Parallel and cross-sectional hamstring injuries in sprint running

This issue of Journal of Sport and Health Science contains a point-counterpoint discussion of hamstring injuries in sprint running by the groups of Drs.Liu and Yu. 1-4They propose different mechanisms of muscle injuries in general, and hamstring injuries in sprint running specifically.Yu et al. 2,4 present evidence suggesting that hamstring injuries are primarily caused by excessive muscle strain during eccentric contraction.In maximal effort sprint running, excessive muscle strains occur at the end of the swing phase.In contrast, Liu et al. 1,3 point out that, in addition to excessive hamstring strain, high stresses in the late swing and early stance phase and the transition between swing and stance may also contribute to hamstring injuries when the hamstrings actively assist hip extension and knee flexion.Both groups provide supporting published, scientific evidence for their contention.Yet, the entire discussion is based on the assumption that the hamstring muscles contract uniformly in all phases of sprint running.However, the different heads of the hamstring muscles have different insertion sites, structure, and fiber type distributions.Therefore, it is safe to assume that the individual heads fulfill different functions and that they do not elongate at the same rate and that stress across them is not uniform.Using magnetic resonance imaging in combination with finite element modeling, Fiorentino and colleagues 5 reported that non-uniformity in fiber strains may also be a contributing factor for hamstring injuries, especially when sprinting at high speeds.We propose that this fiber strain and strain rate nonuniformity at fast sprint speeds could lead to "parallel injury", tissue separation, and misalignment of myofibrils along the muscle fibers (e.g., see Lee and Healy, 2012, for a detailed image 6 ).Along these lines, Morgan 7 proposed the so-called "Popping Sarcomeres Hypothesis", which is based on the idea that repeated high stresses during eccentric contractions leads