Paired corticospinal-motoneuronal stimulation increases maximal voluntary activation of human adductor pollicis

Paired corticospinal-motoneuronal stimulation (PCMS), which delivers repeated pairs of transcranial magnetic stimuli (TMS) and maximal motor nerve stimuli, can alter corticospinal transmission to low threshold motoneurones in the human spinal cord. To determine whether similar changes occur for high threshold motoneurones, we tested whether maximal voluntary activation and force can be affected by PCMS in healthy individuals. On two separate days, healthy participants (n=14) performed brief thumb adduction MVCs before and after a control protocol (TMS only) or PCMS designed to facilitate corticospinal transmission to adductor pollicis. Peripheral nerve stimulation alone was not performed. During each MVC, a superimposed twitch was elicited by a supramaximal stimulus delivered to the ulnar nerve. With muscles relaxed following the maximal contraction, a similar stimulus elicited a resting twitch. Voluntary activation was calculated as (1- superimposed twitch/resting twitch)*100%. While voluntary activation decreased over time in both conditions, the decrease was less after PCMS (-0.4 ± 4.1%) than after the control protocol (-4.9 ± 4.9%, p = 0.007). This was supported by a greater increase in EMG after PCMS than control (7 ± 13% vs. -3 ± 10%; p = 0.043). However, maximal force was not affected. The findings indicate a modest effect of PCMS on maximal neural drive to adductor pollicis, suggesting that PCMS can affect corticospinal transmission to high threshold motoneurones.

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