The shear modulus of the vastus lateralis muscle does not follow the passive residual torque enhancement in the knee extensors.

In vitro experiments define passive force enhancement as the increase in steady-state passive force following the deactivation of an actively stretched muscle, in contrast to a purely passive stretch. This phenomenon, linked to residual force enhancement, is also observed in voluntarily contracted muscles as passive residual torque enhancement (RTEpass). While mechanisms remain unclear, titin stiffness likey plays a key role. Supersonic shear wave elastography (SSI) estimates tissue stiffness via the shear modulus (μ). The study aimed to assess whether RTEpass of the knee extensor muscles is accompanied by an increase in vastus lateralis stiffness (RμEpass) as measured by shear wave elastography. Passive torque was measured in 20 healthy young adults at a knee flexion angle of 100° before and after both isometric contractions (control protocol) and isometric contractions preceded by an eccentric contraction at 30°/s (from 70° to 100°). The comparison of protocols revealed a significant mean RTEpass of 1.03 N·m (16.5 %; p < 0.001), confirming the RTEpass in knee extensors. Although the experimental protocol showed a significant change in μ from the Before- to Post-contraction moment (5.89 %; p = 0.041), no differences in μ were observed between protocols at any post-contraction moments (p ≥ 0.191). Spearman correlation analysis indicated a weak, non-significant correlation between RTEpass and RμEpass (rs = 0.219; p = 0.352). These findings suggest that changes in vastus lateralis stiffness, as measured by SSI, are insufficient to explain RTEpass. While the literature identifies titin as a primary mechanism for passive residual torque enhancement, SSI elastography did not detect this phenomenon through solely vastus lateralis stiffness.