HIIT (High-Intensity Interval Training)

HIIT is interval training in which the work bouts are performed at a "near-maximal" intensity — roughly 80% to 100% of maximal heart rate (HRmax) or maximal aerobic capacity (VO2max) — interspersed with recovery periods of lower-intensity activity or rest. The work bouts are hard but submaximal-to-maximal (not the supramaximal all-out efforts of sprint interval training; see sit).

Work intervals commonly run from ~30 seconds up to ~4 minutes, with recovery scaled to allow repetition. Two well-studied formats:

• Norwegian 4×4: four 4-minute intervals at a hard, near-maximal intensity (≈90–95% HRmax / maximal aerobic speed), each followed by ~3 minutes of active recovery. This is a classic VO2max-builder (see vo2max-intervals).
• Shorter running intervals: e.g., ~140 s of work with ~165 s of recovery, repeated.

A typical prescription is about 3 sessions per week. HIIT improves VO2max at least as effectively as moderate continuous training and often does so in less total time, which is its main selling point. It also improves insulin sensitivity, blood pressure, and other cardiometabolic markers.

Cautions: the high intensity raises cardiovascular and musculoskeletal demand, so HIIT warrants a proper warm-up (see warm-up-cool-down), conservative progression, and screening for higher-risk individuals. Because it is taxing, it should be a minority of weekly volume in most programs (see training-intensity-distribution); piling on too much high-intensity work is a fast route to overreaching (see under-recovery-signs). The post-exercise calorie "afterburn" (EPOC) from HIIT is real but modest — do not overstate it (see epoc-afterburn).

• Work bouts at ~80–100% HRmax / VO2max ("near-maximal"); recovery between bouts.
• Work intervals commonly ~30 s to ~4 min.
• Norwegian 4×4: 4 × 4 min hard with ~3 min active recovery.
• Common dose: ~3 sessions/week.
• Matches or beats MICT for VO2max gains, usually in less time; improves cardiometabolic health.
• Should be a minority of weekly volume; needs warm-up and sensible progression.

• interval-training — HIIT is the near-maximal sub-type.
• sit — the supramaximal (all-out) cousin.
• mict — the moderate continuous comparison.
• vo2max-intervals — long hard intervals (e.g., 4×4) specifically for VO2max.
• epoc-afterburn — the modest post-HIIT calorie bump.
• under-recovery-signs — risk of over-dosing high intensity.