Influence of nitrate supplementation on motor unit activity during recovery following a sustained ischemic contraction in recreationally active young males

Abstract

Purpose: Dietary nitrate (NO₃^-) supplementation enhances muscle blood flow and metabolic efficiency in hypoxia, however, its efficacy on neuromuscular function and specifically, the effect on motor unit (MU) activity is less clear. We investigated whether NO₃^- supplementation affected MU activity following a 3 min sustained ischemic contraction and whether this is influenced by blood flow restriction (BFR) during the recovery period.

Method: In a randomized, double-blinded, cross-over design, 14 males (mean ± SD, 25 ± 6 years) completed two trials following 5 days of supplementation with NO₃^--rich (NIT) or NO₃^--depleted (PLA) beetroot juice to modify plasma nitrite (NO₂^-) concentration (482 ± 92 vs. 198 ± 48 nmol·L^-1, p < 0.001). Intramuscular electromyography was used to assess MU potential (MUP) size (duration and area) and mean firing rates (MUFR) during a 3 min submaximal (25% MVC) isometric contraction with BFR. These variables were also assessed during a 90 s recovery period with the first half completed with, and the second half completed without, BFR.

Results: The change in MUP area and MUFR, did not differ between conditions (all p > 0.05), but NIT elicited a reduction in MUP recovery time during brief isometric contractions (p < 0.001), and during recoveries with (p = 0.002) and without (p = 0.012) BFR.

Conclusion: These novel observations improve understanding of the effects of NO₃^- on the recovery of neuromuscular function post-exercise and might have implications for recovery of muscle contractile function.

Trial registration: The study was registered on clinicaltrials.gov with ID of NCT05993715 on August 08, 2023.

Keywords: Beetroot juice; Electromyography; Motor unit; Nitric oxide; Recovery.

Fig. 1

Experimental procedure and a representative intramuscular electromyographic signal (iEMG). A Schematic of the dynamometer, muscle contraction procedure, blood sample measurements. B An iEMG signal (above) and force tracing (below) from a participant during a sustained isometric contraction at 25% MVC. C A single MUP extracted from the iEMG signal shown in B and several overlaid MUPs (shimmer plot) extracted across multiple MU firings, respectively. Black boxes: brief muscle contractions; boxes with gradient fill: rests between 6 × 20 s contractions; dark grey boxes: contractions with blood flow restriction (BFR); white box: contraction after releasing BFR

Fig. 2

Motor unit potential (MUP) duration (A), MUP area (B), and MU firing rate (C) during the brief isometric contractions (BC), after the BFR_3min contraction, and after brief recoveries with (Rec 1) and without (Rec 2) BFR for the nitrate (NIT) and placebo (PLA) cohorts (n = 14). Data are mean ± SD. #Main effect of supplement p < 0.05. *Main effect of study time point, p < 0.05