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stretching & flexibility


Chapter 1: The Physiology of Stretching

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stretching & flexibility To gain an understanding of a stretch, it is essential to examine the muman musculoskeletal system. This outlines where a stretch begins and what physiological changes occur to assist in the extension of any muscle through its full range of motion. This chapter highlights important physiological concepts that become apparent when a muscle is being stretched. Each sub-section will begin with a general discussion and for those who are interested in furthering their knowledge, more detail is provided on the individual topics. The human musculoskeletal system The muscles and bones in the human body create what is called the musculoskeletal system. Connected to the brain through the central nervous system (CNS), the musculoskeletal system provides protection for the body’s internal organs and assists in the function of respiration, circulation, balance and kinesthetic awareness. Bones provide structural support for body posture while muscles provide contractile tension between bones, giving humans the ability to move. In order to create mobility, bones are connected at joints by ligaments while the muscles themselves are attached to the bones via tendons. Muscle composition The numerous muscles in the body come in an array of sizes and shapes, and all have a specific purpose. Major muscle groups like the legs, back and chest are largely used for compound movements, which require more than one muscle. Then there are the biceps, triceps and calf muscles, which make up some of the smaller muscle groups, but which play an assisting role in the movements of the bigger muscle groups. Then there are the all-important functions of the heart muscle, and those in the hands, feet, and even the ear. They all perform different functions, but they are crucial to every movement made. A muscle consists of what are known as fascicles. These strands of tissue are the same kind of muscle strand that can be seen when we cut open a piece of steak or chicken. Within the fascicle are bundles of muscle fibres, known as fasciculi. These muscle fibres are bound together by tens of thousands of

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stretching & flexibility myofybrils that resemble threads, which in turn, contract, relax and lengthen when required. The myofybrils contain millions of bands that lay end-to-end and are known as sarcomeres. These consist of filaments, some thick and some thin. These are myofilaments and are formed by contractile proteins. Muscle contraction There is a certain structural sequence that makes up the operation of a muscle: Firstly, nerves are connected to the spinal column and then to the muscle. The nerves and muscle meet at a crossroads called the neuromuscular junction. As an electrical pulse passes through the neuromuscular junction, this signal is sent through the inside of the muscle fibres. Within the muscle fibres, this signal triggers the flow of calcium, causing both the thick and thin myofilaments to slide across each other. Consequently, the sarcomere shorten, generating force. As many billions of sarcomeres shorten in the muscle at the same time, the muscle fibre contracts. There is no partial contraction of a muscle fibre. Muscle fibres cannot vary the level of intensity by which they contract according to the load they are under. However, there are strong and weak muscle contractions, caused by the number of muscle fibres that are recruited to successfully perform the required function. As the central nervous system recruits increasing amounts of muscle fibres, the force behind the muscle contraction is enhanced. Muscle fibre types The energy from the mitochondria creates the calcium flow in the muscle fibres. This is the part of the muscle cell that converts glucose, otherwise known as blood sugar, into energy. The type of muscle fibre will determine the amount of mitochondria. The greater the amount of mitochondria in the muscle fibre, the greater the level of energy it can produce. There are two types of muscle fibres: slow-twitch and fast-twitch. If a person has predominantly slow-twitch muscle fibres, their muscles are slow to contract, but also slow to fatigue. These people are normally suited to endurance activities, like marathons, where the body is required

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stretching & flexibility to sustain a constant and even release of energy over a long period of time. Slow-twitch muscle fibres are also known as Type 1 muscle fibres. Fast-twitch muscle fibres are quick to contract and are catergorised in two groups — Type 2A muscle fibres, which fatigue at a steady rate, and Type 2B muscle fibres, which fatigue quickly. People with fast-twitch muscle fibres often do well in activities that require strength and power, like weightlifting or sprinting. The biggest contributor to the slow fatigue rate of the slow-twitch muscle fibres is the amount of mitochondira they contain — more in comparison to the fast-twitch muscle fibres. This enables the slow-twitch fibres to produce more energy for prolonged periods of time. The size of the slow-twitch fibres is smaller in diameter than fast-twitch fibres and they have more blood flow in the capillaries that surround them. This allows the slow-twitch muscle fibres to mobilise more oxygen and remove greater amounts of waste products from the muscle fibres. This in turn lessens the extent of fatigue in the muscle. All muscles contain varying degrees of the three types of muscle fibres. There is a sequence of events that occur when a muscle begins to contract. Firstly, Type 1 fibres are called upon. Following this are the Type 2A and then 2B fibres. Type 2B fibres are more difficult to call upon and will only begin to participate when the majority of Type 1 and 2A fibres have been activated. To recall the differences between muscles that have predominantly slow-twitch fibres and those that have mostly fast twitch fibres, we can use a meat analogy. Dark meat gets its colour because it has a large amount of slow-twitch muscle fibres and therefore a greater number of mitochondria, which are dark. Meat that is light in colour, like white meat, consists mainly of muscle fibres that are at rest for the majority of the time, and are only called upon for momentary energy requirements in intense activity. White meat is light in appearance because it contains a lesser amount of mitochondria.

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stretching & flexibility Connective tissue The purpose of connective tissue is to provide strength and elasticity, while incorporating a base lubricant which enables fibres to slide across one another. This supports the fibres of the tissue to stick together in bundles. Connective tissues surround the muscle and the its fibres. The connective tissue is made up of two types of fibre. The first is collagenous connective tissue which, as its name suggests, is composed of mostly collagen. This is the fibre in the connective tissue which provides tensile strength. The second type of fibre is elastic connective tissue which consists mainly of elastin, and is the fibre within the connective tissue that aids in elasticity. At the base of the two types of fibre is a substance called mucopolysaccharide which provides lubrication and is a gluing agent. With an increasing amount of elastic connective tissue around a joint, the better the range of motion in the joint. Tendons and ligaments make up connective tissues and the fascial sheaths bind muscles into individual groups. Commonly referred to as fascia, these fascial sheaths are recognised by their location in the muscles. The inner-most fascial sheath that groups separate muscle fibres is called endomysium. Next is the perimysium, the fascial sheath that binds together groups of muscle fibres into separate fasciculi. Lastly, the epimysium, is the outermost fascial sheath that envelops the fascicles. Muscle groups When the muscles are activated and begin the process of moving a limb through the joint’s range of motion, the muscles normally co-operate within their groups. Agonists and antagonists As mentioned earlier, agonists cause the movement to occur. They create the normal range of movement in a joint by contracting. Opposite the agonists are the antagonists that are responsible for returning a limb to its initial position.

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stretching & flexibility Synergists These muscles perform, or assist in performing, the same set of joint motion as the agonists. Synergists are sometimes referred to as neutralisers because they help cancel out, or neutralise, extra motion from the agonists to make sure that the force generated works within the desired plane of motion. Fixators These muscles provide the necessary support to assist in holding the rest of the body in place while the movement occurs. Fixators are also known as stabilisers. For example, when the knee is flexed, the hamstring contracts, and, to some extent, so does the gastrocnemius (calf) and lower buttocks. Meanwhile, the quadriceps are inhibited — relaxed and lengthened somewhat — so as not to resist the flexion. In this example, the hamstring serves as the agonist, or prime mover, the quadricep serves as the antagonist and the calf and lower buttocks serve as the synergists. Agonists and antagonists are usually located on opposite sides of the affected joint — like the hamstrings and quadriceps, or the triceps and biceps — while synergists are usually located on the same side of the joint near the agonists. Larger muscles often call upon their smaller neighbours to function as synergists. A list of commonly used agonist/antagonist muscle pairs include: • anterior deltoids/posterior deltoids (front and back shoulder) • trapezius/deltoids (traps and delts) • abdominals/spinal erectors (abs and lower-back) • left and right external obliques (sides) • quadriceps/hamstrings (quads and hams) • shins/calves • biceps/triceps • forearm flexors/extensors

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stretching & flexibility Contraction types When a muscle contracts, it does not necessarily mean that the muscle shortens. Rather, it implies that tension has been created in the muscle. There are different types of muscle contraction, including isometric contractions and isotonic contractions, with the latter being divided into concentric and eccentric contractions. An isometric contraction means that during a contraction there is no movement because the load on the muscle is more than the tension that is generated by the muscle contracting. An example of this is when a muscle attempts to push or pull an object that is immovable. The isotonic contraction occurs when there is a contraction, but with movement. Tension made by the contracting muscle is more than the load on the muscle. This will happen when the muscles succeed in pushing or pulling an object. The two types of isotonic contraction are: Concentric contraction: This is a contraction in which the muscle decreases in length (shortens) against an opposing load, such as lifting a weight up. Eccentric contraction: This is a contraction in which the muscle increases in length (lengthens) as it resists a load. An exercise that clearly exemplifies both a concentric and eccentric contraction is the traditional bicep curl. The concentric contraction occurs when the elbow is flexed, allowing the biceps muscle to contract. The eccentric contraction happens when the elbow is extended and returns the weight back to the starting point. It is the bicep muscle controlling both the concentric and eccentric phases of the exercise. Stretch details When a stretch is performed, the muscle fibres are stretching. This process starts with the sarcomere, which is the base of the contraction in the muscle fibre. When the sarcomere contract, the overlap of the thin and thick myofilaments becomes larger. As it stretches, the overlap less-

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stretching & flexibility ens, allowing the muscle fibre to lengthen. As the muscle fibre elongates to its maximum resting length, extra stretching generates more force on the connective tissue around the muscle fibre. With the tension increasing still, collagen fibres composed within the connective tissue align with the tension. Therefore, when you stretch, you are stretching the muscle fibre sarcomere by sarcomere to its maximum length, and then the connective tissue takes over and makes up any remaining slack. This helps to bring any remaining fibres into line, along the direction of the tension. Just as the strength of a contracting muscle is determined by the number of fibres recruited during the contraction, the maximum length of a stretched muscle is dictated by the number of fibres that are stretched. In this case, when more fibres are stretched, the muscle will elongate further for a particular stretch. Proprioceptors Tiny nerve endings transmit all information regarding the musculoskeletal system to the central nervous system and are known as proprioceptors. These are at the foundation of proprioception, being the perception of one’s body position and movement. Proprioceptors are aware of any alterations in physical posture, movement, or position, along with changes in tension and force in the body. They are located in the nerve endings of joints, tendons and muscles. The proprioceptors that are directly related to stretching are found in tendons and muscle fibres. The two types of muscle fibres are the intrafusal muscle fibres and the extrafusal muscle fibres. The extrafusil fibres contain myofibrils, and are usually what is meant when discussing muscle fibres. The other type of muscle fibres are the intrafusal fibres, also known as muscle spindles, which lie alongside the extrafusal fibres. The muscle spindles, also known as stretch receptors, are the major proprioceptors in the muscle. The other proprioceptor that is triggered when stretching is found in the tendon at the end of the muscle fibre and is referred to as the golgi tendon organ. A third variety of proprioceptor, the pacinian corpus-

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stretching & flexibility cle, is situated near the golgi tendon. It is used for detecting any changes in one’s movement or pressure within the body. Stretch reflex During a stretch, the muscle spindle is stretched along with the muscle. The muscle spindle relays messages to the spine regarding any change in length, and the rate at which this change occurs. The spine conveys the signal and alerts the stretch reflex, which will try to resist the change in the lengthening muscle by making the stretched muscle contract. If the muscle elongates suddenly, the muscle contraction will be stronger. Plyometric training uses this jump technique, which is based upon the basic movement of the muscle spindle, and helps to improve muscle tone and protects the body from injury. When practising static stretching, by making a muscle accustomed to a particular stretched position for a prolonged period of time, the muscle spindle is taught to adjust to being stretched to a new length, which reduces its signaling. With consistency, train the stretch receptors to allow more length of the muscles. Lengthen reaction When muscles contract, usually due to the stretch reflex, tension is created at the place where the muscle connects to the tendon; this is where the golgi tendon is found. This tendon is sensitive to any change in tension, along with the pace at which the change in tension occurs. It then sends messages to the spine, relaying the changes that are occurring. When the tension exceeds a certain level, it creates a lengthening reaction that stops the muscles from contracting, instead causing them to relax. The golgi tendon's primary function is to help protect tendons, ligaments and muscles from injury. The lengthening that occurs is made possible through the strong signaling from the golgi tendon to the spinal cord, which overrides any signaling from the muscle spindles telling the muscle to contract. It is a lot easier to place a muscle in an extended elongated position if it is not trying to contract at the same time.

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