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Hamstring Injury - Proposed Causes and Prevention
Epidemiological studies have demonstrated that acute hamstring injuries pose the greatest injury risk to elite and sub-elite sprinters (Woods et al., 2004; Edouard & Alonso; 2013; Askling et al. 2014). An examination of all injuries sustained at IAAF World Championships since 1987 suggests that acute hamstring injuries were responsible for approximately one quarter of all injuries in female sprinters and one third of all injuries for male sprinters. In addition to their prevalence, acute hamstring injuries also required one of the longest recovery times of all injuries common to sprinters, significantly affecting training availability, competition preparation and sprinting performance (Askling, Saartok & Thorstensson, 2006; Askling et al., 2007).
The most likely mechanism of acute hamstring injury during the sprinting action is a source of debate amongst scholars. The most commonly cited causes of hamstring injury in sprinters are detailed below:
Excessive Muscle Strain in Eccentric Contractions
The original evidence for eccentric muscle contractions being the primary cause of hamstring injuries was originally based on research conducted in rabbits. Lieber & Friden et al. (1993) demonstrated that the primary cause of hamstring injury was not the quantity of force per se, but the magnitude of the strain during the period of eccentric muscle contraction.
To understand how the finding of Lieber & Friden may relate to sprinting in humans, we can turn to Wood (1987), who studied the hamstring muscle lengths, power, flexions and contractions during the running action. Wood demonstrated that the hamstring muscles contract eccentrically in the late swing phase and late stance phase of sprinting.
Based on this finding, it was hypothesized that hamstring muscle strains are most likely to occur in the late swing phase prior to footstrike (Thelen et al., 2005) and in the late stance prior to takeoff. This was studied more recently by Yu et al. (2008), who concurred that the potential for hamstring injury exists during the late stance phase as well as during the late swing phase of sprinting. They added that the hamstring muscles were at a much longer length at the end of the swing phase, which may result in a higher risk of hamstring injury during this phase.
Excessive Muscle Strain in Concentric Contractions
A recent study conducted by Liu et al. (2017) reaffirmed the risk of hamstring injury during the late swing phase, but challenged the existing wisdom by proposing that it was the early stance phase, not the late stance phase, that posed the greater risk of hamstring injury. The authors hypothesized that it was therefore the period of transition between swing and stance phases where hamstring injuries were most likely to occur.
Liu et al. proposed that hamstring injuries during sprinting were most likely caused by large passive forces at the knee and hip joints, which result in the lengthening of the hamstring muscles. These forces were found to be highest during the transition between the swing and stance phases, where high-force eccentric and concentric contractions occur.
In response to this paper, Yu, Liu & Garrett (2017) published a rebuttal stating that there was no theoretical or experimental evidence that supported the view that concentric contraction could contribute to hamstring injury.
Something Different?
An editorial in the Journal of Sport and Health Science written by Walter Herzog (2017) challenged both proposed mechanisms and raised an important consideration regarding eccentric vs. concentric contractions. Herzog posited that it may not be the stretching of the muscle tendon unit that is important when considering hamstring injuries, but the elongation of the muscle fibres and fascicles. Previous research has demonstrated that when muscle force increases, fascicle lengths tend to decrease (de Brito Fontana & Herzog, 2016). During the late swing phase of sprinting there may be both eccentric contraction (the stretching of the muscle tendon unit) and concentric contraction (shortening of the muscle fibres and fascicles).
Herzog also proposed an alternative hypothesis, suggesting that hamstring injuries may primarily be caused by changes in hamstring activation caused by psychological factors during competition. He gives the example of sprinters rarely pulling up with an injury while leading a race - is this because of excessive strain when trying to catch up or from the loss of focus when an athlete considers a race unwinnable?
Ultimately, it appears that at this stage there is not enough evidence to definitively understand the causes of hamstring injuries during maximum-speed running. However, we do have agreement that the late swing phase is a period of high injury risk and as coaches we are able to use this information to assist in the design of prevention strategies to best support our athletes.
Coaching Interventions
Understanding the potential mechanisms of hamstring injury during maximum speed running can assist in the development of injury prevention and rehabilitation strategies. Using the mechanisms proposed by Yu et al. and Liu et al., we will examine the coaching strategies that could be implemented and the current evidence supporting their use.
Planning an Effective Warm Up
Theoretically, if hamstring injuries are exclusively caused by muscle strain during eccentric contractions, a warm up that increases the hamstrings muscle length would assist the prevention of injuries. This is supported by the finding that hamstring injuries tend to occur in the early stages of a training session, competition or season, where muscle lengths are generally shorter (Ekstrand & Gillquist, 1982; Gabbe et al., 2005).
However, while there is some evidence to indicate that increasing the muscle temperature and length is an effective strategy to increase the force that the muscle can handle during eccentric contractions (Safran et al., 1988), other studies have challenged the effectiveness of traditional dynamic warmups in increasing muscle temperature and length (Volkert et al., 2003; O’Sullivan, Murray & Sainsbury, 2009). Research indicates that hamstringspecific warm ups that include static stretching held for 20 to 30 seconds are the most effective for injury prevention (Hartig & Henderson, 1999; Verall et al., 2005).
Warm Up Recommendations from Woods et al. (2017) for Injury Prevention
• Warm-up should induce mild sweating without fatigue -approximately 40-60% of VO 2max.
• Static and / or dynamic stretching should be a long termcomponent of training.
• Stretches of longer duration with lower force provide thegreatest benefit.
• Stretches should be held for at least 3 x 30 seconds foreach muscle group.
• Ideally, the steretching protocol should occur within 15minutes immediately prior to the activity to receive themost benefit.
• Warm up activities should mirror the movement patternsthat will be performed in the main session.
Improving Flexibility
A study comparing the hamstring flexibility between injured and non-injured athletes concluded that hamstring flexibility was the strongest predictor of injury risk (Worrell et al., 1991). There is evidence for reducing the occurrence of hamstring injuries by implementing a regular static stretching program that targets the hamstrings and quadriceps. Outcomes appear to be especially strong when stretching is conducted post-exercise when the muscles are fatigued. Verrall, Slavotinek & Barnes (2005) demonstrated that hamstring stretches conducted during breaks in training and after a training session resulted in a significant decrease in hamstring strain occurrence when implemented as part of a holistic approach to injury prevention. The rationale given for its effectiveness is that stretching improved force absorption, making the muscles more resistant to a stretch injury. This finding supports a study conducted by Dadebo, White & George (2004), who found that among elite British football players, the absence or presence of hamstring stretching was the most important correlate of hamstring strain rates.
Improving Strength
Nelson & Bandy (2004) demonstrated that in addition to static stretching, hamstring flexibility could also be developed effectively by an eccentric strength training method using a TheraBand. This was an important finding, as we know from the mechanism for hamstring strains proposed by Yu et al. (2008) that injury appears to be caused by eccentric loading at high velocity. By improving the hamstrings eccentric strength, we may reduce the likelihood of strains occurring (Guex & Millet, 2013).
Comfort et al. (2009) built on the study by Nelson & Bandy and argued that for competitive athletes eccentric-specific strength exercises should be preferred over static stretching alone, as stretching does not adequately prepare the muscles for the sport-specific movements and forces encountered in competition. Injury prevention and rehabilitation should therefore incorporate a combination of static hamstring stretches with a selection of eccentric-specific exercises conducted to build the necessary range of motion and the specific strength that is required for sprinting.
Potential Hamstring Flexibility and Strength Exercises Recommended by Peer-Reviewed Studies for Injury Prevention
Hip Flex, 30 sec, 3 per week
Hurdler Hamstring Stretch, 30 sec, 2 per day
Eccentric Hamstring Stretch, 30 sec, 3 per week
Eccentric Backwards Step, 10-15 steps, 3 per week
Nordic Hamstring Curl, 2 x 4, 2 per week
Deadlifts (especially single-leg), 2-3 x 4-12, 2-3 per week
Eccentric Box Drop, 3 x 5, 2 per week
Preventing Strength Imbalance
Orchard et al. (1997) examined hamstring injuries in professional Australian Rules Football players and found that there was significant correlation between the occurrence of hamstring injury and two forms of strength imbalance. Injury was primarily encountered in the hamstring muscles on the player’s weaker leg and hamstring injury was significantly more likely in players with a lower hamstring to quadriceps muscle ratio. This finding was supported by Sugiura et al. (2008), who examined hamstring injuries in elite sprinters and noted that sprains always occurred on the athlete’s weaker side. The authors concluded that deficits in hamstring strength was associated with weakness during the eccentric action of sprinting and resulted in an increased likelihood of injury (Figure 3.1).
The implications of these studies is that coaches are able to assist injury prevention by ensuring strength is developed on both sides of the body, using an appropriate balance of unilateral and bilateral exercises. The coach should also ensure that athletes are including an appropriate quantity of hamstring-specific exercises such as Nordic Hamstring Curls and Deadlifts, to prevent imbalance between hamstrings and quadriceps strength.
However, not all evidence supports the view that strength imbalance is a risk factor or cause of hamstring strains. Worrell et al. (1991) found that neither bilateral strength asymmetry nor the ratio of strength between hamstrings to quadriceps had a significant relationship with the occurrence of hamstring injury and Brockett, Morgan & Proske (2004) supported this finding in a study of athletes with a history of hamstring injury.
Early clinical evidence also challenges the cause and effect relationship between strength imbalance and hamstring strains. A single-case study of an elite football player with minimal bilateral asymmetry developed significant asymmetry in hamstring strength 5 days prior to suffering a hamstring strain (Schache et al., 2010). This may suggest that strength imbalance is a symptom rather than a cause of hamstring strains and more research may be required to better understand the relationshbip between the two.
Overall, it is unclear whether strength imbalance is a significant contributor to hamstring strains, but ensuring a sprinter develops strength in their hamstrings and on both sides of their body is important for achieving athletic success and should still be a priority for the coach.
Fatigue and Low Energy
There is evidence to suggest that athlete fatigue is an important contributor to the likelihood of a hamstring strain occurring. Mair et al. (1996) demonstrated that fatigued muscles are able to absorb less energy before reaching the degree of stretch that causes injury than non-fatigued muscles. This is likely to be especially important for hamstring strains caused by the running action, due to the large forces absorbed by the hamstrings during the latter part of the swing phase as the hamstrings decelerate the leg (Garrett, 1990).
Fatigue may also play a role in exacerbating muscle imbalance, which as noted above, may contribute to hamstring injury. Rahnama et al. (2003) studied amateur football players and found that the ratio between hamstring and quadriceps strength became greater as the match went on and fatigue developed. The authors note that the amateur status of the participants from this study may contribute to these results, suggesting that a lack of muscular endurance may contribute to the growing imbalance with fatigue and that a potential implication for coaches is to develop the muscular endurance of endurance athletes to reduce the injury risk.
Optimising Recovery
Ensuring there is adequate recovery between training sessions has been well documented to assist in injury prevention generally (Hootman, Dick & Agel, 2007; Brink et al., 2010) and prevention of hamstring injuries in particular (Heiderscheit et al., 2010). Post-training recovery requires a multifaceted approach that should take into consideration the effectiveness of the following modalities:
Sleep: See Bird (2012) and Robson-Ansley, Gleeson & Ansley (2008). Sleep is understood to be an essential component of recovery and preparation for training and competition. Athletes sleeping less than 8 hours per night are at a higher risk of injury for amateur (Milewski et al., 2014) and elite athletes (Simpson, Gibbs & Matheson, 2016). It is recommended that the coach educates their athletes to the importance of sleep as a form of recovery and to maximise performance as ignorance of the importance of sleep is correlated with lower quantity and quality of sleep (Amschler & McKenzie, 2005).
Nutrition: Adequate nutrition intake pre-and post-training has been shown to aid athlete recovery and assist the prevention of lower limb injuries (Meyer, O’Connor & Shirreffs, 2007; Hausswirth & Le Meur, 2011; Di Fiori et al., 2014).
Athletics coaches are not expected to be nutritionists and should ensure that any advice given is in-line with the Australian Guide to Healthy Eating, unless the coach has additional sport nutrition qualifications. However, it may be beneficial for the coach to understand the potential signs of low energy availability and refer to the athlete to a sports nutrition specialist if symptoms are recognised and can not be explained by other factors. These may include:
• Significant changes in body composition
• Hormonal changes
• Poor gut health (bowl irregularity etc.)
• Poor immunity to illnesses and frequent infections
• Fatigue
• Changes to training quality and consistency.
To find a Sports Dietitian or to learn more, visit Sports Dietitians Australia.
Active Recovery: There is plenty of anecdotal evidence supporting active recovery for the prevention of hamstring strains, especially among the recreational running community. However, peer reviewed evidence supporting the practice is minimal. Coffey, Leveritt & Gill (2004) examined the effectiveness of active recovery after running and found no significant difference between active and passive recovery.
The best evidence for the use of an active recovery is toremove lactate post-exercise (Ahmaidi et al., 1996), howeverno peer-reviewed evidence links lactate with hamstringinjuries. In fact, active recovery may be detrimental to muscleglycogen resynthesis (Choi et al., 1994) increasing the risk ofinjury in subsequent sessions if recovery is incomplete.
Foam Rolling: Foam rolling is becoming an increasingly popular component of a cooldown immediately prior to or after exercise. There is a growing body of evidence to support the use of foam rolling to aid recovery and improve athletes’ range of motion (Peacock et al., 2014; Cheatham et al., 2015; Pearcey et al., 2015), although its effectiveness as a pre-exercise intervention has been questioned (Healey et al., 2014).
The role of foam rolling in aiding prevention of hamstring injuries has yet to be examined directly, but it has been hypothesized that foam rolling between sessions may be an effective tool for increasing the range of motion at the hip and the knee joints, which can contribute to prevention of hamstring strains (Heiderscheit et al., 2010).
A study that examined changes to the hip and knee-joint range of motion after a foam rolling intervention supported this hypothesis, demonstrating that increased range of motion increased and mechnical efficiency increased. The results suggested that foam rolling allowed the athlete to better absorb the forces (including eccentric forces) during the lunge action. While these findings are promising, more research would be required to know if foam rolling can prepare an athlete’s hamstrings to be able to tolerate the high eccentric forces during the late-swing phase of the sprinting action.
Foam rolling may also be an effective tool for aiding rehabilitation from hamstring injuries. In a literature review on the effectiveness of self myofascial release, Schroeder & Best (2015) concluded that foam rolling is likely to be an effective method of rehabilitation once strength and pain-free lengthening of the hamstring has been restored (Heiderscheit et al., 2010).
Quadriceps
Adductors
Hamstrings
Iliotibial Band
Gluteals
Each exercise was conducted for 45 seconds on the left and right side with 15 seconds rest after every change. The authors recommend using this program post-exercise to aid recovery and reduce muscle fatigue.
Cryotherapy: Different forms of cryotherapy, most notably ice baths, have a become a popular recovery method for elite and sub-elite track and field athletes. In addition to aiding recovery, cryotherapy has been explicitly recommended as a method for preventing hamstring injuries for elite athletes (Kujala, Orava & Jarvinen, 1997).
However despite its popularity, recent evidence has questioned the effectiveness of cryotherapy as a tool for hamstring injury prevention and rehabilitation (Jarvinen et al., 2005; Copland, Tipton & Fields, 2009). A meta-analysis conducted by Hohenauer et al. (2015) concluded that while cryotherapy may be an effective method for assisting recovery as measured by subjective self-reporting metrics (e.g. athlete reported muscle soreness), there was no evidence that it is effective for assisting recovery according to objective physiological variables. This led the authors to suggest that some of the benefits dervied from cryotherapy may be placebo related, exacerbated by the prevelance of ice bath usage among elite athletes in public events.
This was supported by Tiidus (2015) who concluded that cryotherapy, as practiced within usual guidelines, would not be sufficient to cool human muscle significantly to assist recovery or prevent injury.
Contrast Temperature Water Immersion: Despite its popularity among professional sporting clubs from a range of different sporting codes, there is little peer-reviewed evidence that contrast temperature water immersion is more beneficial for muscle recovery or hamstring injury prevention than other modalities explored in this article.
Ingram et al. (2009) found that contrast water immersion was less effective than cryotherapy in attenuating muscle soreness and in a review on published literature, Cochrane (2004) concluded that there was insufficient evidence to support the use of contrast water immersion to improve recovery outcomes. Unfortunately, the last 14 years has not added a lot of research that has examined its effectiveness in detail.
The best evidence to support the use of contrast water immersion found that it may allow for faster restoration of strength and power in recreational athletes, which may contribute to prevention of injury caused by fatigue (Vaile, Gill & Blazevich, 2007). However, due to the resources required, it is difficult to recommend for most athletics coaches.
Recommended Implementations
Cryotherapy (Hohenauer et al., 2015)
Optimal Water Temperature: 10 - Optimal Treatment Time: 13 minutes
Contrast Water Immersion (Cochrane, 2004):
Ratio Hot Water : Cold Water: 3 : 1 Optimal Water Temperature Hot: 40 Optimal Water Temperature Cold: 13.5 Optimal Treatment Time: 24 minutes (18 minutes hot : 6 minutes cold)
Synergist Knee Flexor Strength
There appears to be a lack of research on the role of synergist knee flexor strength in affecting hamstring injury, there is a theoretical basis to hypothesize that synergist muscle strength would contribute to hamstring injury prevention.
In the late swing phase of running, where strains are most likely to occur, the hamstrings are functioning to control the knee to bring the foot back to the ground. As the forces during this action are relatively high, improving the synergist muscles that contribute to knee flexion may assist injury prevention by reducing the force required exclusively by the hamstrings.
Despite the lack of direct evidence that synergist knee flexors influence the likelihood of hamstring injury, we can see supporting theoretical evidence in studies by Podraza & White(2010) and Walsh et al. (2012).
While we await more conclusive evidence, incorporating strength exercises that engage the synergist muscles of the lower limb is likely to be an effective strategy for the coach.Even if the theory that it contributes to injury prevention is flawed, there is evidence that sprinting performance is correlated with greater size of synergist muscles such as the sartorius (Handsfield et al., 2016), suggesting that specific exercises to develop these muscles is likely to bean effective coaching strategy.
Running Biomechanics
Poor running technique may also contribute to hamstring strains. Limited evidence supports the correlation between technique and injury, but a 2002 epidemiological review of injuries of professional AFL players observed that overstriding appeared to be a common mechanism of hamstring injury (Orchard & Seward, 2002). Considering the proposed mechanism of hamstring injuries supported by Yu et al.(i.e. caused by high magnitude eccentric contractions), it is interesting to consider what is occurring biomechanically when an athlete overstrides and how it affects the eccentric loading of the hamstrings in the late swing phase and transition between swing and stance phases.
Conclusion
Running-related hamstring injuries are most likely to be caused by high intensity eccentric contractions during the late-swing phase of the running action.
The coach can assist their athlete to avoid hamstring strains by developing their flexibility and strength, managing load to avoid excessive fatigue, implementing effective recovery strategies, avoiding strength imbalance (bilaterally and between hamstrings and quadriceps) and planning and implementing an effective warm up that prepares the athlete for the upcoming training session or competition.
