Author(s): BIZET, B.1,2, NORDEZ, A.2,3, MONTE, A.1, JUBEAU, M.2, LACOURPAILLE, L.2, ZAMPARO, P.1, GENNISSON, J.L.4, ANDRADE, R.2, Institution: UNIVERSITÀ DI VERONA , Country: ITALY, Abstract-ID: 2658

INTRODUCTION:
Unaccustomed eccentric exercise induces acute muscle injury and soreness [1], involving alterations of the myofibrillar ultrastructure [2], muscle fiber membrane [3], and disruption of the extracellular matrix. Such alterations impair force transmission and lead to strength loss, commonly used as an indirect biomarker of muscle damage (EIMD). The along-fiber shear modulus, assessed by ultrasound shear wave elastography (SWE), has been shown to increase in the presence of muscle injury [4]. However, the direction-dependent mechanical and microstructural in-vivo changes in skeletal muscle following EIMD are poorly understood. This study aimed to characterize the time course of these adaptations using angle-resolved SWE and the Nakagami m parameter.
METHODS:
To investigate the evolution of along- and cross-muscle fibers SM (SML - SMX) and m parameters (mL – mT) in the biceps brachii at three elbow joint angles (0° [full extension], 45°, and 90°) following an eccentric exercise protocol. Twenty young adults (9M, 11W) completed four sessions: baseline, 48h, 7d, and 14d post-exercise. The exercise protocol consisted of 3-6 sets of 10 eccentric elbow flexions performed at 120°·s⁻¹ using an isokinetic dynamometer (System 3; Biodex Medical Systems). Ultrasound scans (Mach 30, SuperSonic Imagine) were acquired at the muscle mid-belly in longitudinal and transverse orientations. Angle-resolved SWE was used to assess SM [5]. SM and m values were analyzed within standardized regions of interest.
RESULTS:
Strength significantly decreased at 48 h (−18.2%; p < 0.001) and fully recovered by 14d (p < 0.001), confirming EIMD and subsequent recovery. SML showed a significant session×angle interaction (p < 0.01), increasing at 48h only at 0° (+58.8%; p < 0.001), while remaining unchanged at other angles. SMT was unaffected (p = 0.41), highlighting the directional specificity of the mechanical alterations under passive loading. In contrast, mT and mL were not influenced by joint angle (p > 0.10) but exhibited a delayed temporal response: no change was observed at 48h (p = 0.81), whereas a peak increase occurred at 7d in all angles (+16.1% to +17.8%; p < 0.001).
CONCLUSION:
Our findings show that mechanical (SM) and microstructural (m) adaptations following eccentric-induced muscle damage evolve with distinct temporal patterns, with mechanical changes dependent on muscle length and microstructural alterations consistent across angles, likely reflecting different physiological processes involved in post EIMD remodeling. Notably, the directional-dependent of mechanical responses contrasted with the isotropic behavior of m parameters. These results highlight the importance of assessing muscle tissue anisotropy for understanding structural disorganization and remodeling after EIMD. Thanks to angle-resolved SWE, tensile and shear anisotropy can now be measured, and these data will be analyzed in future work.
[1] Clarkson, 2002
[2] Fridén, 1981
[3] Armstrong, 1990
[4] Lacourpaille 2014
[5] Andrade, 2025