Shear wave elastography reveals passive and active mechanics of triceps surae muscles in vivo: from shear modulus-ankle angle to stress-strain characteristics

Characterizing individual muscle behavior is crucial for understanding joint function and adaptations to exercise, diseases, or aging. Shear wave elastography (SWE) is a promising tool for measuring the intrinsic material properties of muscle. This study assessed the passive and active shear modulus of the triceps surae muscles in 14 volunteers (7 females, 25.9 ± 2.5 yr) using SWE. Ankle moment, surface electromyography, and SWE of the gastrocnemius medialis (GM), gastrocnemius lateralis (GL), and soleus (SOL) muscles were measured from 30° plantar flexion (PF) to 15° dorsiflexion (DF) ankle angles during passive and isometric contractions at 25%, 50%, and 75% of maximum voluntary contraction (MVC). Muscle length, passive and active ankle moment, and passive shear modulus increased from PF to DF (<i>P</i> < 0.001 for all). At 15° DF, the passive shear modulus of the SOL was 76% lower than that of the GM (<i>P</i> < 0.001), suggesting that the SOL operates within a lower strain range. The active shear modulus decreased from PF to DF (e.g., by 36.8% at 75% MVC, <i>P</i> = 0.009) and was lowest in SOL. The decreasing active shear modulus suggests that the muscles operate at shorter-than-optimal to optimal lengths. Contraction intensity also affected the shear modulus (<i>P</i> < 0.001), indicating distinct force-sharing strategies, with GL possibly playing a crucial role at higher-intensity contractions and longer lengths. This study demonstrated SWE's potential to characterize muscle mechanics in vivo. If validated, predictions from SWE could facilitate studying muscle behavior and force-sharing strategies, serving as a diagnostic or monitoring tool for muscle function and performance.<b>NEW & NOTEWORTHY</b> This study assessed the length- and activation-dependent shear moduli of the triceps surae muscles using shear wave elastography. By combining joint moment, muscle fascicle geometry, and electromyography data, we characterize the muscles' in vivo passive and active mechanical behaviors. Our results indicate that the muscles operate at shorter-than-optimal to optimal lengths with soleus force production being least impacted by joint position. We observed muscle-specific shear modulus characteristics, providing insights into stress-strain behavior and force-sharing strategies.