Overnight fasting does not affect overall motor unit firing rate or recruitment threshold, but may selectively facilitate recruitment of low-threshold motor units during low-intensity exercise, suggesting a potential neuromuscular strategy to optimize energy metabolism under fasting conditions.
Key Findings
Results
Overnight fasting enhanced fat oxidation compared to caloric feeding, as confirmed by a lower respiratory exchange ratio.
Participants underwent an 8-10 hour fast before the experiment.
Non-caloric feeding (NCF) condition used a 0 kcal placebo jelly; caloric feeding (CF) condition used a high-carbohydrate jelly containing 7 kcal/kg body mass.
RER was significantly lower under NCF than CF (P < 0.001), confirming enhanced fat oxidation in the fasted/NCF state.
RER was measured during dynamic knee extension at approximately 20% of peak oxygen consumption.
Results
Motor unit firing rate did not differ between the non-caloric feeding and caloric feeding conditions at either exercise intensity.
High-density surface electromyography (HDsEMG) from the vastus lateralis was used during isometric knee extensions at 30% and 70% of maximal voluntary contraction (MVC).
HDsEMG signals were decomposed into individual motor unit action potentials, which were tracked across conditions.
No significant difference in MU firing rate was observed between NCF and CF at either 30% or 70% MVC (both P > 0.05).
Sample size was 18 healthy adults.
Results
Overall motor unit recruitment threshold did not differ between the non-caloric feeding and caloric feeding conditions at either exercise intensity.
MU recruitment threshold was calculated from decomposed HDsEMG signals tracked across both conditions.
No significant difference in MU recruitment threshold was found between NCF and CF conditions at 30% or 70% MVC (both P > 0.05).
Motor units were tracked across conditions to allow within-unit comparisons.
Results
During low-intensity exercise (30% MVC), the change in motor unit recruitment threshold from NCF to CF was positively associated with the recruitment threshold, indicating recruitment threshold-dependent changes.
The difference in MU recruitment threshold from NCF to CF condition was positively associated with recruitment threshold during 30% MVC (P = 0.002).
This positive association suggests that higher-threshold MUs showed greater increases in recruitment threshold under CF relative to NCF, while lower-threshold MUs showed relatively less change or a different directional shift.
The authors interpret this as overnight fasting potentially selectively facilitating recruitment of low-threshold motor units during low-intensity exercise.
No such recruitment threshold-dependent association was observed during 70% MVC (P = 0.110).
Results
No recruitment threshold-dependent changes in motor unit recruitment threshold were observed during high-intensity exercise (70% MVC) between fasting and fed conditions.
The association between the difference in MU recruitment threshold (NCF to CF) and recruitment threshold was not statistically significant at 70% MVC (P = 0.110).
This contrasts with the significant association found at 30% MVC (P = 0.002).
The intensity-specific finding suggests the potential neuromuscular modulation associated with fasting may be limited to low-intensity contractions.
Methods
The study design used a within-subject motor unit tracking approach with two feeding conditions measured two hours after ingestion of either placebo or caloric jelly.
Eighteen healthy adults participated in the study.
After an 8-10 hour fast, participants ingested either 0 kcal placebo jelly (NCF) or high-carbohydrate jelly at 7 kcal/kg body mass (CF).
Measurements were taken 2 hours after ingestion.
HDsEMG was recorded from the vastus lateralis during isometric knee extensions at 30% and 70% MVC.
Individual MU action potentials were decomposed from HDsEMG signals and tracked across conditions to enable paired comparison.
What This Means
This research suggests that going without food overnight (fasting for 8-10 hours) does not broadly change how the nervous system recruits and activates muscle fibers during exercise. The study measured the electrical activity of individual motor units — the nerve-muscle groups that work together to produce movement — in the thigh muscle of 18 healthy adults during both low- and high-intensity knee exercises, comparing a fasted state to a state where participants consumed a carbohydrate-rich meal. While the body clearly shifted toward burning more fat when fasted (as shown by lower respiratory exchange ratios), the overall pattern of when motor units were activated and how rapidly they fired did not change significantly between the fasted and fed states.
However, the study found a more subtle effect: during low-intensity exercise (at 30% of maximum effort), fasting appeared to selectively shift the pattern of motor unit recruitment in a threshold-dependent way. Specifically, lower-threshold motor units — the smaller, more energy-efficient ones that are typically activated first — appeared to be preferentially recruited during fasting. This effect was not observed during higher-intensity exercise (70% of maximum effort). This hints that the nervous system may make small adjustments to favor more efficient muscle fibers when the body is in a low-energy state.
This research matters because it provides new insight into how the neuromuscular system — not just metabolism — may adapt to fasting or exercising in an energy-restricted state. The findings suggest that fasting exercise does not impair the basic mechanics of how muscles are controlled, but there may be subtle shifts in recruitment strategy at lower intensities that could reflect the body's effort to conserve energy. This could be relevant to understanding the effects of popular practices like fasted exercise on muscle function, though more research is needed to confirm and extend these findings.
Check Your Own Numbers
Upload your bloodwork. We'll cross-reference your results against this study and 4,700 others.
Igawa K, Takeda R, Nishikawa T, Ueda S, Mita Y, Watanabe K. (2026). Overnight fasting does not affect motor unit firing but may induce recruitment threshold-dependent changes in motor unit recruitment threshold.. Journal of neurophysiology. https://doi.org/10.1152/jn.00034.2026