A review of the role of muscle strength in hamstring injury

Hamstring injury is one of the most common athletic injuries, typically occurring during forced lengthening of the muscle such as in sprinting or kicking activities. The high recurrence rate of hamstring strain is of particular concern to athletes and medical professionals. However, the aetiology of hamstring strain is not yet clearly understood. This chapter seeks to elucidate one of the key proposed risk factors in hamstring injury - reduced muscle strength. A number of prospective studies have sought to clarify whether hamstring weakness, or alterations in thigh muscle strength ratios, can predispose an athlete to hamstring injury. The evidence to date has been equivocal, in part due to limitations in study design. Retrospective studies have produced similarly conflicting results, with some evidence suggesting that athletes with a history of hamstring strain display impaired ability to generate force, whilst other studies disagree. In particular, there is increasing evidence to suggest that hamstring strain results in a rightward shift in the active length-tension curve of the muscle, likely due to focal damage to the weakest sarcomeres in the muscle. Notwithstanding the contrasting findings in the extant literature, there is some moderate evidence that hamstring strengthening programmes may be effective in reducing the incidence of hamstring injury. One promising mechanism for preventing muscle strain appears to be a shift in the optimum angle for peak torque through addition of sarcomeres in series. Eccentric strength training, particularly when performed at high intensity over long muscle lengths, has been found to produce the greatest adaptation in muscle. However, targeted eccentric programmes have been associated with reduced compliance amongst athletes, in part due to muscle soreness and, occasionally, increased risk of injury. Therefore, it is essential that the intensity of the exercise programme is matched to the stage of the healing process, progressing gradually thereafter to high force muscle contractions. There is an absence of large prospective studies that have measured the force profile of the hamstrings, in particular the length-tension curve, both before and after injury. Research is needed to clarify whether impaired function of the hamstring predisposes an athlete to injury, or indeed whether these features are modified subsequent to injury. Finally, further work should hopefully establish if modifying the force profile of the hamstrings through strength training can minimise the risk of future injury.