Movements of the Hindlimb During Locomotion of the Cat

Information about vertebrate stepping has proliferated enormously in the past few years, but a comprehensive frame of reference is still lacking to describe control mechanisms for the step cycle of even one limb, let alone of the four-legged interlimb sequence. The first section of the chapter surveys previous models and data for the outputs of locomotion. The concept is justified that central, afferent input, and motor output variables are integrated over a broad domain. Events in individual afferents, muscles, and joints merge to a smooth unitary ensemble, the step cycle. From kinematics it is learned that a variety of subcomponents of the step remain relatively constant as velocity changes. These subcomponents include the duration of the swing (interval while the foot is off the surface), the flexion epoch, and the sequence of joint excursions. Under a number of environmental manipulations, these “constants” may shift their values somewhat, perhaps to smooth the cycle. More flexible are the duration of the stance (interval when the foot is on the surface), amplitude of joint angles for the limbs, and excursions of the spine. Interlimb timings and footfall patterns are even more permutable than single limb events, and the relative flexibility of all measures has made it difficult to generate enough data to determine the limits of their operating ranges. Electromyographic and kinetic data are sparse, but reinforce the concept that the electrical and mechanical activities of different muscles are summed to build an uninterrupted step cycle.