Dietary Supplementation with Inorganic Nitrate Attenuates Neuromuscular Fatigue in Skeletal Muscle

Abstract

Dietary supplementation with inorganic nitrate (NO3-) has been shown to have an ergogenic effect on exercise performance. While metabolic effects of supplementation have been well examined, the effects on muscle contractile function in vivo have been less explored. PURPOSE: This study examined the effect of dietary supplementation with NO3- on markers of contractile function during human knee extension. METHODS: In a double-blinded, randomized cross-over design, eighteen (12 M) healthy participants undertook four days of supplementation with either nitrate-rich beetroot juice (NITRATE; days 1-3: 525 mg NO3-, day 4: 1050 mg NO3-) or nitrate-depleted beetroot juice (PLACEBO). On the fourth day, isometric knee extension force was assessed by a series of voluntary and electrically evoked (stimulation) tests. In addition, muscular fatigue was examined in two separate continuous-stimulation fatigue tests (0.8 s tetanus with a 1:1 work:rest ratio for 102.4 s); one with and one without blood flow occlusion. RESULTS: There were no significant differences between the NITRATE and PLACEBO trials for muscle twitch characteristics (peak twitch force, half-relaxation time and the force-frequency relationship for stimulations up to 100 Hz) nor maximum voluntary contraction force (PLACEBO: 492 ± 141 N vs. NITRATE: 482 ± 127; P=0.41). No differences between trials were observed in the non-occluded fatigue test, however NITRATE was found to attenuate the decline in force during the occluded test, such that the force at 80 s (PLACEBO: 66 ± 11 vs. NITRATE 74 ± 9 % of initial force; P=0.01) and 102 s (PLACEBO: 47 ± 8 % vs. NITRATE 55 ± 8 %; P<0.01) was significantly higher. CONCLUSIONS: These data show that four days of NO3- supplementation results in peripheral musculature adaptations that improve fatigue resistance in hypovolemic conditions, independent of any NO3- mediated effects on blood flow or the central nervous system.