Increased corticospinal inhibition following brief maximal and submaximal contractions in humans.

A potentiating conditioning contraction (CC) has been shown to increase silent period duration, an index of corticospinal inhibition; however, it is unknown if the CC must induce potentiation for corticospinal inhibition to increase. Ten healthy, young adults (four females) completed this study to assess potentiation and silent period (SP) duration before and after four types of CCs: voluntary and electrically evoked maximal CCs to optimize potentiation, and voluntary and electrically evoked submaximal CCs (∼40% of maximal voluntary force) that induced minimal potentiation. Stimulation was applied to the ulnar nerve to evoke twitches for the assessment of potentiation and to evoke tetanic CCs of the first dorsal interosseous muscle. The SP was elicited by applying transcranial magnetic stimulation to the motor cortex during brief contractions at 25% of maximal voluntary force. Changes to twitch force and SP duration were not different for voluntary and tetanic contractions, so data were pooled. Twitch force increased by 81.2 ± 35.7% (P < 0.001) and 3.2 ± 6.5% (P = 0.039) following maximal and submaximal CCs, respectively. The SP was prolonged following maximal (12.6 ± 6.3%; P < 0.001) and submaximal (4.8 ± 4.9%; P < 0.001) CCs. Correlations between post-CC twitch force and SP duration were not significant for maximal or submaximal conditions (r = -0.068; r = 0.067; P ≥ 0.780, respectively). Duration of the SP increased not only following maximal-intensity CCs but also after submaximal-intensity CCs that induced virtually no potentiation (∼3%). Thus, we suggest that corticospinal inhibition is not directly related to mechanisms of muscle potentiation per se, but, rather, the level of muscle contraction likely mediates feedback from large diameter afferents that affect the SP.NEW & NOTEWORTHY The transcranial magnetic stimulation-induced silent period reflects a transient state of corticospinal inhibition that is influenced by recent history of muscle activation, which may include an effect of potentiation. We demonstrate that silent period duration increases following both voluntary and electrically evoked maximal and submaximal conditioning contractions, even though the latter intensity produced virtually no muscle potentiation. Feedback from group Ia and Ib muscle afferents is proposed as the cause of the increased corticospinal inhibition.