HIIT Training
topic
High-Intensity Interval Training (HIIT) alternates between brief periods of near-maximal effort (85–95% VO2 max, 20 seconds to 4 minutes) and recovery periods — producing rapid improvements in VO2 max, cardiovascular efficiency, anaerobic capacity, insulin sensitivity, and mitochondrial biogenesis in less total training time than equivalent steady-state aerobic exercise, through the metabolic amplification of high post-exercise oxygen consumption (EPOC) and the intensity-dependent recruitment of fast-twitch motor units not activated by lower-intensity exercise.
Role
HIIT has been validated as the most time-efficient method of improving cardiorespiratory fitness — producing VO2 max improvements comparable to 3x longer sessions of moderate-intensity continuous training in 4–6 week protocols — making it the most practical aerobic exercise approach for people whose primary exercise barrier is time availability. However, HIIT's effectiveness has been so widely publicized that it has been commercialized into high-intensity classes pursued at frequencies (5–7 days/week) that produce overtraining syndrome, adrenal stress, sleep disruption, and injury — undermining the very health outcomes it was shown to improve at 2–3 sessions per week.