The 1 mV TMS intensity and active motor threshold (AMT) were 56.5 ± 7.5% and 36.0 ± 4.7% [mean ± standard deviation (SD)], respectively, of the maximum stimulator output. Figure 1 shows representative waveforms of MEP during muscle contraction of 30% EMG. We were able to clearly observe CSP for all muscle contraction levels and all TMS intensities.
Effects of muscle contraction on MEP amplitude and CSP duration (experiment 1)
Results of one-way analysis of variance (ANOVA) showed that MEP amplitude changed significantly with increased muscle contraction (F(3,30) = 45.758, p < 0.01; Figure 2a). The MEP amplitude [mean ± standard error of the mean (SEM)] was 1.17 ± 0.13 mV at 10% EMG, 4.98 ± 0.43 mV at 30% EMG, 6.86 ± 0.70 mV at 50% EMG, and 7.75 ± 0.94 mV at 70% EMG. The MEP amplitude at 30%, 50%, and 70% EMG was significantly larger than that at 10% EMG (p < 0.01). Furthermore, the MEP amplitude at 50% and 70% EMG was significantly larger than that at 30% EMG (30% EMG, p < 0.01; 50% EMG, p < 0.05). No significant difference in the MEP amplitude was observed between 50% and 70% EMG.
CSP duration [mean ± standard error of the mean (SEM)] was 56.3 ± 1.0 ms at 10% EMG, 55.5 ± 1.6 ms at 30% EMG, 55.0 ± 2.1 ms at 50% EMG, and 54.8 ± 1.6 ms at 70% EMG. No significant differences in CSP duration were observed among the muscle contraction levels (F(2.134,21.342) = 0.466, p > 0.05; Figure 2b).
Effects of TMS intensity on MEP amplitude and CSP duration (experiment 2)
Results of one-way ANOVA showed that MEP amplitude changed significantly with increases in TMS intensity (F(2.356,23.564) = 67.687, p < 0.01; Figure 3a). The MEP amplitude at 10% EMG was 1.19 ± 0.13 mV at 100% AMT, 2.32 ± 0.33 mV at 110% AMT, 3.78 ± 0.50 mV at 120% AMT, 5.67 ± 0.77 mV at 130% AMT, 7.18 ± 0.66 mV at 140% AMT, 7.69 ± 0.64 mV at 150% AMT, and 8.28 ± 0.64 mV at 160% AMT. No significant difference in the MEP amplitude was observed at high-output stimulation intensities of >140% AMT.
Results of one-way ANOVA showed that CSP duration changed significantly with increases in TMS intensity (F(6, 60) = 119.578, p < 0.01; Figure 3b). The mean CSP duration was 57.1 ± 1.0 ms at 100% AMT, 65.6 ± 1.6 ms at 110% AMT, 81.7 ± 4.2 ms at 120% AMT, 107.0 ± 7.1 ms at 130% AMT, 124.8 ± 6.6 ms at 140% AMT, 139.9 ± 6.0 ms at 150% AMT, and 157.8 ± 7.0 ms at 160% AMT. CSP duration was prolonged with increasing stimulus intensity. CSP duration at 160% AMT was significantly prolonged compared with those at all other stimulus intensities.
Effects of paired-pulse TMS on MEP amplitude and CSP duration (experiment 3)
The mean (± SD) conditioning pulse intensity was 32.5 ± 5.8%, and the test pulse intensity was 56.5 ± 7.5%. Figure 4 shows the representative waveforms of MEP induced by paired-pulse TMS at 10% EMG. Results of one-way ANOVA showed that MEP amplitude was significantly changed by paired-pulse TMS (rest, F(2,20) = 18.712, p < 0.01; 10% EMG, F(2,20) = 12.263, p < 0.01; 30% EMG, F(1.785,17.845) = 21.738, p < 0.01; 50% EMG, F(1.770,17.698) = 6.788, p < 0.05). The mean (± SEM) MEP amplitude at rest was 0.90 ± 0.10 mV (single-pulse TMS), 0.51 ± 0.08 mV (SICI), and 1.25 ± 0.18 mV (ICF). The MEP amplitude induced by SICI was significantly smaller than that induced by single-pulse TMS (p < 0.01). Furthermore, the MEP amplitude induced by ICF was significantly larger than that induced by single-pulse TMS (p < 0.05) (Figure 5a). The MEP amplitude at 10% EMG was 7.96 ± 1.02 mV (single-pulse TMS), 6.33 ± 0.84 mV (SICI), and 8.95 ± 1.14 mV (ICF) (Figure 5b) and that at 30% EMG was 10.40 ± 1.09 mV (single-pulse TMS), 9.53 ± 0.98 mV (SICI), and 10.90 ± 1.10 mV (ICF) (Figure 5c). At both 10% and 30% muscle contraction levels, the MEP amplitude induced by SICI was significantly smaller than that induced by single-pulse TMS and ICF (p < 0.05). Furthermore, the MEP amplitude induced by ICF was significantly larger than that induced by single-pulse TMS at 10% and 30% muscle contraction levels (p < 0.05). The MEP amplitude at 50% EMG was 9.67 ± 0.82 mV (single-pulse TMS), 9.06 ± 0.74 mV (SICI), and 9.73 ± 0.74 mV (ICF), and the MEP amplitude induced by SICI was significantly smaller than that induced by single-pulse TMS and ICF (p < 0.05). In contrast, no significant differences were observed in the MEP amplitude induced by single-pulse TMS and ICF (Figure 5d).
Results of one-way ANOVA showed that the decrease and increase ratios changed significantly with increases in muscle contractions (decrease ratios, F(3,30) = 27.744, p < 0.01; increase ratios, F(1.253,12.530) = 11.571, p < 0.01; Figure 6). The decrease ratios with SICI were 42.3 ± 6.7% (rest), 19.2 ± 4.6% (10% EMG), 8.0 ± 1.5% (30% EMG), and 6.7 ± 1.2% (50% EMG). The ratio at rest was significantly higher than that at all muscle contraction levels (p < 0.05), and no significant differences were observed among the different muscle contraction levels. The increase ratios with ICF were 41.5 ± 9.4% (rest), 12.5 ± 2.8% (10% EMG), 6.1 ± 1.8% (30% EMG), and 4.9 ± 1.2% (50% EMG). The ratio at rest was significantly higher than that at 30% and 50% EMG (p < 0.05), and no significant differences were observed among the different muscle contraction levels.
Results of one-way ANOVA showed that CSP duration changed significantly with paired-pulse TMS (10% EMG, F(1.476,14.756) = 12.734, p < 0.01; 30% EMG, F(1.988,19.883) = 15.789, p < 0.01; 50% EMG, F(1.801,18.011) = 24.969, p < 0.01; Figure 7). CSP duration at 10% EMG was 145.5 ± 7.9 ms (single-pulse TMS), 130.8 ± 8.6 ms (SICI), and 145.9 ± 9.6 ms (ICF). CSP duration at 30% EMG was 143.5 ± 6.1 ms (single-pulse TMS), 129.1 ± 6.5 ms (SICI), and 143.1 ± 6.4 ms (ICF). CSP duration at 50% EMG was 140.1 ± 6.5 ms (single-pulse TMS), 127.3 ± 6.5 ms (SICI), and 141.4 ± 7.4 ms (ICF). At all muscle contraction levels, CSP shortened significantly with SICI compared with single-pulse TMS and ICF (p < 0.05). However, no significant differences were observed in CSP duration between single-pulse TMS and ICF.
No significant correlations were observed between MEP amplitude and the duration of CSP induced by single-pulse TMS and after SICI and ICF at 10%, 30%, and 50% EMG (single-pulse TMS, p = 0.261, 0.140, and 0.345; SICI, p = 0.716, 0.234, and 0.312; ICF, p = 0.189, 0.091, and 0.112, respectively; Figure 8).