Meeting Abstract
Traditional one-element Hill-type muscle models are common in musculoskeletal modelling and assume identical biomechanical properties for all fibers of a muscle. The rat medial gastrocnemius (MG) is a pennate and compartmentalized muscle with proximal fibers that are shorter, more angled and less fatigable than distal fibers. These features may be at odds with the assumption of homogeneous fiber mechanics. We used sonomicrometry on an in situ preparation in anesthetized rats (n = 3) to evaluate supra-maximal force-velocity properties of three MG structural units: the whole muscle, proximal fascicles and distal fascicles. Isometric optimum length (L0) was determined for each structural unit and isotonic shortening contractions were elicited at various levels of MG force as controlled and measured by a servomotor. Shortening speed was determined for each structural unit at its specific L0. Fascicle speeds were normalized to their specific L0, and muscle speed to the average fascicle L0. Maximum shortening speed (Vmax) and power ratio (a force-velocity curvature measurement) were calculated. Whole muscle Vmax (12.7±1.9 L0/s; mean±SD) was higher than Vmax of the proximal (9.9±2.2 L0/s) and distal (8.7±1.3 L0/s) fascicles. Power ratio of the whole muscle (0.12±0.01) was similar to that of the proximal (0.12±0.01) and distal (0.12±0.02) fascicles. The presence of relatively slower fibers in the proximal compartment appeared to have negligible effects on fascicle force-velocity characteristics. In line with previous studies, these findings suggest that pennate muscles can partly circumvent fascicle force–velocity constraints and are likely important in considering regional differences in contractile properties. Funded by NIH AR055648 to A.B.
