Dietary nitrate supplementation positively affects cardiovascular function at rest as well as energy metabolism during exercise in humans, and has recently also been reported to markedly enhance the in-vitro contractile properties of mouse fast-twitch muscle. The aim of this study was to investigate the effects of short-term nitrate supplementation on the in-vivo contractile properties of skeletal muscle and voluntary muscle function of humans.
In a double-blind randomized crossover design, 19 healthy untrained men (21 ± 3 years) ingested nitrate-rich concentrated beetroot juice (NIT; nitrate dose: ∼9.7 mmol·day) and a placebo (PLA) for 7 consecutive days. After the last supplementation dose, force was recorded whilst participants completed a series of voluntary and involuntary (electrically evoked) unilateral isometric contractions of the knee extensors.
NIT enhanced the peak force response to low-frequency electrical stimulation: maximal twitch (NIT 149 ± 41 vs PLA 138 ± 37 N; P=0.008, effect size r [ES]=0.56); sub-maximal 1-20 Hz contractions (+5-10%; ES=0.53-0.63). Whilst explosive (rising phase) force production during the first 50 ms of evoked maximal twitch and octet (8 electrical impulses at 300 Hz) contractions was also 3-15% greater after NIT compared to PLA (P=0.023-0.048, ES=0.52-0.59), explosive voluntary force remained similar (P=0.510, ES=0.16). Maximum voluntary force was also unchanged after NIT (P=0.539, ES=0.15).
These results indicate that 7 days of dietary nitrate supplementation enhanced the in-vivo contractile properties of human skeletal muscle. Specifically, nitrate supplementation improved excitation-contraction coupling at low frequencies of stimulation and enhanced evoked explosive force production, but did not affect maximum or explosive voluntary force production in untrained individuals.
Med Sci Sports Exerc. 46(12):2234-2243.