Effects Of Blood Flow Restriction On Motor Unit Behavior During A Single High-Load Contraction

PURPOSE: To determine the effects of blood flow restriction (BFR) on motor unit (MU) behavior of the biceps brachii (BB) during a single high-load contraction. METHODS: Twelve recreationally active males (age = 25 ± 5 yrs) participated in this study. Surface electromyography (sEMG) was recorded from the BB during separate BFR and control (CON) visits. Following 3 maximal voluntary isometric contractions (MVCs) of the elbow flexors, participants randomly completed separate isometric trapezoidal muscle actions at 70% MVC with BFR and without (CON). For BFR, a cuff was inflated to 60% of the pressure required to completely occlude the brachial artery at rest. sEMG signals for the 70% MVCs were decomposed for analysis of MU: recruitment thresholds (RTs), action potential amplitudes (MUAPAMPS), initial firing rates (IFRs), and mean firing rates (MFRs). EMG amplitude (EMGRMS) at steady force was normalized (N-EMGRMS) to MVC EMGRMS. Y-intercepts (y-ints) and slopes were calculated for the MUAPAMP, IFR, and MFR vs. RT linear relationships. A terms and B terms were calculated for the MFR vs. MUAPAMP exponential relationships. Nine paired samples t-tests compared N-EMGRMS, y-ints and slopes for the MUAPAMP, IFR, and MFR vs. RT relationships, and A and B terms for the MFR vs. MUAPAMP relationships between BFR and CON. Alpha was ≤0.05. RESULTS: There were no coefficient differences for the MFR vs. RT relationships, or the MFR vs. MUAPAMP relationships. There were differences for the MUAPAMP (y-ints: p = 0.008, CON = -0.26 ± 0.39 mV, BFR = -0.71 ± 0.36 mV; slopes: p = 0.018, CON = 0.019 ± 0.013 mV/%MVC, BFR = 0.028 ± 0.012 mV/%MVC) and IFR (y-ints: p = 0.008, CON = 14.02 ± 3.39 pps, BFR = 17.52 ± 4.00 pps; slopes: p = 0.006; CON = -0.167 ± 0.053 pps/%MVC, BFR = -0.224 ± 0.058 pps/%MVC) vs. RT relationships, and N-EMGRMS (p = 0.011; CON = 60.73 ± 10.40 %, BFR = 75.53 ± 20.58 %). CONCLUSION: BFR increased IFRs of the lowest-threshold MUs compared to CON. Additionally, increased slopes for the MUAPAMP vs. RT relationships, greater N-EMGRMS at steady force, and no change in MFRs relative to RTs or MUAPAMPS suggests a greater reliance on MU recruitment to modulate force during the steady force-segment with BFR compared to CON. BFR during a single, high-load contraction may increase MU recruitment compared to a non-BFR contraction performed at the same relative load.