Battle between high-intensity interval training and moderate-intensity continuous training in cardiac rehabilitation practice

In a position paper in EJPC by Hansen et al.,1 authors provide detailed and interesting recommendations for cardiac rehabilitation providers concerning the design of exercise programme, one of which is that it should primarily be aimed at optimizing total energy expenditure rather than focusing on exercise intensity. However, we are still confused about how to match total energy expenditure between high-intensity interval training (HIIT) and moderate-intensity continuous training (MICT) in cardiac rehabilitation practice. Despite the priority given to total energy expenditure in this article, exercise intensity remains a key issue in prescribing exercise. Although energy supplies (aerobic and anaerobic energy) and activations of muscle fibres (type I, IIA, and IIB) during almost all exercise activities are not exclusive, there are dominant energy supplies and muscle fibre recruitments with their own attributes in different training zones divided by the first and second ventilation threshold, eliciting in corresponding physiological responses, respectively (Figure 1A).2 One of which is often overlooked that post-exercise metabolic responses to HIIT will remain elevated in an attempt to compensate for oxygen deficit driven by 'high intensity' during exercise.3 Consequently, HIIT may be underestimated despite matching total energy expenditure between HIIT and MICT with isocaloric protocols, which will potentially contribute to maximizing effect size of HIIT.