Abstract
Cross education is the process whereby training of one limb gives rise to increases in the subsequent performance of its opposite counterpart. The execution of many unilateral tasks is associated with increased excitability of corticospinal projections from primary motor cortex (M1) to the opposite limb. It has been proposed that these effects are causally related. Our aim was to establish whether changes in corticospinal excitability (CSE) arising from prior training of the opposite limb determine levels of interlimb transfer. We used three vision conditions shown previously to modulate the excitability of corticospinal projections to the inactive (right) limb during wrist flexion movements performed by the training (left) limb. These were: (1) mirrored visual feedback of the training limb; (2) no visual feedback of either limb; and (3) visual feedback of the inactive limb. Training comprised 300 discrete, ballistic wrist flexion movements executed as rapidly as possible. Performance of the right limb on the same task was assessed prior to, at the mid point of, and following left limb training. There was no evidence that variations in the excitability of corticospinal projections (assessed by transcranial magnetic stimulation (TMS)) to the inactive limb were associated with, or predictive of, the extent of interlimb transfer that was expressed. There were however associations between alterations in muscle activation dynamics observed for the untrained limb, and the degree of positive transfer that arose from training of the opposite limb. The results suggest that the acute adaptations that mediate the bilateral performance gains realized through unilateral practice of this ballistic wrist flexion task are mediated by neural elements other than those within M1 that are recruited at rest by single-pulse TMS.
Highlights
Cross education is the phenomenon whereby training of one limb gives rise to increases in the subsequent performance of its opposite counterpart
Behavioral Outcomes In order to eliminate participants who did not engage fully with the training task in the fashion intended, we excluded from further analysis those who failed to exhibit improvements in performance (i.e., changes in the peak accelerations of the ballistic movements exhibited for the training limb) above a criterion defined empirically by the adjusted boxplot method for outlier rejection
The repetition of 300 ballistic movements, each executed with the intent of maximizing acceleration, gave rise to substantial increases in the performance of the opposite limb
Summary
Cross education is the phenomenon whereby training of one limb gives rise to increases in the subsequent performance of its opposite counterpart. In view of the fact that the execution of many unilateral tasks is associated with increased excitability of corticospinal projections to the opposite limb (Hortobágyi et al, 2003; Carson et al, 2004) it has been proposed that interlimb transfer of training induced performance gains may be subserved by interactions between the primary motor cortices (e.g., Hinder et al, 2011). With respect to ballistic movement tasks, no association has yet been established between variations in degree of cross education expressed across participants, and the extent to which elevations in the excitability of corticospinal projections to the muscles of the untrained limb are manifested (Carroll et al, 2008; Hinder et al, 2011; Dickins et al, 2015; Reissig et al, 2015)
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