Interval exercise refers to the basic pattern of alternating periods of more intense effort with period of less intense effort, or complete rest, within a single training session. Coaches and athletes have employed the practice since the early 20th century, and English language publications on physiological adaptations to interval training date back to the 1960s1. The wide variety of terms used to describe this basic type of exercise has led to a dizzying array of acronyms and a lack of consistency in the literature. In an effort to standardize terminology, a classification scheme was recently proposed to delineate “high intensity interval training” (HIIT) from “sprint interval training” (SIT)2. HIIT generally refers to submaximal exercise protocols in which the workload elicits a relative intensity of ≥80% of peak heart rate. SIT describes protocols in which the intensity corresponds to ≥100% of the workload that elicits maximal oxygen uptake (VO2max). It has long been appreciated that both HIIT and SIT elicit physiological adaptations that resemble, and indeed can be superior to, changes normally associated with traditional moderate-intensity continuous training (MICT)3,4. Studies that have directly compared MICT to HIIT protocols matched for total work or energy expenditure, as summarized in several recent systematic reviews and meta-analyses2,5,6, have generally concluded that interval training elicits superior physiological adaptations in both average healthy individuals and people with lifestyle-induced cardiometabolic disease. Research over the last decade in particular has shed new light on the potency of low-volume interval training, which involves a relatively small total amount of exercise, to elicit physiological adaptations that are comparable to MICT in a time-efficient manner7. Studies that have directly compared MICT to low-volume HIIT or SIT protocols have reported similar improvements in markers of aerobic energy metabolism, as well as clinical indices of health status, despite large differences in total exercise and training time commitment. These findings are noteworthy given that lack of time is the most cited barrier to regular physical activity; however, while the efficacy of interval training is increasingly recognized, it’s effectiveness and potential impact on public health remains controversial8. Recent evidence supports the general contention that exercise intensity is more important than duration for training-induced increases in cardiorespiratory fitness9. In contrast, the specific roles of intensity, duration and volume on aspects of exercise-induced skeletal muscle remodelling, in particular mitochondrial biogenesis, are equivocal10. Recent work suggests that SIT promotes greater and faster mitochondrial adaptations in skeletal muscle of moderately trained men than does HIIT and MICT despite a much lower training volume11. With respect to highly trained endurance athletes, interval training has long been considered an essential component of programs designed to maximize performance, although the underlying mechanisms are likely different compared to less trained individuals12. Inserting a short period of HIIT or SIT for up to several weeks, either by replacing a portion of usual training or in combination with an overall reduction in total training volume, can further enhance performance in highly trained individuals. It has been proposed that a polarized approach, in which ~75% of total training volume be performed at low intensities, with ~10-15% performed at very high intensities, may be the optimal training intensity distribution for elite athletes who compete in endurance events13.