasions are frequently treated by using a local anesthetic gel to reduce pain, an antiseptic scrub, an antibiotic ointment, and an appropriate nonadhesive dressing.

The abrasion, though uncomfortable, will generally not prevent sports participation. The wound should be cleansed and redressed once or twice daily, particularly after each day of competition or activity. If underlying bone is exposed, the sports specialist should place the threat of osteomyelitis above the cyclist’s desire to continue activity and should manage the bone aggressively through debridement, cleansing, intravenous antibiotics if necessary, and so on.

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Conclusion This article has briefly reviewed the biomechanics and etiology of cycling injuries and provided the sports specialist with a brief look at the complexities of cycling. When treating cyclists, the sports specialist must be equally aware of how to resolve the cyclist’s complaint and of the possibility of creating a new injury. Unless the sports specialist is willing to devote a considerable amount of time to analyzing the mechanics, it is best to refer the patient to a clinician with a particular interest in cyclists who will take the time to review the situation thoroughly.

Acknowledgment. Valerie Knight for her assistance with the illustrations.

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4. D AILY P, D AILY G: Eliptical sprockets: power plus or minus? 49er Engineer 9: 2, 1975.

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