Meeting Abstract
Architectural gear ratio (AGR), or the ratio of whole muscle versus myofiber contraction speed, is a phenomenon observed in pennate muscles that results from changes in pennation angle as myofibers contract. AGR has been hypothesized to broaden a muscle’s ability to contract at varying levels of force and speed: in shortening contractions, a low gear ratio favors force output, whereas a higher gear ratio allows a muscle to reach higher speeds than achieved by its individual myofibers. In lengthening contractions, however, higher (as well as low) gear ratios would reduce the speed of myofiber stretch relative to whole muscle lengthening, possibly preventing injury. Until now, AGR and muscle volumetric shape changes have only been measured in isolated muscle experiments, in which the systematic relationship between muscle AGR and force for shortening contractions has been observed. To address if this relationship persists in vivo, we studied the rat medial gastrocnemius muscle and used treadmill locomotion for different gaits (walk, trot, and gallop) and slopes (0°, +20°, -20°) to elicit contractions with different force levels and length trajectories. We measured changes in muscle belly length, and the length of muscle fascicles via fluoromicrometry (n = 6). In separate experiments (n = 3), we measured muscle activation and force production in vivo via electromyography and miniature tendon force transducers, respectively. We found that in vivo AGR was lowest (0.9 ± 0.1) during high-force uphill locomotion and highest (2.6 ± 0.4) during low force downhill locomotion. Hence, the AGR-force relationship of the rat MG in vivo appears to match that of earlier in situ muscle experiments. Our results support the hypothesis that pennate muscles possess a mechanism in the form of variable gearing to broaden their performance during mechanically diverse movements. Funded by NIH AR055648 to A.B.
