Introduction: Transcranial magnetic stimulation (TMS) is a non-invasive research technique used to study the nervous system. When conducting TMS research, the timing between pulses, or the interpulse interval, may influence corticospinal excitability. Previous studies conducted on hand muscles have shown that longer interpulse interval durations result in greater corticospinal excitability.
Purpose: The purpose of the present study was to investigate the effects of different interpulse intervals of single-pulse TMS on corticospinal excitability for the knee extensors. Based on previous studies, it was hypothesized that longer interpulse intervals would produce greater motor evoked potential amplitude and minimize pulse- to-pulse reliability.
Methods: Seventeen healthy, college-aged males and females participated in a single laboratory visit, during which 25 single TMS pulses were delivered to the motor cortex with interpulse intervals of 5,10,15, and 20 seconds. Bipolar surface electromyographic signals were detected from the dominant vastus lateralis and rectus femoris. Friedman\u27s test was used to examine mean differences across conditions. Within each condition, reliability across the 25 pulses was investigated. Results: For the vastus lateralis, a Friedman\u27s test indicated significant differences among conditions (chi-squared [3] = 7.80, p = .050). However, there were no significant pairwise differences (p ≥ .094) and small effect sizes (d ≤ 0.269). For the rectus femoris, Friedman\u27s test results showed no significant differences among conditions (chi-squared [3] = 2.44, p = .487). For both muscles and all four conditions, low intraclass correlation coefficients and high standard errors of measurement were suggestive of poor reliability.
Conclusion: Unlike resting muscles of the hand, interpulse interval duration has little influence on corticospinal excitability for the knee extensors during active contractions
