Nitrate Supplement Benefits Contractile Forces in Fatigued but Not Unfatigued Muscle

TY - JOUR

T1 - Nitrate Supplement Benefits Contractile Forces in Fatigued but Not Unfatigued Muscle

AU - Tillin, Neale

AU - Moudy, Sarah

AU - Nourse, Kirsty

AU - Tyler, Christopher

PY - 2018/10/1

Y1 - 2018/10/1

N2 - 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. © 2018, American College of Sports Medicine. This is an author produced version of a paper published in MEDICINE & SCIENCE IN SPORT & EXERCISE uploaded in accordance with the publisher’s self- archiving policy. The final published version (version of record) is available online at the link. Some minor differences between this version and the final published version may remain. We suggest you refer to the final published version should you wish to cite from it.

AB - 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. © 2018, American College of Sports Medicine. This is an author produced version of a paper published in MEDICINE & SCIENCE IN SPORT & EXERCISE uploaded in accordance with the publisher’s self- archiving policy. The final published version (version of record) is available online at the link. Some minor differences between this version and the final published version may remain. We suggest you refer to the final published version should you wish to cite from it.

KW - low-frequency fatigue

KW - rate of force development

KW - force-frequency relationship

KW - beetroot juice

KW - excitation-contraction coupling

KW - explosive strength

U2 - 10.1249/MSS.0000000000001655

DO - 10.1249/MSS.0000000000001655

M3 - Article

SN - 0195-9131

VL - 50

SP - 2122

EP - 2131

JO - Medicine & Science in Sport & Exercise

JF - Medicine & Science in Sport & Exercise

IS - 10

ER -