HIIT (High-Intensity Interval Training) = short bouts of high-intensity work (≥85% maxHR) alternated with short low-intensity recovery, repeated in cycles.
Classic protocols:
•Tabata 1996 (*Med Sci Sports Exerc*): 20 s all-out + 10 s rest × 8 rounds = 4 minutes, originally designed for Olympic speed skaters•Norwegian 4×4: 4 min at 90-95% maxHR + 3 min easy, × 4 rounds•30-15 intervals: 30 s high + 15 s moderate•Sprint Interval Training (SIT): 30 s all-out + 4 min rest × 4-6 rounds
HIIT's selling points:
•VO2max gains comparable to — or slightly larger than — steady-state (*Milanovic 2015* meta-analysis, 28 controlled trials, 723 participants)•Usually shorter per session — but note that this meta-analysis never compared training duration, so no specific time saving can be attributed to it•EPOC (excess post-exercise oxygen consumption) is slightly higher, but the absolute amount is dramatically overstated (the difference is less than 100 kcal per session)