Retention of hindlimb stepping ability in adult spinal cats after the cessation of step training.

Adult spinal cats were trained to perform bipedal hindlimb locomotion on a treadmill for 6-12 wk. After each animal acquired the ability to step, locomotor training was withheld, and stepping was reexamined 6 and 12 wk after training ended. The performance characteristics, hindlimb muscle electromyographic activity patterns, and kinematic characteristics of the step cycle that were acquired with training were largely maintained when training was withheld for 6 wk. However, after 12 wk without training, locomotor performance declined, i.e., stumbling was more frequent, and the ability to consistently execute full weight-bearing steps at any treadmill speed decreased. In addition, the height that the paw was lifted during the swing phase decreased, and a smaller range of extension in the hindlimbs occurred during the E3 phase of stance. When three of the spinal cats underwent 1 wk of retraining, stepping ability was regained more rapidly than when trained initially. The finding that stepping ability in trained adult spinal cats can persist for 6 wk without training provides further evidence that training-induced enhancement of stepping is learned in the spinal cats and that a memory of the enhanced stepping is stored in the spinal networks. However, it appears that the spinal cord can forget how to consistently execute stepping if that task is not practiced for 12 wk. The more rapid learning that occurred with retraining is also consistent with a learning phenomenon. These results in conjunction with our earlier findings suggest that the efficacy of the neural pathways that execute a motor task is highly dependent on the periodic activation of those pathways in a sequence compatible with that motor task.