Fatigue-induced changes in hamstrings' active muscle stiffness: Effect of contraction type and implications for strain injuries

Hamstring strain injuries may occur due to selective fatigue and compromised mechanical properties within this muscle group. We examined 1) the effect of fatigue on hamstrings active muscle stiffness, and 2) whether contraction type affects these responses. Nine healthy males completed 99 submaximal knee flexions in isometric (ISO), concentric (CON) and eccentric (ECC) conditions. We measured knee flexor maximal voluntary torque (MVT) (pre/post), shear wave velocity (SWV) during contraction and transverse relaxation times (T2) (pre/post) in biceps femoris long head (BFlh), semitendinosus (ST) and semimembranosus (SM) muscles. MVT decreased substantially after all conditions, while SWV interindividual responses were highly variable across muscles and contraction types. In ISO, BFlh SWV tended to increase while ST and SM SWV remained unchanged. In contrast, in CON, SWV decreased markedly in ST and SM (p< 0.01), but not in BFlh. Finally, in ECC, SWV decreased in BFlh (p< 0.01) but it remained largely constant in ST and variable in SM. In conclusion, fatigue differentially affects the mechanical properties of the constituent hamstring muscles, depending on contraction type. We found evidence that BFlh is selectively fatigued during eccentric contractions, which may explain its susceptibility to strain injuries.