Facilitation of descending excitatory and spinal inhibitory networks from training of endurance and precision walking in participants with incomplete spinal cord injury

After incomplete spinal cord injury (iSCI), training of walking function that emphasizes both endurance and speed may produce different changes in spared neural pathways compared to precision training that emphasizes walking over obstacles and precise placement of the foot. To examine this, 16 participants with iSCI received 2 months of endurance or precision training, in random order, with 2 months of rest before crossing-over to the other type of training. Both forms of training increased the maximum motor-evoked potential (MEPmax) elicited by transcranial magnetic stimulation over the motor cortex, but only in tibialis anterior (TA) muscles that had small (< 0.5 mV) MEPmax values before training, no matter when the specific type of training was performed. A similar pattern of training-induced increases in maximum voluntary contractions was also observed. Although walking function was improved by both forms of training, a positive correlation between MEPmax and clinical measures of walking function only occurred after endurance training. Endurance and precision training also increased the excitability of inhibitory spinal networks, as demonstrated by an increase in the suppression of TA MEPs by a prior, low-threshold stimulation to the common peroneal nerve and by increases in the inhibitory component of the cutaneomuscular reflex. The increase in the descending excitation of the spinal cord and the increase in excitability of inhibitory spinal networks may mediate the improved volitional control of walking and reduction of involuntary muscle spasticity, respectively, that are observed in response to intensive motor training in participants with incomplete spinal cord injury.