Meeting Abstract
The operating length of a muscle during movement is a key determinant of its ability to produce force in vivo. Relative to the peak force plateau on a force-length curve, muscles that operate at longer lengths can produce higher forces throughout shortening, whereas muscles that operate at shorter lengths may be safer from sudden length perturbations and subsequent damage. As connective tissues change during aging, the mechanics of muscle length changes must also be affected. In this study we investigated the relationship of in vivo muscle operating length to in situ muscle properties in young (7 mos) and aged (30-32 mos) rats. Specifically, we examined the relationship of passive and active force-length relationships to muscle operating length during walking and trotting using fluoromicrometry. Muscle lengths were measured via small (< 1.0 mm) radio−opaque markers surgically implanted into the muscle and placed along muscle fascicles in the medial gastrocnemius, and markers were visualized during locomotion and in situ muscle contractions using high−speed x−ray videography. An active force-length relationship was characterized in situ by tetanically stimulating the muscle while visualizing the same muscle markers and simultaneously measuring force with an ergometer. Despite inter-individual variation in passive muscle stiffness, rats of both age groups demonstrated a clear (R2>0.7; p<0.001) relationship between passive stiffness and in vivo operating length. Our results suggest that, rather than tuning muscle lengths relative to an optimal force plateau, the operating lengths of locomotor muscles during sub-maximal activity do not exceed lengths that result in significant passive forces.
