Meeting Abstract
The lower hind limb muscles in cursorial vertebrates have been hypothesized to be specialized for meeting the varying mechanical demands of ground-based locomotion. The mechanical action of one such muscle, the medial gastrocnemius (MG), is well studied for bipeds but less so for quadrupeds. We used fluoromicrometry to measure MG muscle fiber length changes as rats (N = 5) galloped on a treadmill set at different slopes (-20°, 0°, +20°). These slopes were expected to elicit different demands for force production and fiber contractile length changes as the muscle would need to act as a brake, strut, or motor, respectively. Muscle fibers were expected to lengthen and dissipate energy, acting as brake on a decline. On the level, muscle fibers were expected to remain near-isometric, letting the Achilles tendon elastically cycle mechanical energy between strides. Muscle fiber shortening was expected to be greatest on an incline, allowing the muscle to act as motor to perform mechanical work. Consistent with these hypotheses, we found that MG muscle fibers lengthened on a decline to dissipate energy in early stance and shortened on an incline. However, our results do not support the hypothesis of near-isometric contraction allowing the muscle to act as a strut during level galloping. The unexpected small amount of muscle fiber shortening on the level supports prior suggestions that the rat Achilles tendon may be overbuilt for elastic energy cycling. Fiber shortening may be necessary to produce additional work as forelimb muscles potentially dissipate energy in comparison to hindlimb muscles. Future research will aim to identify those fore and hind limb interrelationships. Funded by NIH AR055648 to A.B.
