HIIT Engine: 30 on, 45 off — why exactly those numbers

The interval days are written as 30 seconds of work and 45 seconds of rest. Almost everyone who runs them changes one of those numbers, and it is nearly always the rest — usually to 30 and 30, because it sounds harder.

It is harder. It also trains something different, and inside a calorie deficit it is the wrong trade. The ratio is the prescription here, in exactly the way the weight on the bar is the prescription in a lifting session.

What the rest period is doing

A hard 30-second effort runs mostly on fuel stored inside the working muscle, and the rest period is when that store gets rebuilt. Most of it comes back in the first minute or two, and it is largely restored after a few minutes — which means the length of your rest decides how much power you can produce on the next round.

Forty-five seconds is a deliberate partial refill. Long enough that round twelve still looks like round two, short enough that fatigue accumulates across the session rather than being wiped clean each time.

It is also long enough to do something with. Forty-five seconds is enough to get your breathing back under control, reset your position on the bike or the rower, and start the next round on time rather than fumbling into it four seconds late. Treat it as a countdown you are preparing during, not a break.

Three families of ratio

Every interval protocol you have seen falls into one of a few bands, and each band trains something different. None of them is wrong; they are answers to different questions.

• Long rest, 1:4 and beyond — near-complete recovery between efforts, so every round is close to peak power. This is sprint and power training, and it produces very little total work
• Moderate rest, 1:1.5 to 1:2 — you start each round partly recovered, output stays high across the session, and hard work accumulates
• Short rest, 1:1 or less — power falls round by round and the session becomes a test of fatigue tolerance. It feels brutal and the average intensity is lower than it feels
• No meaningful rest — that is steady-state cardio with a stopwatch attached to it, and it belongs on a different day

Why 1:1.5 at a genuine nine out of ten

Do the arithmetic. Each round is 75 seconds, so a 22-minute block holds roughly seventeen of them — about eight and a half minutes of genuinely hard work, spread thinly enough that all of it can be genuinely hard.

The alternative protocols fail this test in opposite directions. Longer rests would leave you with four minutes of work in the same 22. Shorter rests would give you more work on paper and a far weaker version of it, because rounds eight through seventeen would be performed by someone who is simply exhausted.

The modality matters less than the ratio, and the best one is whichever you can go genuinely hard on without a skill or an impact cost. A bike, a rower or a hill are all excellent because you can produce maximum effort safely while tired. Sprinting on flat ground is the one to be careful with — it is where hamstrings get hurt, particularly in round fourteen.

“The rest is not the part where nothing happens. It is what decides how hard the next thirty seconds can be.”

What a nine out of ten actually feels like

It means you could not have gone meaningfully faster, and the last five seconds are ugly. Talking is out of the question. By the end of the rest you are ready but not comfortable — if you are recovered and waiting, the effort was not a nine.

The most common self-deception is running the effort at a seven and the rest at a strict 45 seconds, which produces a session that is unpleasant and not very productive. If every round feels manageable, add resistance rather than adding time.

That is also how the session progresses across the phases. You do not shorten the rest as you get fitter — you hold the ratio and raise the output, or you add a round or two. Shortening the rest changes what the session is; adding resistance keeps it the same session, done harder.

Fitting it around everything else

Interval work is the most fatiguing thing per minute in the plan, which is why it appears a couple of times a week rather than daily and why it is never scheduled the day after Lower Body Power. More sessions is not a better answer; harder sessions are, up to the point where the lifting starts to suffer.

Intervals are also demanding on the heart and lungs by design. If you are new to exercise, have a heart or lung condition, are pregnant, or take medication that affects heart rate or blood pressure, get clearance from a clinician before running maximum-effort intervals — and use the zone 2 alternative in the meantime.