It has long been known that practicing a task with one limb can result in performance improvements with the opposite, untrained limb. Hypotheses to account for cross-limb transfer of performance state that the effect is mediated either by neural adaptations in higher order control centers that are accessible to both limbs, or that there is a "spillover" of neural drive to the opposite hemisphere that results in bilateral adaptation. Here we address these hypotheses by assessing performance and corticospinal excitability in both hands after unilateral practice of a ballistic finger movement. Participants (n = 9) completed 300 practice trials of a ballistic task with the right hand, the aim of which was to maximize the peak abduction acceleration of the index finger. Practice caused a 140% improvement in right-hand performance and an 82% improvement for the untrained left hand. There were bilateral increases in the amplitude of responses to transcranial magnetic stimulation, but increased corticospinal excitability was not correlated with improved performance. There were no significant changes in corticospinal excitability or task performance for a control group that did not train (n = 9), indicating that performance testing for the left hand alone did not induce performance or corticospinal effects. Although the data do not provide conclusive evidence whether increased corticospinal excitability in the untrained hand is causally related to the cross-transfer of ballistic performance, the finding that ballistic practice can induce bilateral corticospinal adaptations may have important clinical implications for movement rehabilitation.
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