HIIT (High-Intensity Interval Training)

HIIT: what is it?

The appeal of HIIT is efficiency: a hard interval session can drive central cardiovascular adaptations in fifteen or twenty minutes that would take an hour of steady work. That efficiency is real, and it is also why the format gets over-prescribed. The cost of a HIIT session is not paid in the session, it is paid in recovery, and a client doing four a week is usually accumulating fatigue faster than adaptation. The coaching skill is not writing intervals - it is deciding how many the client can absorb.

What counts as HIIT

Two conditions. The work intervals must be near-maximal, high enough that the client could not hold the effort much longer than the interval lasts. And there must be genuine recovery between them, long enough that the next interval can be performed at the same quality. Take away either one and you have a hard circuit, not HIIT - useful, but a different stimulus.

That distinction matters because most sessions marketed as HIIT are not. A twenty-minute class of burpees and mountain climbers with ten seconds of rest is continuous moderate-hard work: too hard to be recovery, not hard enough or recovered enough to be a true interval. The heart rate trace of that session is a plateau, not a series of peaks.

The protocols worth knowing

Different work durations serve different purposes. Longer intervals accumulate more time at a high percentage of VO2max; shorter ones let the client hit higher intensities with less accumulated discomfort.

Choosing the modality

Cyclical, low-skill, low-impact modalities work best: bike, rower, ski erg, incline treadmill walking or running, and hill repeats. They let the client reach a genuinely high intensity without technique or joint impact becoming the limiter, and they make the intervals repeatable, which is the whole point.

Loaded and technical movements are a poor fit for true HIIT. If a client's squat technique degrades on the third interval, the limiter is no longer cardiovascular and the session has become something else. Save barbell work for the strength part of the plan and let the conditioning be conditioning.

• Warm up properly. A cold client cannot reach the intended intensity in interval one.
• Judge the session by whether the last interval matched the first, not by how destroyed the client feels.
• Rest is prescribed, not optional. Cutting the recovery lowers the quality of the work.
• One to three HIIT sessions a week for most clients, with easy work filling the rest.

HIIT for fat loss, honestly

HIIT burns fewer calories per session than most people assume, because the sessions are short. Where it earns its place in a fat-loss plan is time efficiency and the cardiovascular fitness it builds, not raw expenditure - and the post-exercise elevation in metabolism, while real, is modest relative to the meal that follows.

The honest framing for a client is that fat loss is driven by the diet and by total daily movement, HIIT is an efficient way to get fitter in limited time, and steady work is a better way to accumulate energy expenditure without eating into recovery. Most clients do best with a mix rather than a choice between the two.