Hamstrings are most susceptible to injury during the early stance phase of sprinting.

Br J Sports Med February 2012 Vol 46 No 2 88 The fi rst publications which considered the time of onset in the gait cycle for hamstring strains concluded that early stance was the highest risk period.1,2 The rationale proposed was that external joint moments were much higher in stance phases than swing (table 1) because of the presence of high hip and knee joint reaction forces secondary to the ground reaction force (GRF).3–6 Ralph Mann’s original argument has been rejected by many subsequent authors, perhaps because of the dogma that muscles probably strain during eccentric contractions (table 1). This is a widely held belief despite experimental muscle strains being able to be produced during concentric (shortening) contractions.7 Table 1 shows that the high knee fl exion moment (sum of angular forces) in late swing occurs because the hamstrings are highly active and angular forces in the opposite direction (ie, due to quadriceps activity and external forces) are minimal. The hamstring does work hard in late swing to reverse the inertia of the shank angular movement in the opposite direction, but inertia (momentum) itself is not a force. However, in early stance the hamstring is also working hard but against potentially large opposing forces.1–4,6 A ‘smoking gun’ video of a case of calf strain has shown the exact time of disruption of gastrocnemius fi bres during late stance/push-off,8 which is conveniently a stance phase but also when the calf is contracting eccentrically. No such direct evidence is available for hamstring strains. Cases which claim to show a hamstring strain occurring in late swing9,10 do so using indirect evidence, looking at change in body mechanics (athlete reaction) to a strain and speculating when the injury occurred prior to the fi rst available indication. For example, in the Heiderscheit et al case, the fi rst indication of change of mechanics is actually seen