An appropriate and individualized dose of regular aerobic exercise is recommended to promote and preserve cardiorespiratory health throughout life. The exercise dose is defined by four ingredients: frequency, intensity, time and type of exercise. Among these, intensity is the most elusive term of the exercise “prescription.” It is typically determined from an external load (such as speed, watt or pace) that elicits a desired metabolic intensity (e.g., a fraction of maximal or reserve oxygen consumption).
Whenever this approach is not feasible, heart rate (HR) is commonly used as a convenient intensity measure in both clinical practice and sport. Exercise prescriptions that use HR targets rely on the existence, and constancy over time, of a linear relationship with metabolic intensity. However, during constant-load exercise lasting more than 10 minutes, a mismatch between HR and oxygen consumption emerges over time as a result of a slow rise in HR independent of metabolism. This phenomenon has been known for years under the name of “cardiovascular drift” and, more recently, “HR slow component.” In 2018, Zuccarelli et al. suggested a practical implication of this phenomenon. They showed the use of HR targets for exercise-intensity implementation can lead to an unanticipated and undesired reduction in work rate and metabolic activation during a prolonged session rather than the desired constant training intensity. This phenomenon has been consistently observed in males, both healthy and those with obesity. However, its physiological quantification and relationship with oxygen consumption across exercise intensities and in different populations are missing, along with its physiological underpinnings.
In our study, published in the April 2022 issue of Medicine & Science in Sports & Exercise®, we tested the hypothesis that an HR increase over time also occurs in postmenopausal women, possibly with an intensity-dependent amplitude. Eighteen postmenopausal women performed several constant-work exercises at relative intensities from 40 to 80% maximal oxygen consumption on a cycle ergometer. During each test, we measured cardiorespiratory responses (HR and oxygen consumption) and body temperature over 30 minutes. This investigation demonstrated that increments in HR over time are present at all relative exercise intensities, their amplitude being larger with increasing intensity. The observed HR slow component was significantly correlated with relative intensity (r2 = 0.66) and increase in body temperature (r2 =0.52). However, its amplitude was about twice as large as that of oxygen consumption, implying a mismatch between HR and metabolic intensity that becomes larger both with intensity and over time. To deal with this mismatch, we proposed an equation that adjusts HR targets over time based on relative exercise intensity to account for the HR drift.
The take-home message is that whenever the implementation of the metabolic intensity of prolonged exercise is done using HR targets, one needs to be mindful of the mismatch between HR and metabolic intensity that emerges over time. Intensity- and time-adjusted HR targets are needed to assure that the desired stimulus is maintained throughout the exercise session in a given individual.
Massimo Teso, M.Sc., is a PhD, student in exercise physiology at the University of Verona in Italy. His research is focused on the slow component of oxygen consumption and heart rate kinetics and their implications for the exercise intensity prescription.
Silvia Pogliaghi, MD, PhD, FACSM, is an associate professor of human physiology at the University of Verona and an adjunct associate professor at the University of Western Ontario and the University of Calgary, the latter both in Canada. Specializing in sports medicine, she studies the physiological mechanisms that regulate and limit the acute and adaptive responses to aerobic exercise in physiological and pathological conditions.
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