Acute changes in motor unit behavior 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) behavior in response to limited oxygen supply. Fourteen healthy participants (means ± SD age, 29 ± 5 yr; height, 175 ± 7 cm; mass, 75 ± 11.1 kg) were recruited. Neuromuscular function, perceived effort, muscle pain, and MU behavior were monitored during isometric contractions of the dominant ankle dorsiflexors at 60% of maximal voluntary contraction (MVC), with blood flow restriction (BFR) and without (Control). High-density surface electromyography was used to investigate MU behavior of the tibialis anterior muscle. MU were tracked across contractions and classified as relatively lower-threshold (≤30% MVC) and higher-threshold (> 30% MVC). During exercise with BFR, heart rate, perceived effort, and muscle pain were higher (<i>P</i> < 0.001). BFR induced greater neuromuscular fatigue, reduced maximal muscle activation, and muscle contractile function (<i>P</i> < 0.001). The discharge rate of lower-threshold MU decreased (<i>P</i> < 0.001), whereas it increased for higher-threshold MU (<i>P</i> ≤ 0.003). Both MU types exhibited reduced recruitment and derecruitment thresholds with BFR (<i>P</i> < 0.001). These results show disparate adjustment between lower- and higher-threshold MU during exercise with limited oxygen supply. Higher discharge rate of relatively higher-threshold MU might be required to compensate for the lower discharge rate of relatively lower-threshold MU. This suggests that the nervous system adopts an acute neural strategy to maintain force production during contractions with limited oxygen supply.<b>NEW & NOTEWORTHY</b> The behavior of motor units during high-intensity fatiguing contractions with limited oxygen supply is poorly investigated. Here, we show that during fatiguing exercise with blood flow restriction, motor units are recruited at lower force levels with inhibition of relatively lower-threshold motor units and increased activity of relatively higher-threshold motor units. These changes in motor unit behavior might represent an acute neural adaptation to produce force during high-intensity contractions with limited oxygen supply.