The many faces of exercise intensity: a call to agree on definitions and provide standardized prescriptions

Abstract

Irrefutable evidence supports adapted physical exercise as medicine for primary and secondary prevention of many chronic diseases.1,2 A specific prescription of exercise, considering frequency, intensity, duration, type, volume as well as progression is necessary to optimally improve fitness and gain clinical benefits while minimizing risks.3 When considering endurance training, exercise intensity represents one of the most critical components for exercise prescription (Ex Rx); heart rate, cycling work rate, walking speed, and perceived effort are parameters used to prescribe specific exercise intensities. Low, moderate, or high intensity can be preferred based on training objectives, desired physiological adaptations, also evaluating individual experience with sports and exercise, health status, contraindications, and functional limitations to exercise. But how are exercise intensities currently prescribed? High quality international literature provides a surprisingly heterogeneous spectrum of ranges and definitions. Since exercise intensity represents a cornerstone component of Ex Rx, this mini-review aims to provide an overview and raise awareness regarding this clinically relevant issue in order to further address this apparently granted topic with the scientific community. When exercise is prescribed without a cardiopulmonary exercise test (CPET), different parameters have been proposed for Ex Rx, estimating peak oxygen uptake (VO2 peak), maximum heart rate (HRmax), and heart rate reserve (HRreserve), with indirect formulas based on subject’s sex, age, weight, and other parameters. A certain heterogeneity regarding these methods should be expected for the indirect nature of the measurements [e.g. the most commonly used equations for estimating HRmax: Fox et al.: 220 − age | Astrand et al.: 216.6 − (0.84 · age) | Tanaka et al.: 208 − (0.7 · age) | Gellish et al.: 207 − (0.7 · age) | Gulati et al.: 206 − (0.88 · age) | Nes et al.: 211 − (0.64 · age)]. Such an anchor measurement like HRmax and its different associated training ranges (e.g. 45–60% and 60–80%) are then used in clinical routine to determine exercise intensity for Ex Rx. Although better accuracy can be definitely achieved by directly measured HRmax, estimated methods might remain acceptable in order to respond to the high demand of Ex Rx for a large population of patients with low cardiovascular risk;4 these are considered ‘relative percentage methods’ or ‘range-based methods’. Once more, as for the aforementioned formulas, some heterogeneity arises regarding exercise intensity ranges for prescription (i.e. moderate intensity: 55–74%HRmax ESC Guidelines, 64–76% HRmax ACSM Guidelines; 40–69% HRreserve ESC Guidelines, 40–59% HRreserve ACSM Guidelines). All these range-based indirect methods are likely to fail when considering individual metabolic response and different functional limitations to exercise, since not all subjects experience similar (patho)physiological adaptations to the same relative intensity, especially when affected by several chronic diseases.5–7 Although there are different valuable direct approaches to estimate individual intensity thresholds, such as by blood lactate measurement, CPET represents also the gold standard for cardiorespiratory fitness and efficiency assessment.8 This method is less likely to fail in intensity determination by the identification of ventilatory thresholds (VTs), which reflect the individual response to exercise comprehensively evaluating the whole oxygen transport system, and leading to a ‘ventilatory threshold-based method’ for Ex Rx. Indeed, VTs represent a modification in the metabolism related to energy production due to an increase of external work rate and a subsequent higher demand in the turnover of energetic substrates.9–11 The first VT or anaerobic threshold (VT1 or AT) represents the exercise intensity corresponding to the transition from prevalently aerobic metabolism to a point where blood lactate begins to rise thereby activating the metabolic buffer system, which stimulates ventilation because of lactate-related CO2 production; the second VT or respiratory compensation point (VT2 or RCP) represents the exercise intensity at which blood lactate accumulates rapidly, resulting in an imbalanced increase in ventilation (VE) relative to CO2 elimination for a concomitant increase in H+ beyond the subject’s metabolic buffer capacity.12 The vast majority of the general population as well as most patients are able to achieve both VTs during an incremental CPET.13 Bearing in mind the limitations of different types of indirect estimation methods and the range-based methods using directly measured HR or VO2 with an ECG exercise test or CPET, the individually identified VTs have always been proposed as a feasible and pivotal method for Ex Rx.14 In fact, a ‘threshold-based’ prescription rather than a ‘range-based’ approach is recommended to improve the benefits of an individually adapted exercise training intervention in patients with cardiovascular diseases.15,16 Moreover, a threshold-based Ex Rx has been reported to yield 100% of responders on VO2 peak improvement when compared to 42–60% of responders by relying on a range-based Ex Rx.17 But how are exercise intensities defined for this threshold-based Ex Rx method and what is actually ‘moderate intensity’ according to VTs? Figure 1 visually shows the variability of intensity range definitions and Table 1 provides the current evidence from literature to this question, which may give rise to a call to agree on, and simplify definitions of intensities, thereby providing and facilitating standardized exercise intensity prescriptions. Exercise intensity definitions according to ‘ventilatory threshold-based methods’. Figure 1 shows the current heterogeneous evidence for exercise intensity definitions according to the identification of VTs with CPET. Colours are associated with exercise intensities as follow: light blue-very low intensity, green-low intensity, yellow-moderate intensity, orange-high intensity, and red-very high intensity. At the bottom our standardized proposal for clinical practice. CP, critical power; VT1, first ventilatory threshold; VT2, second ventilatory threshold. Literature overview on ‘ventilatory-threshold-based’ exercise intensity definitions The table provides the original reported text of current evidence for the definition of exercise intensities according to VTs. CP, critical power; GET, gas exchange threshold; HR, heart rate; LT1, first lactate threshold; LT2, second lactate threshold; MLSS, maximal lactate steady state; VO2, oxygen uptake; RCP, respiratory compensation point; VT1, first ventilatory threshold; VT2, second ventilatory threshold; WR, work rate. It could be agreed that the current heterogeneity regarding this pivotal methodology for Ex Rx might not be acceptable in times of precision medicine, also considering that individualized exercise therapy is an evidence-based class I recommendation (level of evidence A) for the treatment of different chronic diseases. Although CPET is an established valid and reliable diagnostic and prognostic tool, the definition of ‘low’, ‘moderate’, and ‘high’ intensity does not seem to be well defined in literature while the easily determinable VTs are only partially and differently considered for the definition of intensity ranges. In view of these remarks, the definition of VT-based exercise intensities must become consistent in literature and feasible for daily practice. Therefore, it should be aimed to reach a consensus to guide and standardize Ex Rx of endurance training to improve clinical and functional outcomes of exercise-based treatment interventions as well as scientific evidence by comparable training intensities. We thus propose a homogenous definition of exercise intensity domains following a physiological ‘threshold-based’ classification (Figure 1), which is also easy to implement in clinical practice: i.e. VT1 as the lower limit of moderate intensity and VT2 as the lower limit of high intensity. This approach is in line with recent papers on this issue and advocates a simple language and methodology when formulating and providing individualized Ex Rx. D.N., F.B., S.F., A.C., and G.Q. contributed to the conceptualization of the work. S.F. and A.C. performed the literature formal analysis. S.F., A.E., and M.V. contributed to the interpretation of data. S.F., D.N., and F.B. drafted the manuscript. All authors critically revised the manuscript, approved its final version, and agreed to be accountable for all aspects of the work ensuring integrity and accuracy. None. There are no new data associated with this article.

Topics & Concepts