The effect of dietary nitrate on knee extensor peak isometric force and rate of force development in health adults

INTROUDCTION: Dietary nitrate (NO3-) has received considerable attention over the last decade with focus largely placed on the benefits for health and short-duration exercise performance. However, evidence suggests that NO3- supplementation can sever to promote the contractile properties of skeletal muscle, particularly when considering rapid and forceful actions involving fast-twitch muscle fibres. Whilst studies have investigate the benefits of NO3- for promoting muscle function, it remains that much of the focus has been placed on involuntary contractions. Further, few studies have reported the peak force, force (Fo) at specific time intervals and rate of force development (RFD). Therefore, the aim of this study was to investigate the effects of dietary NO3- on the aforementioned characteristics of the knee extensors. METHODS: Using double-blind, randomised, crossover design, 14 healthy participants completed two 5-day supplement period with either nitrate-rich (BR, ~800 mg/d) or nitrate-depleted beetroot (PLA) juice consumed. After each period, isometric knee extensor force was recorded during three maximal efforts interspersed with 20 s passive recovery. For the analysis, the raw data was inserted into a custom-spread sheet to determine maximal voluntary contraction (MVC), Fo at 50, 100, 150 and 200 ms, and RFD (∆Force/∆time). Data was analysed using effect sizes and 95% confidence intervals. These were supplemented with magnitude-based descriptors using a post-only-crossover spreadsheet. RESULTS: The magnitude of difference in MVC (BR, 964 ± 445 N, vs PLA, 919 ± 426 N) and Fo50 and RFD50 between BR and PLA was trivial (ES = 0.07 to 0.08) and considered unclear to most likely trivial. There was a small beneficial effect of BR on Fo at 100, 150 and 200 ms (ES = 0.28 – 0.35), with these findings considered possibly to likely positive. Similarly, RFD at 100, 150 and 200 ms was possibly to likely higher after BR supplementation with effect sizes considered small (ES = 0.28 – 0.35). CONCLUSION: The results of this study revealed that short-term BR supplementation has no beneficial effect on MVC and Fo50 and RFD50. Whilst results for the MVC agreed with previous research, those for Fo50 and RFD50 do not. Our results do, however, indicate that BR supplementation is beneficial for contraction of knee extensors at 100-200 ms. Such findings may have important implication for sporting actions that involved knee extensors and are executed within 100-200 ms.