Hamstrings muscle fibre composition may be predominantly fast-twitch and could explain the high incidence of hamstrings strain injuries. However, hamstrings muscle composition in vivo, and its influence on knee flexor muscle function, remains unknown. We investigated biceps femoris long head (BFlh) myosin-heavy chain (MHC) composition from biopsy samples, and the association of hamstrings composition and hamstrings muscle volume (using MRI) with knee flexor maximal and explosive strength. Thirty-one young men performed maximal (concentric, eccentric, isometric) and explosive (isometric) contractions. BFlh exhibited a balanced MHC distribution (mean±SD (min-max); 47.1±9.1% (32.6-71.0%) MHC-I, 35.5±8.5% (21.5-60.0%) MHC-IIA, 17.4±9.1% (0.0-30.9%) MHC-IIX). Muscle volume was correlated with knee flexor maximal strength at all velocities and contraction modes (r= 0.62–0.76, P< 0.01), but only associated with late phase explosive strength (time to 90 Nm; r= -0.53, P< 0.05). In contrast, BFlh muscle composition was not related to any maximal or explosive strength measure. BFlh MHC composition was not found to be ‘fast’, and therefore composition does not appear to explain the high incidence of hamstrings strain injury. Hamstrings muscle volume explained 38-58% of the inter-individual differences in knee flexor maximum strength at a range of velocities and contraction modes, while BFlh muscle composition was not associated with maximal or explosive strength.