Intra-set Changes In Surface Electromyography Due To High Or Low Load Resistance Exercise To Failure

PURPOSE: The purpose of this study was to examine the load-dependence of changes in surface electromyography during lower body resistance exercise taken to failure. METHODS: Thirty strength-trained college-aged males (M; n = 14) and females (F; n = 16) completed crossover design where subjects completed 3 sets of unilateral leg extensions to failure with 2 min. rest at 80% (HL) or 30% (LL) of 1RM. Each visit was separated by 24 hours and the leg used for each visit was randomized and counter-balanced across subjects. EMG amplitude of the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) were recorded during the first, middle, and last rep of each set was selected for analysis using custom written signal processing software and EMG amplitudes were normalized to a warm-up repetition at equal load. RESULTS: For reps completed there were no sig. 3-way interactions, but there were sig. interactions for sex × set, sex × condition, and set × condition (p < .001-0.041). Post-hoc analysis indicated that when collapsed across sex, reps in LL set 1 (S1; Mean [95% CI]; 29.1 [27.3-31.0] reps) were sig. greater (ps < 0.001) than set 2 (S2; 22.7 [20.9-24.6] reps) and set 3 (S3; 19.3 [17.5-21.3], which was sig. lower than S2 (p = 0.025). There was no sig. difference in reps between sets in HL (ps > 0.05). When collapsed across set, there was no sig. difference in reps completed in HL between sexes (p = 0.359); however, M completed sig. more reps in LL than F (27.1 [24.9-29.3] vs. 20.1 [18.4-22.4] reps; p < 0.001). When collapsed across condition, M completed sig. more reps than F in S1 (22.1 [20.5-25.2] vs. 16.8 [14.7-19.0] reps; p < 0.001) and S2 (18.7 [16.3-21.0] vs 13.0 [10.9-15.1]; p < 0.001), but not S3. There were no sig. interaction effects for normalized EMG amplitude (ps > 0.05); however, there was a main effect for condition (p < 0.001). Post-hoc analyses indicated that when collapsed across rep, set, and sex, HL normalized amplitudes were 219% higher than those during LL (384% [357-412%] vs. 165% [137-193%]). CONCLUSIONS: Males completed more reps than females at low loads. Relative load, not fatigue, is the main driver of increased EMG amplitude across fatiguing sets.