Effective targeting of specific training outcomes requires the careful selection and control of training characteristics. This foundational principle of training science has driven exercise physiologists and sport scientists to continually evaluate and refine methods for prescribing and monitoring exercise intensities, with the goal of balancing safety and efficacy. While these challenges apply to various forms of exercise, including high-intensity interval training, this editorial focuses on continuous endurance exercise. Previous research on this topic has primarily aimed at optimizing univariate, unidimensional approaches to exercise intensity prescription. Efforts have centered on identifying the most appropriate type and value of the exercise intensity reference for various applications, ranging from traditional physiological anchors (e.g., maximal oxygen consumption) to more advanced threshold-based methods and subjective measures (e.g., rate of perceived exertion) (Marcora 2009; Mann et al. 2013). In other words, researchers have sought a single dimension of exercise-induced strain that is both valid and reliable, while also practically usable (e.g., Faude et al. 2009). However, exercise intensity is a complex, multivariable construct, encompassing various dimensions, such as cardiorespiratory, metabolic, and neuromuscular strain, as well as subjective exertion. A univariate approach to prescribing exercise intensity relies on the assumption, often implicit, that the relationships between these dimensions remain fairly constant across individuals. This assumption contrasts with the considerable inter-individual variability in the relationship between different dimensions of exercise-induced strain, which has been consistently observed in prior research (Scharhag-Rosenberger et al. 2010; Egger et al. 2016). Consequently, standardizing one dimension of exercise intensity [e.g., cardiocirculatory strain by measures like the percentage of peak or reserve heart rate (HR)] cannot be expected to standardize exercise intensity across the other dimensions (Katch et al. 1978; Pettitt et al. 2008). Furthermore, drift effects from continuous exercise complicate the relationship between various dimensions of exercise intensity. Prescriptions that are equivalent at the beginning of exercise may not remain so over time. Specifically for HR-derived targets, the ‘slow component’ phenomenon—where HR gradually drifts upward during continuous exercise at a constant power output (Zuccarelli et al. 2018)—further complicates the standardization of other measures of exercise intensity, such as oxygen consumption (V̇O₂).

In this issue of the European Journal of Applied Physiology, Mitchinson and colleagues (2024) extend the discussion on exercise intensity prescriptions by evaluating the HR-clamp model for prescribing continuous cycling exercise within the vigorous-intensity range, as defined by the American College of Sports Medicine (Garber et al. 2011). The study focuses on the acute exercise response, specifically VO2, during three HR-clamp trials performed to exhaustion (up to 60 min) at the lower, middle, and upper limits of this intensity range. The findings reveal substantial discrepancies between HR, V̇O2, and power output when expressed as percentages of their respective peak values. This finding reinforces the evidence that HR cannot be used as a valid proxy for targeting adaptation stimuli across the various dimensions of exercise-induced strain during sustained vigorous exercise. Although the initial cardiorespiratory responses for the lower and middle HR targets generally aligned with current guidelines for vigorous-intensity exercise, rapid reductions in power output led to declines in V̇O₂. By the end of exercise, the lower HR target notably fell below the recommended range. These reductions in power output reflect physiological adjustments to maintain HR within target ranges. The authors associate these adjustments with inefficiencies at the muscle level, evidenced by an increasing V̇O2 cost per Watt over time. However, they also identify HR drift—likely due to exercise-related reductions in stroke volume and mean arterial pressure—as the primary driver of power output alterations. These findings are consistent with the concept of a slow component for HR kinetics, raising questions about the actual cardiorespiratory stimulus provided by the HR-clamp exercise and the more general implications of this intensity prescription method for adaptive responses. The study also identifies specific challenges with the lower and higher HR targets when applied across individuals. The lower target failed to elicit the expected V̇O₂ response in 40% of participants, whereas the higher target proved unsustainable for at least 20 min in all but two participants. Although the middle HR target, on average, approached the desired V̇O₂ range for the intended minimum duration, substantial variability was observed within the sample—only approximately 60% of participants remained within the recommended range for at least 20 min.

Taken together, the findings of Mitchinson and colleagues contribute to the growing body of evidence that current approaches to exercise intensity prescription fail to standardize the acute exercise response across dimensions. This limitation not only hinders the ability to standardize the stimulus for adaptation but also raises safety concerns, particularly for vulnerable populations. These findings serve as a call to action for researchers, clinicians, and practitioners to reconsider the metrics and models used to prescribe and monitor exercise intensity as part of a complex, dynamic construct. In particular, the dynamic interplay between the dimensions of exercise-induced strain during continuous exercise highlights the need for a more holistic perspective on training characteristics (Fig. 1). Future research should focus on developing refined exercise prescription models that embrace this complexity. Such models may benefit from incorporating dynamic, individualized assessments of physiological—and potentially perceptual—responses to exercise, with the ultimate goal of targeting specific adaptations while minimizing risk, maximizing adherence, and avoiding harm. In this context, we propose that the prescription for an individual exercise session should be appraised as a comprehensive package encompassing at least intensity, duration, and mode.

Intensity as a complex construct in endurance exercise prescription