Abstract
The present study was designed to provide further insight into the mechanistic basis for the improved exercise tolerance following dietary nitrate supplementation. In a randomized, double-blind, crossover design, twelve recreationally active males completed a dynamic time-to-exhaustion test of the knee extensors after 5 days of consuming both nitrate-rich (NITRATE) and nitrate-depleted beetroot juice (PLACEBO). Participants who improved their time-to-exhaustion following NITRATE performed a time-matched trial corresponding to the PLACEBO exercise duration with another 5 days of dietary nitrate supplementation. This procedure was performed to obtain time-matched exercise trials with (NITRATEtm) and without dietary nitrate supplementation (PLACEBO). Neuromuscular tests were performed before and after each time-matched condition. Muscle fatigue was quantified as percentage change in maximal voluntary torque from pre- to post-exercise (ΔMVT). Changes in voluntary activation (ΔVA) and quadriceps twitch torque (ΔPS100) were used to quantify central and peripheral factors of muscle fatigue, respectively. Muscle oxygen saturation, quadriceps muscle activity as well as perceptual data (i.e., perception of effort and leg muscle pain) were recorded during exercise. Time-to-exhaustion was improved with NITRATE (12:41 ± 07:18 min) compared to PLACEBO (09:03 ± 04:18 min; P = 0.010). NITRATEtm resulted in both lower ΔMVT and ΔPS100 compared to PLACEBO (P = 0.002; P = 0.001, respectively). ΔVA was not different between conditions (P = 0.308). NITRATEtm resulted in reduced perception of effort and leg muscle pain. Our findings extend the mechanistic basis for the improved exercise tolerance by showing that dietary nitrate supplementation (i) attenuated the development of muscle fatigue by reducing the exercise-induced impairments in contractile muscle function; and (ii) lowered the perception of both effort and leg muscle pain during exercise.
Highlights
The capacity to maintain physical activity is crucial for endurance athletes, but is at least as important for the general population since it has been shown that poor aerobic fitness is linked to cardiovascular disease and overall mortality (Myers et al, 2002; Kodama et al, 2009)
The present findings extend the mechanistic basis for the improved exercise tolerance following dietary nitrate supplementation by investigating key-determinants of endurance performance
For the first time, that 5 days of dietary nitrate supplementation were associated with reduced levels of muscle fatigue compared to the time-matched placebo condition
Summary
The capacity to maintain physical activity (i.e., exercise tolerance) is crucial for endurance athletes, but is at least as important for the general population since it has been shown that poor aerobic fitness is linked to cardiovascular disease and overall mortality (Myers et al, 2002; Kodama et al, 2009). Exercise tolerance is determined by various physiological [e.g., cardiovascular, respiratory, metabolic, and neuromuscular mechanisms (McKenna and Hargreaves, 2007)] and psychological factors [e.g., external motivation, mental fatigue (McCormick et al, 2015)]. Increased dietary nitrate intake has been shown to improve vascular (Ferguson et al, 2013), metabolic (Bailey et al, 2010), and skeletal muscle function in response to exercise (Hernández et al, 2012). Effort perception and muscle pain are considered as important factors that determine exercise tolerance (Noakes, 2008; Marcora and Staiano, 2010; Mauger, 2014)
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