From cat to man: basic aspects of locomotion relevant to motor rehabilitation of SCI

On the assumption that locomotion is partly produced by a central pattern generator (CPG) in the spinal cord of both cat and man, it is essential to learn more about how such a CPG is controlled by sensory input produced during gait. For the cat there is evidence that load receptor input both from extensor muscles and from cutaneous receptors from the foot, is able to reinforce the ongoing extensor activity in the stance phase and delay the ensuing swing phase. Original data on electrical stimulation of nerves in walking premammillary cats with one hindlimb fixed, support the notion that this type of load afferent input acts directly on the CPG. A second potential source of sensory input on the CPG is derived from sensory signals related to hip position. One would therefore expect that hip position is a more tightly controlled variable than the position of other joints. This was investigated by measuring these angles under conditions of constrained gait (crouch). It was found that cats indeed maintained the maximum excursions of hip flexion and extension within stricter limits than the corresponding angles at other joints. Finally, experiments on hip joint denervation show that there is very little effect on step cycle parameters, thereby supporting the idea that the important hip signal is unlikely to be derived from hip joint afferents. It is suggested that procedures aimed at activating the locomotor CPG in SCI patients could benefit from the use of periodic stimulation of ankle muscle load afferents and hip flexor stretch receptors.