Crossed facilitation and post-contraction depression of abductor pollicis brevis motor neurons

Crossed facilitation of abductor pollicis brevis (APB) motor neurons by voluntary contraction of the contralateral APB was investigated using F response analysis. The integrated F response EMG showed a mean 4-fold increase during contraction. This enhancement was maintained until the onset of subjective fatigue and then declined. After isometric or isotonic contraction of the ipsilateral APB there was a significant depression of F responses with a progressive return to baseline values over a 6 minute period. The significance of the findings is discussed.