Effect of nitrate supplementation on skeletal muscle motor unit activity during isometric blood flow restriction exercise

Background Nitrate (NO₃^-) supplementation has been reported to lower motor unit (MU) firing rate (MUFR) during dynamic resistance exercise, however its impact on MU activity during isometric and ischemic exercise is unknown.

Purpose To assess the effect of NO₃^- supplementation on knee extensor MU activities during brief isometric contractions and a 3-min sustained contraction with blood flow restriction (BFR).

Methods Sixteen healthy active young adults (six females) completed two trials in a randomized, double-blind, crossover design. Trials were preceded by 5 days of either NO₃^- (NIT) or placebo (PLA) supplementation. Intramuscular electromyography was used to determine the m.vastus lateralis MU potential (MUP) size, MUFR and near fibre (NF) jiggle (a measure of neuromuscular stability) during brief (20 s) isometric contractions at 25% maximal strength and throughout a 3 min sustained BFR isometric contraction.

Results Plasma nitrite (NO₂^-) concentration was elevated after NIT compared to PLA (475±93 vs.198±46 nmol·L^-1, p<0.001). While changes in MUP area, NF jiggle and MUFR were similar between NIT and PLA trials (all p>0.05), MUP duration was shorter with NIT compared to PLA during brief isometric contractions and the sustained ischemic contraction (p<0.01). Additionally, mean MUP duration, MUP area and NF jiggle increased, and MUFR decreased over the 3 min sustained BFR isometric contraction for both conditions (all p<0.05).

Conclusion These findings provide insight into the effect of NO₃^- supplementation on MUP properties and reveal shorter MUP duration after short-term NO₃^- supplementation which may have potential positive implications for skeletal muscle contractile performance.