Interval exercise refers to the basic pattern of alternating periods of more intense effort with periods of less intense efforts, or complete rest, within a single training session. HIIT (High Intensity Interval Training) generally refers to submaximal exercise protocols in which the workload elicits a relative intensity of about 80% of HRmax, whereas SIT (Sprint Interval Training) refers to protocols in which the intensity corresponds to 100% of the workload that elicits maximal oxygen uptake (VO2max).
Numerous studies over the last decade have examined physiological adaptations to HIIT, SIT and traditional Moderate Intensity Continuous Training (MICT) matched for total work or energy expenditure. While these studies have returned equivocal outcomes, several systematic reviews and meta-analyseshave concluded that both HIIT and SIT elicit superior physiological adaptations in both healthy individuals and people with lifestyle-related cardiometabolic disease.
Research in the last decade in particular has shed new light on the potency of low-volume high intensity interval training, which involves a small amount of exercise, to elicit physiological adaptations comparable to moderate intensity continuous training but that result more time-efficient compared to the latter. Other studies that have compared MICT to low volume SIT and HIIT protocols have shown similar improvements in aerobic energy metabolism, and clinical markers of health status, despite the great differences in training duration and committment.
Recent evidence supports the general consensus that exercise intensity is more important than exercise time for exercise-induced enhancement of cardiorespiratory fitness. The specific roles of exercise intensity, duration and volume on skeletal muscle remodelling, in particular mitochondrial biogenesis, are equivocal. Recent work suggests the potential for SIT to promote greater and faster mitochondrial adaptations in skeletal muscle than HIIT or MICT does, despite the greatly reduced training volume. Muscle fibre-type specific responses where incredibly similar to MICT despite the lower training volume.
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