Meeting Abstract
Limb muscles must produce force to match the mechanical demands of locomotion. As muscle undergoes contractile length changes it remains isovolumetric. Hence, it has been hypothesized that during a high-force contraction muscle bulges in width, as contracting fibers restrict bulging in thickness. By contrast, during a low-force contraction muscles are hypothesized to bulge in thickness as connective tissue stiffness resists bulging in width. This interaction between contractile and connective tissue components of pennate muscles has been shown to govern volumetric constraints in situ but is unexplored in vivo. The rat medial gastrocnemius (MG) is a unipennate hind limb extensor from which in vivo measurements of 3D shape changes and force production can be obtained, allowing us to determine how interactions between muscle width and thickness are shaped by variations in force. We used fluoromicrometry to measure changes in MG width and thickness as rats (N = 5) walked, trotted, and galloped on different treadmill slopes (-20°, 0°, +20°). These nine gait-slope combinations provided a gradient in muscle force and elicited changes in muscle width, thickness, or both. Consistent with our hypothesis, we found bulging in width of the rat MG to nearly double, with a comparable decrease in thickness bulging from the weakest to the strongest contractions. This result indicates that contractile and connective tissues have a balancing role in determining how a pennate muscle resists bulging during variable force contractions in vivo. Insight into how 3D muscle bulging is shaped by variations in force production may inform muscle-centered rehabilitation strategies and improve the functionality of prosthetic limbs. (Supported by NIH AR055648).
