Pharmacological sodium nitrate supplementation has been reported to reduce the O₂cost of submaximal exercise in humans. In this study, we hypothesized that dietary supplementation with inorganic nitrate in the form of beetroot juice (BR) would reduce the O₂cost of submaximal exercise and enhance the tolerance to high-intensity exercise. In a double-blind, placebo (PL)-controlled, crossover study, eight men (aged 19–38 yr) consumed 500 ml/day of either BR (containing 11.2 ± 0.6 mM of nitrate) or blackcurrant cordial (as a PL, with negligible nitrate content) for 6 consecutive days and completed a series of “step” moderate-intensity and severe-intensity exercise tests on the last 3 days. On days 4–6, plasma nitrite concentration was significantly greater following dietary nitrate supplementation compared with PL (BR: 273 ± 44 vs. PL: 140 ± 50 nM; P < 0.05), and systolic blood pressure was significantly reduced (BR: 124 ± 2 vs. PL: 132 ± 5 mmHg; P < 0.01). During moderate exercise, nitrate supplementation reduced muscle fractional O₂extraction (as estimated using near-infrared spectroscopy). The gain of the increase in pulmonary O₂uptake following the onset of moderate exercise was reduced by 19% in the BR condition (BR: 8.6 ± 0.7 vs. PL: 10.8 ± 1.6 ml·min⁻¹·W⁻¹; P < 0.05). During severe exercise, the O₂uptake slow component was reduced (BR: 0.57 ± 0.20 vs. PL: 0.74 ± 0.24 l/min; P < 0.05), and the time-to-exhaustion was extended (BR: 675 ± 203 vs. PL: 583 ± 145 s; P < 0.05). The reduced O₂cost of exercise following increased dietary nitrate intake has important implications for our understanding of the factors that regulate mitochondrial respiration and muscle contractile energetics in humans.

For high-intensity muscular exercise, the time-to-exhaustion (t) increases as a predictable and hyperbolic function of decreasing power (P) or velocity (V ). This relationship is highly conserved across diverse species and different modes of exercise and is well described by two parameters: the "critical power" (CP or CV), which is the asymptote for power or velocity, and the curvature constant (W') of the relationship such that t = W'/(P - CP). CP represents the highest rate of energy transduction (oxidative ATP production, V˙O2) that can be sustained without continuously drawing on the energy store W' (composed in part of anaerobic energy sources and expressed in kilojoules). The limit of tolerance (time t) occurs when W' is depleted. The CP concept constitutes a practical framework in which to explore mechanisms of fatigue and help resolve crucial questions regarding the plasticity of exercise performance and muscular systems physiology. This brief review presents the practical and theoretical foundations for the CP concept, explores rigorous alternative mathematical approaches, and highlights exciting new evidence regarding its mechanistic bases and its broad applicability to human athletic performance.

We tested the hypothesis that the asymptote of the hyperbolic relationship between work rate and time to exhaustion during muscular exercise, the “critical power” (CP), represents the highest constant work rate that can be sustained without a progressive loss of homeostasis [as assessed using³¹P magnetic resonance spectroscopy (MRS) measurements of muscle metabolites]. Six healthy male subjects initially completed single-leg knee-extension exercise at three to four different constant work rates to the limit of tolerance (range 3–18 min) for estimation of the CP (mean ± SD, 20 ± 2 W). Subsequently, the subjects exercised at work rates 10% below CP (CP) for as long as possible, while the metabolic responses in the contracting quadriceps muscle, i.e., phosphorylcreatine concentration ([PCr]), P_iconcentration ([P_i]), and pH, were estimated using³¹P-MRS. All subjects completed 20 min of CP exercise was 14.7 ± 7.1 min. During CP exercise, however, [PCr] continued to fall to the point of exhaustion and [P_i] and pH changed precipitously to values that are typically observed at the termination of high-intensity exhaustive exercise (end-exercise values = 26 ± 16% of baseline [PCr], 564 ± 167% of baseline [P_i], and pH 6.87 ± 0.10, all P < 0.05 vs. <CP exercise). These data support the hypothesis that the CP represents the highest constant work rate that can be sustained without a progressive depletion of muscle high-energy phosphates and a rapid accumulation of metabolites (i.e., H⁺concentration and [P_i]), which have been associated with the fatigue process.

Dietary supplementation with beetroot juice (BR), containing approximately 5–8 mmol inorganic nitrate (NO₃⁻), increases plasma nitrite concentration ([NO₂⁻]), reduces blood pressure, and may positively influence the physiological responses to exercise. However, the dose-response relationship between the volume of BR ingested and the physiological effects invoked has not been investigated. In a balanced crossover design, 10 healthy men ingested 70, 140, or 280 ml concentrated BR (containing 4.2, 8.4, and 16.8 mmol NO₃⁻, respectively) or no supplement to establish the effects of BR on resting plasma [NO₃⁻] and [NO₂⁻] over 24 h. Subsequently, on six separate occasions, 10 subjects completed moderate-intensity and severe-intensity cycle exercise tests, 2.5 h postingestion of 70, 140, and 280 ml BR or NO₃⁻-depleted BR as placebo (PL). Following acute BR ingestion, plasma [NO₂⁻] increased in a dose-dependent manner, with the peak changes occurring at approximately 2–3 h. Compared with PL, 70 ml BR did not alter the physiological responses to exercise. However, 140 and 280 ml BR reduced the steady-state oxygen (O₂) uptake during moderate-intensity exercise by 1.7% ( P = 0.06) and 3.0% ( P < 0.05), whereas time-to-task failure was extended by 14% and 12% (both P < 0.05), respectively, compared with PL. The results indicate that whereas plasma [NO₂⁻] and the O₂ cost of moderate-intensity exercise are altered dose dependently with NO₃⁻-rich BR, there is no additional improvement in exercise tolerance after ingesting BR containing 16.8 compared with 8.4 mmol NO₃⁻. These findings have important implications for the use of BR to enhance cardiovascular health and exercise performance in young adults.

It is well known that physiological variables such as maximal oxygen uptake ( ), exercise economy, the lactate threshold, and critical power are highly correlated with endurance exercise performance. In this review, we explore the basis for these relationships by explaining the influence of these “traditional” variables on the dynamic profiles of the response to exercise of different intensities, and how these differences in dynamics are related to exercise tolerance and fatigue. The existence of a “slow component” of during exercise above the lactate threshold reduces exercise efficiency and mandates a greater consumption of endogenous fuel stores (chiefly muscle glycogen) for muscle respiration. For higher exercise intensities (above critical power), steady states in blood acid–base status and pulmonary gas exchange are not attainable and will increase with time until is reached. Here, we show that it is the interaction of the slow component, , and the “anaerobic capacity” that determines the exercise tolerance. Essentially, we take the view that an appreciation of the various exercise intensity “domains” and their characteristic effects on dynamics can be helpful in improving our understanding of the determinants of exercise tolerance and the limitations to endurance sports performance. The reciprocal effects of interventions such as training, prior exercise, and manipulations of muscle oxygen availability on aspects of kinetics and exercise tolerance are consistent with this view.

Dietary nitrate is growing in popularity as a sports nutrition supplement. This article reviews the evidence base for the potential of inorganic nitrate to enhance sports and exercise performance. Inorganic nitrate is present in numerous foodstuffs and is abundant in green leafy vegetables and beetroot. Following ingestion, nitrate is converted in the body to nitrite and stored and circulated in the blood. In conditions of low oxygen availability, nitrite can be converted into nitric oxide, which is known to play a number of important roles in vascular and metabolic control. Dietary nitrate supplementation increases plasma nitrite concentration and reduces resting blood pressure. Intriguingly, nitrate supplementation also reduces the oxygen cost of submaximal exercise and can, in some circumstances, enhance exercise tolerance and performance. The mechanisms that may be responsible for these effects are reviewed and practical guidelines for safe and efficacious dietary nitrate supplementation are provided.