Using the HR-V̇O2 relationship in prescribing aerobic exercise intensity: examining the true nature of the association between HR and V̇O2 during incremental exercise and its transferability to steady-state exercise

The aim of the present work was to delve deeply into the research topic of aerobic exercise intensity prescription by investigating several of its known potential pitfalls and unaddressed issues. To this end, four different studies were carried out. The first two studies focused on incremental exercise: STUDY 1 investigated the actual relationships between HR and V̇O 2 , while the influence of individual characteristics on the HRR-V̇O 2 R relationship was examined in STUDY 2. In the following two studies the issue of the transferability of the HRR-V̇O 2 R relationship derived from the incremental exercise test to constant intensity exercise was investigated in both steady-state exercise (SSE) of moderate intensity and short duration (STUDY 3) and SSE with different intensities and durations (STUDY 4). STUDY 1 showed that both the %HRR-%V̇O 2 R and %HRR-%V̇O 2max relationships were different from the identity line (i.e., y = x), which is in contrast with the literature on which the current ACSM exercise prescription guidelines are based. In addition, the intercepts and slopes of the regressions of the two relationships showed high standard deviations, which highlights high variability among individuals and can cause a substantial error when the relationship is used to predict HR and/or V̇O 2 values for a single person. STUDY 2 showed that although several subject characteristics influenced the relationship between %HRR and %V̇O 2 R, these characteristics did not explain most of the variance of the slopes and intercepts. Indeed, the prediction models of this study showed low precision and high error. Therefore, STUDY 1 and STUDY 2 show that the relationship between %HRR and %V̇O 2 R is not 1:1 as indicated by the current ACSM guidelines, and that using a single equation for the whole population does not appear to be suitable for representing the equation of a given subject and has low predictive ability, even when several confounding factors are accounted for. Hence, individual relationships between the %HRR and %V̇O 2 R are preferable when prescribing the intensity of aerobic exercise in order to avoid the potentially high error associated with using a standardized relationship for the whole population. STUDY 3 demonstrated the lack of external validity associated with using the %HRR-%V̇O 2 R relationship derived from incremental exercise for SSE. Indeed, during SSEs of relatively short duration the 1:1 %HRR-%V̇O 2 R relationship did not appear to be suitable for predicting the intensity of SSEs; hence, the individual relationship derived from a prior GXT is preferable. Nonetheless, the prediction error of both the GXT and 1:1 relationship during SSEs was relatively high and should be taken into account, particularly when exercise intensity is an important consideration from a safety standpoint. STUDY 4 showed that during running SSEs the 1:1 relationship between %HRR and %V̇O 2 R seems to be preserved when exercise duration is relatively short. On the contrary, the 1:1 relationship between %HRR and %V̇O 2 R did not appear to be preserved in SSEs of longer duration, suggesting an effect of SSE duration on the %HRR-%V̇O 2 R relationship. The effect of SSE duration on the %HRR-%V̇O 2 R relationship appeared to be influenced by SSE intensity. Indeed, during SSEs of 45 min, the higher the intensity was (i.e., 80% of HRR) the higher the difference between %HRR and %V̇O 2 R, pointing to a possible interaction effect of SSE intensity and duration. Therefore, STUDY 4 suggests that the 1:1 relationship between %HRR and %V̇O 2 R is valid during relatively short running SSEs at different intensities, whereas during SSEs of long duration the percentages of the reserve values start to dissociate, and the magnitude of the dissociation grows larger at higher exercise intensities. The present work reveals how the current literature regarding the nature of the association between HR and V̇O 2 during incremental exercise and its transferability to steady-state exercise is still inadequate and based on certain assumptions that have not yet been fully investigated. Additionally, the present work reveals that the current methods used to prescribe aerobic exercise intensity yield a relatively high error and might be inappropriate in certain circumstances.