The Influence of Transcranial Magnetic Stimulation Interpulse Interval Duration on Knee Extensor Corticospinal Excitability

Background/Purpose: To examine the influence of interpulse interval duration on knee extensor corticospinal excitability. Methods: Seventeen college-aged males and females participated in a single laboratory visit, during which 25 single transcranial magnetic stimulation pulses were delivered to the motor cortex with interpulse intervals of 5, 10, 15, and 20sec. Surface electromyographic signals were detected from the dominant vastus lateralis and rectus femoris. Motor evoked potential amplitude was compared across the four conditions. Results: For the vastus lateralis, the Friedman test indicated significant differences among conditions (chi-squared [3]=7.80, p=0.050); however, there were no pairwise differences (p >= 0.094) and small effect sizes (d <= 0.269). For the rectus femoris, the Friedman test results showed no significant differences among conditions (chi-squared [3]=2.44, p=0.487). Across all muscles and conditions, low intraclass correlation coefficients and high standard errors of measurement were suggestive of poor reliability. Conclusion: Unlike resting hand muscles, interpulse interval duration has little influence on corticospinal excitability for the knee extensors during active contractions. Impact statement Transcranial magnetic stimulation (TMS) data vary both within and across participants. When designing TMS studies, one variable that is rarely considered is the time between pulses, or the interpulse interval. Previous studies conducted in resting hand muscles show that longer interpulse intervals are associated with greater corticospinal excitability. We compared interpulse intervals of 5, 10, 15, and 20sec on corticospinal excitability of the knee extensors during active contractions. Unlike previous studies, our findings suggest that interpulse interval duration has little influence on corticospinal excitability. TMS researchers studying the knee extensors should select an interpulse interval that suits their needs and be consistent.