Delving deeper into the methods used for assessing aerobic exercise intensity: New insights on the maximal metabolic steady-state concept and the transferability of incremental exercise responses to prolonged exercise

The main aim of this work was to delve deeper in the methods used for prescribing aerobic exercise intensity, investigating several of its known potential pitfalls and unaddressed issues to increase exercise prescription accuracy. To achieve this goal, we carried out four separate investigations using a combination of meta-analytical (STUDY1) and experimental (STUDY 2, STUDY 3, and STUDY 4) approaches. These investigations aimed to assess: a) whether different parameters used for prescribing exercise intensity (power output [PO], oxygen consumption [V̇O2], heart rate [HR], and rating of perceived exertion [RPE]) have an impact on aerobic exercise intensity; b) the accuracy of various methods employed to determine MMSS; and c) the transferability of the relationships found during incremental exercise to steady-state. STUDY 1 investigated the association and proximity between CP and MLSS based on external (i.e., PO) and internal (i.e., V̇O2, HR, and blood lactate concentration ([BLa-])) exercise intensity indicators and the potential influence of individuals characteristics (i.e., age and fitness level) through a meta-analytical approach. STUDY 1 showed that there is a considerable variability among individuals in the differences between CP and MLSS, indicating potential inaccuracies in estimating one parameter from the other on an individual basis. Additionally, the findings of this study showed that it is possible that the issues behind these inaccuracies and high variability found among studies could derive from a lack of standardization in the procedures used for determining both CP and MLSS. STUDY 2 aimed to determine if the boundary between heavy and severe exercise domains can be identified through a single laboratory visit using a Step-Ramp-Step protocol. Additionally, it explored how changes in pedaling cadence at 60 rpm versus 100 rpm affect the association between CP and RCP in both V̇O2 and PO. STUDY 2 suggests that the metabolic intensity at which CP and RCP occur is similar regardless of pedaling cadence. However, when considering the external load, CP tends to be around 7-10 W higher than RCP. Interestingly, both indices show a consistent change in response to variations in pedaling cadence, suggesting a mechanistic connection between CP and RCP rather than mere proximity. STUDY 3 had the goal of assessing the validity of the single visit Step-Ramp-Step protocol during treadmill running and its ability of accounting for physiological adjustments during prolonged exercises (i.e., slow components and mean response time) and predicting the metabolic stimulus induced by an external load during prolonged constant load exercise. STUDY 3 confirmed that, when prescribing prolonged aerobic exercise among healthy individuals, it is of paramount importance to account the dissociation between V̇O2 and PO or speed caused by the lag in V̇O2 (i.e., MRT). In this context, STUDY 3 pointed out that during running exercise considering the MRT is crucial. STUDY 4 investigated whether the relationships between RPE, HRR, and V̇O2R found during incremental exercise are valid during prolonged exercise; hence if RPE-based aerobic exercise intensity methods are accurate. In fact, although RPE is proposed as a valid tool for prescribing aerobic exercise, due to its association with physiological parameters during incremental exercise, the validity of these relations during prolonged exercise is still an understudied issue. STUDY 4 revealed that, when the relationships between psychological (i.e., RPE) and physiological (i.e., HR and V̇O2) parameters were analyzed, the relationships found during incremental exercise are not preserved during prolonged aerobic exercise. The findings of this work can improve the accuracy of aerobic exercise prescription and efficacy of aerobic exercise programs, by maximizing the benefits and reducing the risks associated with exercise.