Excitability of Spinal Motor Neurons in the Contralateral Arm during Voluntary Arm Movements of Various Difficulty Levels

[Purpose] This study evaluated the effects of voluntary arm movements performed at various difficulty levels on the excitability of spinal motor neurons in the contralateral arm using F-wave analysis. [Subjects] A total of 20 healthy volunteers participated in this study after giving their written informed consent. [Methods] With the subject seated on a chair, F-waves were elicited from the right abductor pollicis brevis during movement tasks performed by the left arm. The tasks were repetitive movements between two targets placed 20 cm apart on a desk. The subject was instructed to accurately touch the targets with the tip of a pen. In order to adjust the difficulty level, the size of the targets was changed as follows: 5 cm x 15 cm (width x length) for task 1, 0.5 cm x 15 cm for task 2, and 0.25 cm x 15 cm for task 3. The control task was to remain in the sitting position on the chair. [Results] The amplitude ratio of F/M significantly increased during tasks 2 and 3 compared with that during the control task. [Conclusion] Voluntary arm movements performed at a high difficulty level increase the excitability of spinal motor neurons in the contralateral arm.