Effects of blood flow restriction on motor unit recruitment patterns during fatiguing isometric leg extensions

Objective: Despite growing evidence supporting low-load resistance training with blood flow restriction (LL+BFR), the neuromuscular recruitment patterns during fatiguing LL+BFR exercise remain unclear. This investigation examined motor unit recruitment patterns of the vastus lateralis before and after fatiguing exercise. Methods: Fourteen women performed unilateral isometric leg extension at 50% of maximal voluntary isometric contraction (MVIC) to failure with and without BFR. Repetitions to failure were recorded, and pre- and post-exercise MVIC was assessed. Before and after each exercise intervention, participants completed isometric trapezoidal contractions while surface electromyography (sEMG) was recorded from the vastus lateralis. sEMG signals were decomposed to identify individual motor units to analyze the relationship between recruitment threshold (RT; %MVIC) vs. mean firing rate (MFR, pps) and peak motor unit amplitude (MUamp, mV) vs. MFR. Results: Both interventions resulted in similar decreases in MVIC peak torque from pre- (203.13±53.97 Nm) to post-exercise (143.61±36.93 Nm), but fewer repetitions were needed during BFR (20.3±8.2) compared to non-BFR (25.7±15.4). Similar increases occurred from pre- to post-exercise in slope (-0.21±0.07 pps/%MVIC; -0.27±0.07 pps/%MVIC) and y-intercept (17.60±3.87 pps; 20.02±3.83 pps) for the RT and MFR relationship, and y-intercept increased from pre- (22.46±3.00 pps) to post-exercise (25.81±4.49 pps) for the MUamp and MFR relationship. Conclusion: These results suggest both exercise interventions led to the early recruitment of higher-order motor units and increased firing rate of active motor units post-exercise. However, LL+BFR achieved these changes in fewer repetitions, suggesting LL+BFR is a repetition-efficient mode of exercise for eliciting acute changes in motor unit recruitment.