Reduced functional recovery by delaying motor training after spinal cord injury.

The purpose of this study was to examine if a delay in rehabilitative motor training after spinal cord injury affects functional motor recovery. We studied a skilled motor task in which rats traversed a raised horizontal ladder and we quantified errors in accurate stepping, i.e., foot slips between rungs. After lesions to the dorsal quadrant of the thoracic (T8) spinal cord that aimed to unilaterally sever the corticospinal and rubrospinal tracts, rats were re-trained to walk across the ladder, either immediately after injury or after a 3-mo delay. Before training, the error rate in accurate stepping of the affected hindlimb was similar in the immediately (69.4 +/- 5.3%) and delay (62.7 +/- 4.1%; means +/- SE)-trained animals (not significantly different), suggesting that accurate stepping did not improve spontaneously if rats were not exposed to the ladder. After a 3-wk course of training (30 runs across the ladder per day, 5 day/wk), improvements in accurate stepping performance were greater if training was implemented immediately after injury. On average, immediately trained animals improved stepping performance by 61.5 +/- 28.2%, whereas the delay trained group improved by only 34.9 +/- 28.8% (significantly different). The degree of damage to the corticospinal and rubrospinal tracts was very similar in the two groups of animals, indicating that differences in lesion size did not contribute to the differences in performance improvement. Animals with large lesions to the corticospinal and rubrospinal tracts (>70%) displayed poor recovery from training (especially for delay-trained animals), suggesting that these two pathways were important in mediating improvements in accurate stepping. In addition, recovery of stepping-like reflexes appeared not to contribute to the recovery of accurate stepping given that the time course of reflex recovery was not related to the time course of recovery of accurate stepping. We conclude that training of a skilled motor task that relies on descending control is more beneficial when initiated immediately after a partial spinal cord injury.