prime Mover of finger extension
This study is among the first to provide evidence that use-dependent plasticity may rely on different cortical mechanisms depending on the muscles involved in the task. After repetitive performance of a lightly resisted finger tracking task, there was a decrease in intracortical inhibition of the lower threshold corticospinal neurons (SICIL) targeting EDC, the finger extensor muscle, which was not observed in the FDS, the finger flexor muscle. Further, there was an increase in corticospinal excitability targeting the FDS that was not observed in the EDC. Our findings in the EDC are remarkably similar to a result reported by, who measured CE and SICI in the extensor carpi radialis before and after practice of a waveform tracking task similar to that used in the present study. The tracking task required wrist flexion and extension and was performed with the forearm pronated, so that gravity resisted the extensors, requiring both concentric and eccentric contractions to control the cursor. Like the effects we observed in the EDC, the authors report no change in CE but a significant decrease in SICI targeting the extensor carpi radialis. No measurements of wrist flexors were reported however. Together these studies provide evidence that, in extensors of the wrist and fingers, adaptation to repetitive performance of a precision tracking task occurs more via modulation of intracortical inhibition than corticospinal excitability.
Role of muscle in tracking task.
Of course, the specific role of the muscle in the tracking task may also be influencing the results. Others have reported use-dependent effects that were specific to the role of the muscle in the task performed. For example, following a repetitive, ballistic finger extension task, MEPs from the agonist muscle (EDC) have been reported to increase, while those from the antagonist FDS decreased. A similar finding was associated with a ballistic wrist extension task. These findings may seem to conflict with the present finding of increased CE in the FDS after repetitive movement, with no CE change in EDC, regardless of which muscle was resisted during task performance. However, we suggest that this apparent conflict is likely related to another task parameter: the goal of the task. The ballistic motor tasks used in the studies by and require quick, forceful, concentric contractions of the agonist muscle, while the task in the present study required low-force, precise contractions that were both concentric and eccentric, and required more co-contraction. The former are likely to show larger agonist/antagonist effects, while the present task may not.