Effects Of Left Forearm Muscle Metaboreflex Activation On Right Vastus Lateralis Motor Unit Behavior

Muscle metaboreflex is a physiological response triggered by metabolite accumulation during muscle activity, which increases sympathetic nervous system activity. Although muscle metaboreflex activation (MMA) and cardiovascular responses are well studied, less is known regarding its effect on motor unit (MU) behavior. PURPOSE: To investigate the effects of MMA on MU recruitment and firing rate patterns. METHODS: Sixteen lower-body resistance trained males (21 ± 2 yrs, 179.52 ± 7.87 cm, 84.87 ± 13.42 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 40% MVC trapezoidal muscle action for the right knee extensors. For control (CON), blood flow was not occluded during the hand grip- 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 40% MVCs, EMG was decomposed for analysis of MU: recruitment thresholds (RT), action potential amplitudes (MUAPAMPS), and mean firing rates (MFR). EMG amplitude (EMGRMS) was normalized (N-EMGRMS) to MVC EMGRMS. MMA was assessed via mean arterial pressure (MAP) changes. Y-intercepts (y-ints) and slopes were calculated for the MUAPAMP 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 MUAPAMP and MFR vs. RT relationships, and N-EMGRMS between treatments. Alpha was ≤0.05. RESULTS: There were no significant differences for the y-ints or slopes of the MUAPAMP or MFR vs. RT relationships. However, there were significant differences for MAP (MMA = 21.75 ± 13.75 mm Hg, CON = 41.98 ± 17.73 mm Hg; p < 0.001) and N-EMGRMS (MMA = 43.37 ± 7.76%, CON = 48.34 ± 11.91%; p = 0.015). CONCLUSIONS: MMA of the left forearm decreased N-EMGRMS of the VL, suggesting a downward shift in the N-EMGRMS-torque relationship. Therefore, less excitation to the MU pool of the VL was required to match the same submaximal torque of the knee extensors during MMA.