The effect of nitrate supplementation on muscle contraction in healthy adults

This study examined the effect of dietary supplementation with inorganic nitrate (NO-3) on markers of contractile function in human knee extensors. In a double-blinded, randomized cross-over design, 18 (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 NO-3) or nitrate-depleted beetroot juice (PLACEBO). On the fourth day, isometric knee extension force was assessed during a series of voluntary and electrically evoked (stimulation) tests. In addition, muscular fatigue was examined in two separate continuous-stimulation (0.8 s tetanus with a 1: 1 work: rest ratio for 102.4 s) fatigue tests, one with and one without blood flow restriction. There were no differences for maximum voluntary contraction, peak twitch force, halfrelaxation time and the force-frequency relationship for stimulations up to 100 Hz between the NITRATE and PLACEBO trials. No differences between trials were observed in the non-restricted fatigue test, however NITRATE was found to attenuate the decline in force during the restricted test, such that the force at the 80 s mark (PLACEBO: 66 +/- 11 vs. NITRATE 74 +/- 9% of initial force; P =.01) and 102 s mark (PLACEBO: 47 +/- 8% vs. NITRATE 55 +/- 8%; P <.01) were significantly higher. These results suggest that four days of NO-3 supplementation elicits peripheral responses in muscle that attenuate muscular fatigue during exhaustive exercise under hypovolemic conditions. This ergogenic action is likely attributable to improved Ca2+ handling in the muscle, or enhanced perfusion during ischemia.