HIIT: What It Actually Is, and What It Is Not
Intervals are a tool, not a personality. Here is how to use them two or three times a week without turning every session into a grinding middle gear.
High-intensity interval training became a marketing word somewhere around 2014, and it has been drifting ever since. A 45 minute class where you never stop moving is not HIIT. It is continuous moderate work, and it will make you tired without making you fast.
The Actual Definition
HIIT is repeated bouts of near-maximal effort, typically 20 to 90 seconds long, separated by rest long enough to let you repeat that effort at close to the same quality. The rest is not a break from the training. The rest is what makes the work possible.
The physiology is straightforward. Short, very hard efforts push you well above the intensity you can sustain, which drives adaptations in how efficiently your heart moves blood and how well your muscles handle the by-products of hard work. You cannot access that zone if you never leave the middle gear.
Three Formats That Work
- 30 on, 90 off, eight rounds. Bike, row or ski. Each 30 seconds should be genuinely hard, and round eight should be within a few percent of round one. If round eight collapses, your first round was too aggressive.
- Four by four minutes at hard-but-repeatable, three minutes easy between. Longer, less brutal, brilliant for aerobic power. This is the classic protocol used in most of the research on VO2 max improvement.
- Ten by one minute, one minute easy. The most forgiving version, and a good entry point if you are new to intervals.
The Mistakes We See Most
Cutting the rest
It feels productive and it destroys the session. Less rest means lower output, and lower output means you spent the whole workout in exactly the medium zone you were trying to escape.
Doing it five days a week
Interval training is a stressor, and stressors need to be dosed. Two or three sessions a week, plus strength and easy aerobic work, produces better results than five hard days ever will. The people who plateau hardest are almost always the people training hardest.
Choosing complex movements
Near-maximal effort degrades technique. Bikes, rowers, skis, sleds and running are ideal because they stay safe when your form gets ragged. Heavy barbells and technical gymnastics under maximal fatigue are how people get hurt.
Skipping the warm-up
You cannot go from a desk to a maximal effort. Ten minutes of progressive work costs you nothing and protects the session.
Where It Fits in a Week
A balanced week for most people at our gym looks like two strength sessions, two easy aerobic sessions, and two to three conditioning pieces of which one or two are genuinely high intensity. That distribution builds a body that is strong, has an engine, and still wants to train in March.
The pacing question sits underneath all of this, and it is the skill most people never develop. Our guide on how to pace a workout is the natural next read, and the recovery guide covers the other half of the equation.
If you would rather have someone else decide how hard today should be, that is exactly what coaching is for. Book a movement assessment and we will show you what a properly programmed week feels like.
A 2026 original rewrite of a topic first covered on Dai Manuel's archive. Coaching guidance, not medical advice.