Intra- and intermuscular shear wave speed distributions in the human hamstrings during isometric and eccentric...

Hamstring strain injuries frequently occur near the biceps femoris long head (BFlh) proximal myotendinous junction. Unequal intra- and/or intermuscular mechanical properties may increase injury susceptibility by generating localised gradients in tissue mechanics and strain concentration; however, this remains unexplored under eccentric loading. We examined local shear wave speed (SWS), a measure reflecting tissue stiffness and force transmission, along the BFlh and semitendinosus (ST) during isometric (ISO) and eccentric (ECC) contractions.

Fifteen healthy males performed ISO and ECC knee flexions at 10%, 30%, and 50% of maximal voluntary torque (MVT). BFlh and ST SWS was measured at 30% (prox) and 50% (mid) of the thigh length using ultrasound elastography. BFlh pennation angle (PA), fascicle length (FL), and muscle thickness (MT) were also assessed.

Absolute SWS was lower proximally in BFlh across intensities and conditions, whilst ST showed lower proximal SWS only in ISO at 30% and 50%MVT. When normalised to passive values, intramuscular SWS differences disappeared during ECC but persisted in BFlh during ISO ≤ 30%MVT. Intermuscular SWS ratio (BFlh/ST) was 0.63-0.94 at rest and 10%MVT, increasing to 0.78-1.01 at higher intensities. Moderate correlations were found between BFlh architecture and proximal SWS during ECC only (PA: r = 0.62-0.65; FL: r = - 0.64 to - 0.66; MT: r = 0.57).

Hamstring muscles exhibit region-specific SWS profiles, largely influenced by passive mechanical properties. This heterogeneity may contribute to localised strain concentrations and injury risk, highlighting a potential target for prevention strategies.