Overnight fasting does not affect motor unit firing, but may induce recruitment threshold-dependent changes in motor unit recruitment threshold

Exercise while under energy deficit alters metabolic responses, such as promoting fat oxidation; however, its effects on the neuromuscular system, which consumes fuel, remain unclear. This study investigated motor unit (MU) firing characteristics during low- and high-intensity exercise with or without energy intake following overnight fasting. Eighteen healthy adults participated. After an 8-10 hour fast, participants ingested either placebo jelly containing 0 kcal (Non-caloric feeding [NCF]) or high-carbohydrate jelly containing 7 kcal/kg body mass (Caloric feeding [CF]). Two hours later, the respiratory exchange ratio (RER) during dynamic knee extension at ~20% of peak oxygen consumption and high-density surface electromyography (HDsEMG) from the vastus lateralis during isometric knee extensions at 30% and 70% of maximal voluntary contraction (MVC) were measured. HDsEMG signals were decomposed into individual MU action potentials, which were tracked across the conditions, and the MU firing rate and recruitment threshold were calculated. RER was lower under the NCF than CF (p<0.001), indicating enhanced fat oxidation. In both 30 and 70% MVC, the MU firing rate and recruitment threshold did not differ between conditions (both p > 0.05). During 30% MVC, the difference in the MU recruitment threshold from the NCF to CF condition was positively associated with recruitment threshold (p=0.002), whereas no recruitment threshold-dependent changes were observed during 70% MVC (p = 0.110). These findings suggest that overnight fasting may modulate MU recruitment patterns during low-intensity exercise. These results may highlight a potential neuromuscular strategy to optimize energy metabolism under fasting conditions.