Fatigue and Motor Unit Activation Are Determined by Neither Load Nor Time Under Tension During Resistance Exercise
Bibliographic record
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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".