Fatigue and Motor Unit Activation Are Determined by Neither Load Nor Time Under Tension During Resistance Exercise
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abstract
Recent evidence has established that repetition load determines neither acute rates of muscle protein synthesis or resistance training‐induced hypertrophy. Our objective was to extend on these findings by examining if skeletal muscle hypertrophy following resistance exercise training is instead determined by the degree of motor unit activation. Ten recreationally active young men (mean ± SD; 21 ± 3 y, 177 ± 6 cm, 82 ± 9 kg) completed three sets of unilateral knee extension exercise to volitional failure under four conditions that varied in the speed of contraction (time under tension [TUT]) and load (% one repetition‐maximum [%RM]): 80%RM Regular (80R; 1 eccentric: 1 pause: 1 concentric), 80%RM Slow (80S; 3:1:3), 30%RM Regular (30R; 1:1:1) and 30%RM Slow (30S; 3:1:3). Each participant completed two training sessions separated by one week, performing two of the four conditions in each session (one on each leg). Electromyography (EMG) and maximum voluntary forces (MVF) were used to assess motor unit activity and fatigue, respectively. Load was higher in the heavy‐load (80R and 80S) conditions (145 ± 18 vs. 55 ± 7 kg), the TUT per repetition was higher in the slow (80S and 30S) conditions (5.4 ± 0.6 vs. 2.8 ± 0.5 s), repetitions per set were different in each group such that 80S < 80R < 30S < 30R (80S: 6 ± 1, 80R: 9 ± 2, 30S: 14 ± 4, 30R: 20 ± 4), volume was lower in the slow conditions (2556 ± 776 vs. 3668 ± 1076 kg) and the total TUT including each repetition was different in all conditions such that 80R < 80S < 30R < 30S (80R: 76 ± 20, 80S: 99 ± 17, 30R: 158 ± 19, 30S: 225 ± 52, p<0.05). The EMG amplitude was higher in the heavy‐load conditions during the first (65 ± 15 vs. 46 ± 18 %MVE) and last (89 ± 26 vs. 72 ± 32 %MVE, p<0.05) repetitions of each set. The EMG amplitude increased during each condition (23 ± 18 %MVE) but the integrated EMG (iEMG) signal was highest in the light‐load (30R and 30S) conditions (27 ± 13 vs. 16 ± 6 %MVE·s, p<0.01). MVF decreased between each set in every condition (pre: 240 ± 32, set 1: 218 ± 33, set 2: 204 ± 29, set 3: 194 ± 34 N, p<0.01). Total TUT was significantly correlated with iEMG (r=0.41, p=0.009). These data demonstrate that fatigue during resistance exercise training is unrelated to load or TUT when resistance exercise is performed until volitional failure. In addition, significant motor unit activation occurs when resistance exercise is performed until volitional failure even at lower loads and, although amplitude may be higher with heavier loads, the total (integrated) EMG response is greater during resistance exercise using lighter loads.
