Each method has different advantages and disadvantages. For instance, using work rate can be problematic because a single velocity can represent varying metabolic demands depending on the terrain and environment, and some speeds may not be attainable in certain conditions such as high winds, steep hills, or oppressive heat.
However, using THR to gauge exercise intensity is complicated by a phenomenon known as cardiovascular drift, whereby a progressive increase in HR occurs over time despite no change in work rate. Under conditions in which cardiovascular drift occurs, work rate must be lowered to maintain THR, which can compromise the training stimulus and, subsequent adaptations Wingo, ; Morales-Palomo et al.
RPE is an appealing method of prescribing exercise intensity because it requires no equipment, allows the individual to adjust the intensity based on how the intensity of exercise is perceived, and in young healthy individuals, is directly related to HR Borg, RPE has been repeatedly shown to be a valid method to gauge exercise intensity in temperate conditions Eston and Williams, ; Edwards et al. During constant-intensity exercise in the heat, however, RPE is elevated compared to cooler environments Maw et al.
Therefore, like THR, to maintain target RPE in hot environments, work rate must be lowered to a larger extent compared to that in cooler environments Tucker et al. Even so, the magnitude by which work rate needs to be lowered to maintain target RPE in the heat appears to be less than that to maintain THR Tucker et al.
Consequently, using target RPE to gauge exercise intensity during HIIT in the heat may be advantageous compared to using THR because a higher work rate can be maintained and thereby a greater training stimulus, but this has not been evaluated. An unintended, but important consequence of this, will likely be higher core body temperature and amplified cardiovascular strain indexed as cardiovascular drift , but no study has addressed the extent to which this may occur.
