Left–right coordination from simple to extreme conditions during split‐belt locomotion in the chronic spinal adult cat

Abstract

Key pointsr Coordination between the left and right sides is essential for dynamic stability during locomotion.r The immature or neonatal mammalian spinal cord can adjust to differences in speed between the left and right sides during split-belt locomotion by taking more steps on the fast side.r We show that the adult mammalian spinal cord can also adjust its output so that the fast side can take more steps.r During split-belt locomotion, only certain parts of the cycle are modified to adjust left-right coordination, primarily those associated with swing onset.r When the fast limb takes more steps than the slow limb, strong left-right interactions persist. r Therefore, the adult mammalian spinal cord has a remarkable adaptive capacity for left-right coordination, from simple to extreme conditions. Abstract Although left-right coordination is essential for locomotion, its control is poorly understood, particularly in adult mammals. To investigate the spinal control of left-right coordination, a spinal transection was performed in six adult cats that were then trained to recover hindlimb locomotion. Spinal cats performed tied-belt locomotion from 0.1 to 1.0 m s −1 and split-belt locomotion with low to high (1:1.25-10) slow/fast speed ratios. With the left hindlimb stepping at 0.1 m s −1 and the right hindlimb stepping from 0.2 to 1.0 m s −1 , 1:1, 1:2, 1:3, 1:4 and 1:5 left-right step relationships could appear. The appearance of 1:2+ relationships was not linearly dependent on the difference in speed between the slow and fast belts. The last step taken by the fast hindlimb displayed longer cycle, stance and swing durations and increased extensor activity, as the slow limb transitioned to swing. During split-belt locomotion with 1:1, 1:2 and 1:3 relationships, the timing of stance onset of the fast limb relative to the slow limb and placement of both limbs at contact were invariant with increasing slow/fast speed ratios. In contrast, the timing of stance onset of the slow limb relative to the fast limb and the placement of both limbs at swing onset were modulated with slow/fast speed ratios. Thus, left-right coordination is adjusted by modifying specific parts of the cycle. Results highlight the remarkable adaptive capacity of the adult mammalian spinal cord, providing insight into spinal mechanisms and sensory signals regulating left-right coordination.

Lack of adaptation during prolonged split‐belt locomotion in the intact and spinal cat

“…Moreover, although there was some variability between animals, similar to spinal cats (Frigon et al . ), all intact cats performed 1:2, 1:3 or 1:4 coordination patterns. The cats that performed the most stable locomotion were those that seemed unperturbed by the high difference in speed between the slow and fast belts, whereas those that seemed perturbed or were looking at the treadmill surfaces had more difficulty producing a consistent pattern.…”

“…; Frigon et al . ). In other words, the left and right rhythms become dissociated, consistent with some degree of independence between the spinal circuits generating locomotion for the left and right sides (Yang et al .…”

“…; Frigon, ; Frigon et al . ). However, in human adults, even with high left‐right split‐belt differences, the left and right rhythms rarely become dissociated (Dietz et al .…”

“…As part of our effort to reduce the number of animals that we use in research, all cats participated in other studies to address different scientific questions (Frigon et al . , , , ; Thibaudier et al . , ; D'Angelo et al .…”

“…At the time of data collection, intact and spinal cats had been trained for >6 weeks to step on the treadmill and had performed several sessions of short bouts (20–30 cycles) of split‐belt locomotion at various left–right split‐belt speeds (Frigon et al . , , ; D'Angelo et al . ).…”