Hamstring strains can occur during a variety of athletic maneuvers and situations, resulting in several distinct types of injuries, each with a unique mechanism. The first occurs during a stretching of the muscle at extreme joint positions, such as in a Rockette style high kick.19–20
These injuries generally occur to the proximal free tendon of the semimembranosus tendon and appear to be less severe initially but ultimately require a longer recovery time than hamstrings strained by other mechanisms.21
The second mechanism of hamstring strain occurs during high speed running.1,22
There remains some debate in the literature as to which phase of the sprinting cycle in which hamstring strains occur: the early stance phase or the late swing phase. Proponents of hamstring injury during early stance phase of sprinting suggest it is during this phase in which the muscle absorbs the most force as a result of high ground reaction forces.23
In vivo studies of the Achilles tendon in sprinting24
and patella tendon in jumping25
show that the forces are much higher in the concentric stance phase as opposed to the eccentric swing phase and this may apply to the hamstrings as well. There is also evidence to suggest hamstrings may be susceptible to injury in the late swing phase. Previous studies demonstrate that the hamstrings are under a large amount of stress in the terminal swing phase as the hamstrings eccentrically contract to absorb the kinetic energy and slow the lower limb.27
In a biomechanical study Schache et al28
found that peak musculotendonous strain occurred during terminal swing phase of the sprinting cycle, suggesting that this period may pose the greatest risk for injury. They went on to recommend a rehabilitation program focusing on eccentric loading at longer muscle lengths.
To assess whether a reduction in force production at longer muscle lengths exists in athletes who have sustained a hamstring strain, Brockett et al29
examined the angle torque curves of previously injured subjects and compared them with the subjects' uninvolved leg as well as those of uninjured control subjects. The authors showed that the peak hamstring torque occurred at a significantly shorter muscle length in the previously injured hamstring when compared to controls, indicating what may be termed a shift in the length-tension curve. It is possible that when an athlete sustains a hamstring strain they potentially return to play with weakness at longer muscle lengths possibly predisposing them for a second hamstring strain during the eccentric terminal sprinting movement.
It has been well established in the literature that eccentric training is effective in the prevention of hamstring strains.1,30–33
The authors feel that the eccentric training should be done not just in the seated position from 90 degrees to full knee extension, but should include training in the lengthened state. We hypothesize that training in the lengthened state may help shift the curve to acquire the necessary eccentric strength at the end of the range of motion to avoid susceptibility to further injury. The absence of rehabilitation focusing on lengthened state eccentric training may explain the disproportionally high rate of recurrence. Therefore, it is the belief of the authors that complete rehabilitation of a strained hamstring should include lengthened state eccentric training in order to minimize exposure to further muscle strain. Unfortunately, despite the best prevention programs hamstring strain injuries still occur.