LOCOMOTOR CAPACITY ATTRIBUTABLE TO STEP TRAINING VERSUS SPONTANEOUS-RECOVERY AFTER SPINALIZATION IN ADULT CATS

Locomotor performance, hindlimb muscle activity and gait patterns duri ng stepping were studied in step-trained and nontrained female, adult spinal cats. Changes in locomotor characteristics relative to prespina lization bipedal and quadrupedal stepping patterns were used to evalua te the effects of step training on the capacity to execute full weight -bearing stepping after spinalization. Step training consisted of full weight-bearing stepping of the hind-limbs at the greatest range of tr eadmill speeds possible at any given stage of locomotor recovery. In t he initial stages of training the limbs were assisted as needed to exe cute successful steps. On the basis of two behavioral criteria, the ma ximum speed of treadmill stepping and the number of successful steps p er unit time, the ability to step was at least 3 times greater in anim als trained to step versus those allowed to recover spontaneously, i.e ., the non-trained. The greater success in stepping was reflected in s everal physiological and kinematic properties. For example, the amplit ude of electromyograph (EMG) bursts in the tibialis anterior (an ankle dorsiflexor), the amount of extension at the end of both the stance ( E3) and swing (El) phases of the step cycle, and the amount of lift of the hindlimb during swing were greater in step-trained than in nontra ined spinal cats. The changes that occurred in response to training re flected functional adaptations at specific phases of the step cycle, e g, enhanced flexor and extensor function. The improved stepping capaci ty attributable to step training is interpreted as a change in the pro bability of the appropriate neurons being activated in a temporally ap propriate manner. This interpretation, in turn, suggests that step tra ining facilitated or reinforced the function of extant sensorimotor pa thways rather than promoting the generation of additional pathways. Th ese results show that the capacity of the adult lumbar spinal cord to generate full weight-bearing stepping over a range of speeds is define d, in large part, by the functional experience of the spinal cord afte r supraspinal connectivity has been eliminated. These results have obv ious implications with regards to 1) the possibility of motor learning occurring in the spinal cord; 2) the importance of considering ''moto r experience'' in assessing the effect of any post-spinalization inter vention; and 3) the utilization of use-dependent interventions in faci litating and enhancing motor recovery.