ROBERTS, T.J.*; GABALDON, A.M.; Brown Uniiversity; Colorado State, Pueblo: Does rate of force production increase with running speed in individual muscles?

It has been proposed that increases in metabolic energy cost of running with speed are explained by the higher rates of energy consumption in muscle fibers that produce force more rapidly. This idea is supported by a close correlation between metabolic energy cost and duration of stance force, as measured by the time of foot contact in each step. However, a direct correlation between the intrinsic speed of recruited muscle fibers and time of foot contact has not been demonstrated. We measured force production in the wild turkey lateral gastrocnemius to test for a correlation between foot contact time and rate of force development. Muscle force was measured by strain gages attached to bony tendons. We quanitified the rate of force development as the time from start of force to 50% of peak force output (t50%P). In vitro experiments have demonstrated that this value correlates with unloaded maximum shortening velocity, Vmax, and it may therefore be an indicator of fiber types recruited in vivo. We hypothesized that the t50%P would decrease with running speed, in proportion to the decrease in foot contact time. This hypothesis was not supported. Foot contact time decreased by approximately 50% across the measured speed range, from .37�.03 s at 1 m/s to .18�.03s at 3.5 m/s, while t50%P was independent of speed (.014�.004s at 1 m/s and .012�.004s at 3.5 m/s, mean�s.d.). If our measure of rate of force development is a reliable indicator of the speed of recruited fibers, these results suggest that the correlation between foot contact time and recruited fiber type may not always hold within individual muscles. Supported by NIH grant AR46499.