Effects of the metaboreflex on motor unit behavior profiles

Muscle metaboreflex activation (MMA) is a physiological response to metabolite accumulation during sustained contractions, leading to increased sympathetic nervous system activity. While the cardiovascular effects of MMA are well studied, its systemic influence on motor unit (MU) behavior remains unclear. PURPOSE: To examine the effects of MMA, induced at the upper arm, on MU behavior of the contralateral, non-occluded vastus lateralis (VL). METHODS: Sixteen participants (21 ± 2 yrs; 179.27 ± 7.39 cm; 84.87 ± 13.42 kg) performed maximal voluntary contractions (MVCs) of the right knee extensors. Participants then completed isometric handgrip MVCs of the left hand, followed by a 2-min 30% MVC isometric handgrip contraction. For MMA, arterial occlusion of the left arm (≥ 250 mm Hg) was initiated during the final 10 sec of the 30% MVC, and remained applied during a subsequent 40% MVC trapezoidal knee extensor contraction. For the control (CON) condition, no occlusion occurred. Conditions were randomized. Surface electromyography (EMG) was recorded from the VL during the knee extensor contractions. For the 40% MVCs, EMG was decomposed to extract MU recruitment thresholds (RT), action potential amplitudes (MUAPₐₘₚ), and mean firing rates (MFR). EMG amplitude (EMGRMS) during the 40% MVC was normalized to MVC EMGRMS (N-EMGRMS). MMA was confirmed via mean arterial pressure changes (ΔMAP). Y-intercepts (y-ints) and slopes were calculated for the MUAPAMP and MFR vs. RT relationships. Seven paired t-tests compared ΔMAP, y-ints, slopes, and N-EMGRMS between conditions (α ≤ 0.05). RESULTS: There were no differences for y-ints or slopes from the MUAPₐₘₚ or MFR vs. RT relationships. However, ΔMAP (MMA = Δ21.75 ± 13.75 mm Hg, CON = Δ41.98 ± 17.73 mm Hg; p < 0.001) and N-EMGᵣₘₛ (MMA = 43.37 ± 7.76%, CON = 48.32 ± 11.91%; p = 0.015) differed between conditions. CONCLUSION: MMA at the upper arm decreased N-EMGᵣₘₛ of the VL, without altering MU recruitment or firing rate patterns. Thus, less input excitation was required to maintain the same submaximal torque during MMA, while the similar recruitment and firing rate patterns may suggest increased motorneuron excitability. Evidence is provided that MMA has a systemic effect on MU behavior.