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

KEY POINTS: Coordination between the left and right sides is essential for dynamic stability during locomotion. 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. We show that the adult mammalian spinal cord can also adjust its output so that the fast side can take more steps. During split-belt locomotion, only certain parts of the cycle are modified to adjust left-right coordination, primarily those associated with swing onset. When the fast limb takes more steps than the slow limb, strong left-right interactions persist. 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.