Acute changes in motor unit behaviour to fatiguing isometric contractions with blood flow restriction in healthy individuals.

Fatiguing contractions performed with limited oxygen supply develop a higher neuromuscular fatigue, perceived effort, and muscle pain that reduce exercise capacity. Despite these consistent observations, there is limited information about motor unit (MU) behaviour in response to limited oxygen supply. Fourteen healthy participants (mean ± SD age, 29 ± 5 years; height, 175 ± 7 cm; mass, 75 ± 11.1 kg) were recruited. Neuromuscular function, perceived effort, pain and MU behaviour were monitored during isometric contractions of the tibialis anterior muscle at 60% of maximal voluntary contraction (MVC), with blood flow restriction (BFR) and without (CON). High-density surface electromyography was used to investigate MU behaviour. MU were tracked across contractions and classified as relatively lower- (≤ 30% MVC) and higher-threshold (> 30% MVC). During exercise with BFR, heart rate, perceived effort, and muscle pain were higher ( p < 0.001). BFR induced greater neuromuscular fatigue as shown by reductions in MVC, maximal muscle activation, and muscle contractile function ( p < 0.001). The firing rate of lower-threshold MU decreased ( p < 0.001), whereas it increased for higher-threshold MU ( p ≤ 0.003). Both MU types exhibited reduced recruitment and de-recruitment thresholds with BFR ( p < 0.001). These results show disparate adjustment between lower- and higher-threshold MU during exercise with limited oxygen supply. Higher firing rate of higher-threshold MU might be required to compensate for the lower firing rate of lower-threshold MU. This suggests that the nervous system adopts an acute neural strategy to maintain force production during contractions with limited oxygen supply.