Muscle fiber activity as a function of speed and gait

These experiments were designed to test the hypothesis that locomotory gait transitions occur when some critical cross-sectional area of active muscle is reached as animals increase speed within a gait. We used glycogen depletion as evidence of muscle fiber activity and selected an experimental animal in which all muscle fibers in the locomotory muscles rely extensively on glycogen as a substrate--the lion. We found a high correlation between biochemically and histochemically determined rates of glycogen depletion (r = 0.906). Rates of glycogen depletion in the biceps femoris and triceps brachii muscles increased logarithmically with speed with no discontinuities at the gait transitions. However, we found large discontinuities both in the total cross-sectional area of muscle that showed depletion and in the rates at which the different types of fibers depleted glycogen at the trot-gallop transition. Our results indicate that 1) gait transition did not occur at a maximum tension level either for a particular type of fiber or the whole muscle, and 2) different configurations of motor units within an individual muscle may be recruited as an animal changes gait.