Interaction of Factors Determining Critical Power

Sports Medicine https://doi.org/10.1007/s40279-022-01805-w REVIEW ARTICLE Interaction of Factors Determining Critical Power Richie P. Goulding1 · Simon Marwood2 Accepted: 16 December 2022 © The Author(s) 2023 Abstract The physiological determinants of high-intensity exercise tolerance are important for both elite human performance and morbidity, mortality and disease in clinical settings. The asymptote of the hyperbolic relation between external power and time to task failure, critical power, represents the threshold intensity above which systemic and intramuscular metabolic homeostasis can no longer be maintained. After ~ 60 years of research into the phenomenon of critical power, a clear understanding of its physiological determinants has emerged. The purpose of the present review is to critically examine this contemporary evidence in order to explain the physiological underpinnings of critical power. Evidence demonstrating that alterations in convective and diffusive oxygen delivery can impact upon critical power is first addressed. Subsequently, evidence is considered that shows that rates of muscle oxygen utilisation, inferred via the kinetics of pulmonary oxygen consumption, can influence critical power. The data reveal a clear picture that alterations in the rates of flux along every step of the oxygen transport and utilisation pathways influence critical power. It is also clear that critical power is influenced by motor unit recruitment patterns. On this basis, it is proposed that convective and diffusive oxygen delivery act in concert with muscle oxygen utilisation rates to determine the intracellular metabolic milieu and state of fatigue within the myocytes. This interacts with exercising muscle mass and motor unit recruitment patterns to ultimately determine critical power. Key Points Critical power represents the threshold intensity above which steady-state metabolism is no longer attainable, and within the last ~ 15 years, experimental data have emerged that illuminate its underpinning physiological determinants. Here, we summarise these experimental data to demonstrate that critical power is a parameter of aerobic function that is affected by alterations in the capacities of each step in the oxygen transport and utilisation pathways. * Richie P. Goulding 1 Laboratory for Myology, Faculty of Behavioural and Movement Sciences, Vrije Universiteit Amsterdam, Amsterdam Movement Sciences, O|2 Labgebouw, Vrije Universiteit, De Boelelaan 1108, 1081 HZ Amsterdam, The Netherlands 2 School of Health Sciences, Liverpool Hope University, Liverpool, UK Convective/diffusive oxygen delivery and intracellular oxygen utilisation rates interact with muscle fibre composition and motor unit recruitment profiles to determine the upper limit for steady-state exercise. Vol.:(0123456789) R. P. Goulding, S. Marwood 1 Introduction The determinants of exercise tolerance are of clear interest because of the strong relationships between exercise capacity and athletic performance [1, 2], health in the general population, and clinical outcomes in disease populations [3, 4]. Exercise intensity is, of course, a key factor that determines the tolerability of a given task. Moreover, for individuals or groups of individuals, partitioning the exercise intensity spectrum into domains where the physiological responses to a given task share common qualitative characteristics is an effective approach that can yield insight into the physiological determinants of exercise tolerance. Accordingly, the mechanisms of fatigue and determinants of exercise intolerance are not ubiquitous across the spectrum of exercise intensities [5]. However, above a particular individual-specific power output, the consistent feature of exercise intolerance (and hence, impending task failure) is the inability for pulmonary oxygen uptake (V̇ O2) and [lactate] ( L−) to attain a steady state [6–9]. Thus, for each individual, there exists a range of intensities for which a steady state in pulmonary V̇ O2 is attainable, and a range for which it is not [6, 9–12], with the duration of sustainable exercise in the latter being significantly limited compared with the former. The threshold intensity that separates these two ranges of system behaviour, and its position relative to other landmarks of aerobic function (i.e. maximal V̇ O2 [V̇ O2max] and the lactate threshold), is therefore a fundamental determinant of the ability to sustain exercise [6, 13–15]. This threshold intensity can be determined by undertaking three to five high-intensity, constant-power output cycle ergometer tests to the point of task failure on separate days. The tests should be selected to last no less than 2 and no more than 15 min in duration [16–19], with the precise time to task failure and power output at which each test is conducted recorded. These durations are recommended for a valid determination of this intensity, as it is essential that V̇ O2max is attained at the end of trial in order to meet the requirement for all prediction trials to be performed within the severe-intensity domain. When time to task failure is plotted against power output, the relationship is curvilinear, with the ability to sustain exercise falling away more rapidly at higher power outputs (Fig. 1). This power-time relationship is well described by a hyperbolic function [20], with an asymptote known as critical power (CP) and the curvature constant termed W' (i.e. W prime). This relationship is described by the following equation: T= W� , P − CP where T is the tolerable duration and P is the power output of a given exercise task [6, 20, 21]. When intensity is measured Fig. 1  Hyperbolic power-duration curve that defines the sustainable duration of exercise in the severe-intensity domain. This hyperbolic relationship is defined by two parameters: the power asymptote, known as the critical power (CP), and the curvature constant W′ (denoted by the rectangular dashed blue lines above CP and expressed in kilojoules). Critical power defines the boundary between the heavy- and severe-intensity exercise domains and represents the highest power output for which a metabolic steady state may be attained. The W' comprises a fixed and finite volume of work that is expendable above CP. During severe-intensity exercise, task failure occurs when W′ = 0. GET gas exchange threshold in units of speed, the asymptote is termed critical speed (CS) and the curvature constant D’ (i.e. with units of distance). This power-time relationship appears to be a universal feature of high-intensity exercise tolerance, being apparent in every species [22–26] and mode of exercise (with appropriate units of force, torque or velocity [15, 27–30]) in which it has been studied. This relationship can also be converted to its linear equivalents, either with work plotted against time: � W = CP ⋅ T + W , where W is work, CP is the slope and W′ is the intercept of the equation, or with power plot (...truncated)