Effects of Practicing Difficult Movements of the Unilateral Arm on the Excitability of Spinal Motor Neurons in the Contralateral Arm

Background: The excitability of the spinal motor neurons of the contralateral upper limb increases during voluntary movement of the upper limbs. However, reports on the changes in the facilitation effects of the movements of the unilateral upper limb on the spinal motor neurons in the contralateral upper limb that are associated with motor learning are few. Methods: Sixteen right-handed healthy adults were randomly assigned to either the control group or the practice group. The F-waves were derived from the right abductor pollicis brevis muscle during the tasks before and after practice. The tasks included repetitive movements of the left upper limb between two small targets placed 20 cm apart on a desk. The subjects were instructed to accurately touch the targets with the tip of a pen. The practice of the practice group included repetitive movements using the same targets. The practice of the control group included repetitive movements without targets. The practice was performed for five sessions with each session consisting of 30 movements. The F-waves were analyzed for the amplitude ratio of F/M and latency. In addition, the number of times the tip of the pen touched the outside of the target was counted. Results: The amplitude ratio of F/M during post-practice significantly decreased compared with that during prepractice in the practice group. Latency showed no significant differences. The number of failures during post-practice decreased significantly compared with that during pre-practice in the practice group. Conclusion: This study suggests that the facilitation effects of the voluntary movements of the unilateral upper limb that were performed at a high difficulty level on the spinal motor neurons in the contralateral upper limb decreased with motor learning.