The Influence of TMS Interpulse Interval Duration on Lower Limb Corticospinal Excitability

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

The effects of resistance training to near failure on strength, hypertrophy, and motor unit adaptations in previously trained adults

Abstract Limited research exists examining how resistance training to failure affects applied outcomes and single motor unit characteristics in previously trained individuals. Herein, resistance‐trained adults (24 ± 3 years old, self‐reported resistance training experience was 6 ± 4 years, 11 men and 8 women) were randomly assigned to either a low‐repetitions‐in‐reserve (RIR; i.e., training near failure, n = 10) or high‐RIR (i.e., not training near failure, n = 9) group. All participants implemented progressive overload during 5 weeks where low‐RIR performed squat, bench press, and deadlift twice weekly and were instructed to end each training set with 0–1 RIR. high‐RIR performed identical training except for being instructed to maintain 4–6 RIR after each set. During week 6, participants performed a reduced volume‐load. The following were assessed prior to and following the intervention: (i) vastus lateralis (VL) muscle cross‐sectional area (mCSA) at multiple sites; (ii) squat, bench press, and deadlift one‐repetition maximums (1RMs); and (iii) maximal isometric knee extensor torque and VL motor unit firing rates during an 80% maximal voluntary contraction. Although RIR was lower in the low‐ versus high‐RIR group during the intervention (p < 0.001), total training volume did not significantly differ between groups (p = 0.222). There were main effects of time for squat, bench press, and deadlift 1RMs (all p‐values < 0.05), but no significant condition × time interactions existed for these or proximal/middle/distal VL mCSA data. There were significant interactions for the slope and y‐intercept of the motor unit mean firing rate versus recruitment threshold relationship. Post hoc analyses indicated low‐RIR group slope values decreased and y‐intercept values increased after training suggesting low‐RIR training increased lower‐threshold motor unit firing rates. This study provides insight into how resistance training in proximity to failure affects strength, hypertrophy, and single motor unit characteristics, and may inform those who aim to program for resistance‐trained individuals