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

Abstract

Purpose: 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.

Methods: 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.

Results: 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).

Conclusion: 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.