Why Change Gaits? Recruitment of Muscles and Muscle Fibers as a Function of Speed and Gait

Abstract

Muscles shorten, stay the same length and are stretched while they are active during normal modes of terrestrial locomotion. The relative importance of these different types of muscular activity changes as animals change gait. Energy is conserved during a walk by an alternate storage and recovery of gravitational potential energy within each stride, as in an inverted pendulum. In order for this transfer of energy to take place, muscular activity is required to hold the limb rigid while the animal rotates over it. Energy is conserved by a spring mechanism during running, trotting, galloping, and hopping. Energy is stored when active muscles and their tendons are stretched and recovered as they subsequently shorten. The recruitment patterns of motor units as a function of speed therefore, depends on the type of muscular activity as well as the force exerted. Discontinuities in the cross sectional area of active fibers with increasing speed have been observed at the trot-gallop transition. It is suggested that at this point the trunk is recruited as an additional spring enabling more energy to be stored elastically. It is concluded that we must consider what muscles are doing during normal modes of locomotion before we become too involved in designing schemes of motor unit recruitment.