Influence of internal muscle properties on muscle shape change and gearing in the human gastrocnemii.

Skeletal muscles bulge when they contract. These three-dimensional shape changes coupled with fibre rotation, influence a muscle's mechanical performance by uncoupling fibre velocity from muscle belly velocity (i.e., gearing). Muscle shape change and gearing is likely mediated by the interaction between internal muscle properties and contractile forces. Muscles with greater stiffness and intermuscular fat, due to aging or disuse, may limit a muscle's ability to bulge in width, subsequently causing higher gearing. The aim of this study was to determine the influence of internal muscle properties on shape change, fibre rotation, and gearing in the medial (MG) and lateral gastrocnemii (LG) during isometric plantarflexion contractions. Multi-modal imaging techniques were used to measure muscle shear modulus, intramuscular fat, and fat-corrected physiological cross-sectional area (PCSA), at rest, as well as synchronous muscle architecture changes during submaximal and maximal contractions in the MG and LG of 20 young (24±3y) and 13 older (70±4y) participants. Fat-corrected PCSA was positively associated with fibre rotation, gearing, and changes in thickness during submaximal contractions, but negatively associated with changes in thickness at maximal contractions. Muscle stiffness and intramuscular fat were related to muscle bulging and reduced fibre rotation, respectively, but only at high forces. Further, the MG and LG had varied internal muscle properties, which may relate to the differing shape changes, fibre rotations and gearing behaviours observed at each contraction level. These results indicate that internal muscle properties may play an important role in mediating muscle shape change and gearing, especially during high force contractions.