<qa>  What is the purpose of this protocol?In a previous study, we have shown that locomotor training with frequent changes in pedal resistances complex training elicits significant improvement in locomotor performance compared to training consisting of constant pedal resistance simple training. Behavior gains associated with complex training correlate with down-regulation in the excitability of the alpha-motoneurone pools controlling muscles involved in the training task, as measured by changes in the soleus H-reflex, raising the hypothesis of a cause-effect link between performance improvements and modulation of spinal cord function. The purpose of this protocol is to determine if down-regulation in the excitability of the alpha-motoneuron pool is required for the improvement in locomotor performance. To address this issue, we hypothesize that up-regulation of alpha-motoneuron excitability will cancel improvements in locomotor performance elicited by complex training more than in subjects undergoing a period of simple training. The results of this study would provide novel information on the spinal substrates underlying learning of a locomotor task in intact humans and could be relevant to the design of rehabilitative strategies in patients with locomotor disorders. 