(8) INFLUENCE OF FOREARM MUSCLE METABOREFLEX ACTIVATION ON VASTUS LATERALIS MOTOR UNIT BEHAVIOR

Purpose: The purpose of this study was to investigate the effects of muscle metaboreflex activation (MMA) on motor unit (MU) recruitment and firing rate patterns. Methods: Seventeen lower-body resistance trained males (21 ± 2 yrs, 180.14 ± 8.38 cm, 84.84 ± 12.38 kg) performed isometric maximal voluntary contractions (MVCs) of the right knee extensors. Participants then completed isometric handgrip dynamometer MVCs of the left hand, followed by a 2 minute 30% MVC isometric handgrip muscle action. For the MMA treatment, arterial occlusion of the left arm at supra-systolic pressure (≥ 250 mm Hg) was initiated the last 10 secs of the 30% MVC, and remained on during a subsequent 70% MVC trapezoidal muscle action for the right knee extensors. For control (CON), blood flow was not occluded during the handgrip- or knee extensor-muscle actions. Treatments were randomized. Surface electromyographic (EMG) signals were recorded from the vastus lateralis (VL) during the knee extensor muscle actions. For the 70% MVCs, EMG was decomposed for analysis of MU: recruitment thresholds (RT), action potential amplitudes (MUAp_AMPS), and mean firing rates (MFR). EMG amplitude (EMG_RMS) was normalized (N-EMG_RMS) to MVC EMG_RMS. MMA was assessed via mean arterial pressure (MAP) changes. Y-intercepts (y-ints) and slopes were calculated for the MUAp_AMP and MFR vs. RT relationships during MMA and CON. Six paired samples t-tests were used to compare MAP, y-ints and slopes for the MUAp_AMP and MFR vs. RT relationships, and N-EMG_RMS between treatments. Alpha was ≤ 0.05. Results: There were no significant differences for the y-ints of the MUAp_AMP or MFR vs. RT relationships, slopes of the MUAp_AMP vs. RT relationship, or N-EMG_RMS. However, there were significant differences for the slopes of the MFR vs. RT relationship (MMA = -0.500 ± 0.224 pps/%MVC, CON = -0.581 ± 0.204 pps/%MVC; p = 0.042) and MAP (MMA = 18.68 ± 15.29 mm Hg, CON = 34.13 ± 12.88 mm Hg; p < 0.001). Conclusions: MMA of the forearm increased motorneuron firing excitability of the VL, such as the same amount of muscle excitation resulted in greater higher-threshold MU firing rates, while MU recruitment patterns were unchanged. Practical Applications: These findings indicate that initiating MMA on an upper-body muscle can alter MU firing rate behavior for a lower-body muscle during a high-intensity contraction. This may provide clinicians/strength and conditioning coaches increased opportunities to utilize MMA, such as situations where restricting/occluding blood flow to an exercising muscle isn’t feasible (discomfort, exercise type, etc.). Future research should investigate if initiating MMA in one muscle can increase MU recruitment of another muscle during a series of contractions. Acknowledgements: None.