P04 Investigating the effects of muscle contraction and conditioning stimulus-intensity on short-interval intracortical inhibition
Hendy AM, Ekblom MM, Latella C, Teo WP
- Institute for Physical Activity and Nutrition (IPAN), School of Exercise and Nutrition Sciences, Deakin University, Burwood, Australia.
- The Swedish School of Sport and Health Sciences, GIH, Stockholm, Sweden.
- School of Medical and Health Sciences, Edith Cowan University, Joondalup, Australia.
Short-interval intracortical inhibition (SICI) is typically assessed using paired transcranial magnetic stimulation (TMS) over the primary motor cortex (M1)(1). A reduction in SICI has been shown to precede long-term potentiation after motor practice(2), and occur with enhanced muscle strength after resistance training(3). This study assessed the effect of muscle contraction intensity and conditioning stimulus parameters on modulations in SICI.
Single and paired-pulsed TMS was applied to the M1 and motor-evoked potential (MEP) amplitude was recorded from the biceps brachii in 16 adults. Stimuli were delivered while participants performed 10, 20, 40 and 75% of maximal voluntary isometric contraction (MVIC). At each contraction level, the test stimulus was assessed at 120% of active motor threshold (AMT), while the conditioning stimulus was set to three different conditions: 60, 70 and 80% of AMT.
The single pulse MEP increased with force output, with the highest values occurring at 75% MVIC. In contrast to previous literature(4), the current study did not identify a reduction in SICI from 10 to 40 % MVIC, but rather from 40 to 75 % MVIC (p < 0.01). There was no significant interaction between conditioning stimulus intensity and force level (p = 0.558), or main effect for conditioning stimulus intensity (p = 0.752).
Even during high force outputs (75% MVIC), SICI was still present, possibly allowing for even greater withdrawal of inhibition at higher contraction intensities. Future studies might consider assessing SICI not only at rest or during low contraction intensities, but also at high or maximal intensity.
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