The rest is not a break between the efforts. The rest is part of the prescription.
Intervals are repeated efforts of a single movement, each one hard, each one separated by a defined recovery. Six times four hundred metres with ninety seconds of walking between them. Five times five hundred metres on the rower with a minute off. Four rounds of four minutes hard with three minutes easy.
The rest isn't incidental. Changing ninety seconds to three minutes changes what you're training even if the work stays identical, because the recovery decides how fresh each effort starts and therefore how hard each effort can actually be. An interval prescription that doesn't state its rest isn't a prescription.
Three numbers define the session: the work, the rest, and the target intensity. Get one wrong and the other two stop meaning anything. Short work with long rest trains speed and power. Long work with short rest trains the ability to hold a hard pace. The classic middle ground, four minutes hard against three minutes easy, is a well-established way to load the aerobic ceiling and appears in our library as the Norwegian four by four.
Consistency across the set is the entire measure. Our own six by four hundred metre protocol says it plainly in its coach note: the pace should feel like something you could hold for six to eight minutes flat out, and the splits should be consistent across all rounds. Our five by five hundred metre row version puts a number on it and says the splits shouldn't fall more than three seconds from round one.
Which means the most common interval mistake is going too hard on the first rep. If round one is your best round by a distance, you didn't run six intervals. You ran one interval and five progressively worse imitations of it. The correct feeling on rep one is slight disappointment.
Intervals are expensive. They cost more recovery per minute than anything except a genuine maximal test, and the appropriate weekly dose for most people is one or two sessions on top of a base of easier aerobic work. Turning every conditioning session into intervals produces a plateau that looks like a training problem and is actually a recovery problem.
Our VO2 max interval protocol is six rounds of four hundred metres at hard effort, Zone 4 to 5 and roughly RPE 8 to 9, with ninety seconds of walking rest between rounds. Twenty minutes total including recovery. The pacing instruction is the useful part: pick a pace you could hold flat out for six to eight minutes, which for most people is somewhere near their five-kilometre race pace rather than a sprint. A well-run session looks like 1:34, 1:35, 1:35, 1:36, 1:36, 1:37. A badly-run one looks like 1:26, 1:32, 1:38, 1:44, 1:47, 1:52, which averages almost the same and is worth far less, because only the first two reps were anywhere near the intended intensity and the last three were a slow grind at a pace that trains nothing in particular. The ninety seconds is doing work too. Stretch it to three minutes and you'll hit faster splits, but you'll be training a different quality. Cut it to forty-five and the splits collapse.
6 rounds: 400m run at Zone 4-5, RPE 8-9, + 90 sec walking rest
Intervals run in one of two modes depending on how the work is prescribed, and the app picks without asking. When the effort is a duration, four minutes hard, the clock paces it and advances by itself. When the effort is a distance, four hundred metres, the app can't know when you crossed the line, so you tap at the end of each rep and the split's recorded from your tap. In both cases the rest counts itself down and rolls into the next rep automatically.
Each rep is stored as its own split, and the record keeps your best and average alongside the total. That's what makes the pacing shape visible afterwards, which is the only thing worth reading from an interval session. A total time for six by four hundred tells you almost nothing.
Alternating-modality prescriptions are handled properly. Our Norwegian four by four alternates ski erg and rower round by round, and the player shows only the modality due on the current rep rather than listing both and leaving you to work it out. For time is not a separate structure here: it normalises into intervals, since a single all-out effort is one interval with nothing after it.
The papers behind the empirical claims on this page, with the population each one studied. Where we could confirm only what a study set out to measure and not the size of what it found, the note says so rather than borrowing the paper's authority for a number nobody read.
Jan Helgerud et al. (2007). Aerobic high-intensity intervals improve VO2max more than moderate training. Medicine & Science in Sports & Exercise.
High-intensity aerobic interval training significantly increased VO2max compared with long slow distance and lactate-threshold training intensities (P<0.01). The comparison was matched for total work and frequency, which is what makes it a statement about intensity rather than volume.
Adult males, Norway. Training arms matched for total work and frequency.Repeating a single movement at a hard effort, with a defined recovery between efforts. The three numbers that define it are the work, the rest and the target intensity, and all three are part of the prescription. Changing the rest changes what the session trains even when the work stays identical.
Long enough that the last rep matches the first. For short hard efforts of 200 to 500 metres, 90 seconds to two minutes is typical. For four-minute efforts near your ceiling, around three minutes of easy movement is the established pattern. If your splits are falling, the rest is too short before the work is too hard.
One or two for most people, sitting on top of a larger volume of easy aerobic work. Intervals are the most expensive conditioning per minute you can do, and the common plateau where hard sessions stop producing progress is usually too many hard sessions and not enough easy ones rather than insufficient effort.
How many movements are inside one round. Intervals repeat a single movement with recovery between reps. A circuit puts several distinct movements inside one round and rests between rounds. That is the boundary our own classifier uses, because the rest structure and the duration can look identical across both.
Build a plan and Fire Your Coach will schedule the structure, run the clock, and record what actually happened round by round, so next time the prescription starts from your real numbers.