The first 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 flexion 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 …