Nitrate Supplement Benefits Contractile Forces in Fatigued but Not Unfatigued Muscle

• Published in: Medicine and science in sports and exercise 2018-10, Vol.50 (10), p.2122-2131
• Main Authors: Tillin, Neale A, Moudy, Sarah, Nourse, Kirsty M, Tyler, Christopher J
• Format: Article
• Language: English
• Subjects: Adult; Dietary Supplements; Double-Blind Method; Electromyography; Humans; Male; Muscle Contraction; Muscle Fatigue; Muscle, Skeletal - physiology; Nitrates - administration & dosage; Nitrates - blood; Nitrites - blood; Sports Nutritional Physiological Phenomena; Young Adult
• ISSN: 0195-9131; 1530-0315
• DOI: 10.1249/MSS.0000000000001655

PURPOSEEvidence 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. METHODSIn 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. RESULTSIn 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). CONCLUSIONNitrate 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.