Nitrate Supplement Benefits Contractile Forces in Fatigued but Not Unfatigued Muscle

Purpose Evidence suggests dietary nitrate supplementation enhances low-frequency (≤20 Hz) involuntary, but not voluntary, forces in unfatigued human muscle. We investigated the hypotheses that nitrate supplementation would also attenuate low-frequency fatigue and the loss of explosive-voluntary forces in fatigued conditions. Methods In a counterbalanced double-blinded order, 17 male participants completed two experimental trials following 7 days of dietary supplementation with either nitrate-rich (NIT) or nitrate-depleted (PLA) beetroot juice. Each trial consisted of measuring isometric knee-extension forces during a series of explosive-maximal voluntary contractions (MVCs) and involuntary-tetanic contractions (at 10, 20 50, and 100 Hz) in unfatigued conditions, followed by a fatigue protocol of 60 MVCs and a repeat of the tetanic contractions immediately post the 60 MVCs. Results In unfatigued conditions, there was no effect of NIT on any of the measured dependent variables; including maximal voluntary force, explosive-impulse, and tetanic peak forces or peak rate of force developments (RFDs) at any frequency. In contrast, the percentage decline in explosive-voluntary impulse from the first to the last 6 MVCs in the fatigue protocol was lower in NIT (51.1 ± 13.9%) than PLA (57.3 ± 12.4%; P=0.039; d=0.51). Furthermore, low-frequency fatigue determined via the percentage decline in the 20:50 Hz ratio was attenuated in NIT, for tetanic peak force (NIT, 12.3 ± 12.0% vs. PLA, 17.0 ± 10.1%; P=0.110; d=0.46), and tetanic peak RFD (NIT, 12.3 ± 10.4% vs. PLA, 20.3 ± 9.5%; P=0.011; d=0.83). Conclusion Nitrate supplementation reduced the decline in explosive-voluntary forces during a fatiguing protocol, and attenuated low-frequency fatigue, likely due to reduced disruption of excitation-contraction coupling. However, contrary to previous findings, nitrate supplementation had no effect on contractile performance in unfatigued conditions. Correspondence: Neale A. Tillin, University of Roehampton, Whitelands College, Holybourne Avenue, London, UK, SW5 4JD, Email: neale.tillin@roehampton.ac.uk This work was funded by the University of Roehampton. Conflict of Interest: The authors have no professional relationships with companies or manufacturers that may benefit from the results of the present study. The results of the present study do not constitute endorsement by ACSM. The Authors declare the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. Accepted for Publication: 24 April 2018 © 2018 American College of Sports Medicine

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