What HIIT promises, in zone language
High-intensity interval training alternates short blocks of hard work with deliberate recovery. In the five-zone model, explained in full in our zones guide, the work belongs in zone 4 and, in short doses, zone 5: above 80 percent of the member's maximum. The recovery belongs in zones 1 and 2. The training effect lives in that contrast: repeated exposure to high intensity, made repeatable by real recovery. Flatten the contrast and you flatten the adaptation.
The two mistakes every screen reveals
Work intervals that never reach the zone. Members pace themselves through the "sprint" and finish the class having toured zone 3, moderately tiring, minimally adaptive, while believing they did HIIT. Heart rate lags effort by its nature, short intervals may peak just after the block ends, but across a session the pattern is unmistakable on the screen.
Recoveries that never recover. The enthusiast who jogs through every rest block arrives at interval four unable to lift intensity, and the session collapses into continuous zone 3 with pauses for branding. Coaching the recovery down is the least intuitive and most valuable instruction in interval training.
Both errors have the same cure: make the zones visible and coach the colour. "Red for thirty seconds, then get back to blue" is an instruction a mixed room can execute; effort adjectives are not, which is the standing argument of group heart rate training.
Ratios that work, by intent
1:2 and longer rests, for true intensity. Thirty seconds hard, sixty-plus easy. Protects the quality of each rep; right for sprints and for members chasing top-end power.
1:1, the workhorse. Forty seconds on, forty off, repeated in blocks. Accumulates solid zone 4 time; the default for general fitness classes.
2:1, for capacity, sparingly. Longer work than rest drives threshold hard and demands honest pacing; save it for conditioned rooms.
Guideline for beginners: fewer, longer, calmer. Deconditioned members get more from intervals touching zone 4 briefly with generous recovery than from being thrown at zone 5. Their high-intensity dose grows over weeks, on a base built with zone 2 work.
Dosage across the week
HIIT is a strong drug with a real recovery cost, part of it visible in the afterburn physiology described in our EPOC explainer. One or two genuinely hard interval sessions a week is the productive dose for most members; the rest of the week belongs to easier work. Members who HIIT daily are not training harder, they are recovering never, and their flattening zone response, plus a drifting recovery number, will say so before their motivation does. A club schedule that pairs interval formats with base sessions is quietly protecting its members and its retention curve at once.
Running the class with zones on screen
Brief the journey. Tell the room the session's shape in colours before it starts: where they should be in work, where in rest, what the finisher looks like.
Narrate the board. A coach who calls out zone discipline, including praise for the member who got back to blue fastest, is coaching recovery as a skill, which it is.
Watch device honesty. Fast intervals stress wrist optics; a member whose watch lags badly deserves a loaner strap, the trade-offs are in strap vs watch.
Let the effort score the session. Time in high zones converts into effort points, so the interval class feeds the same streaks and challenges as everything else in the club. Intensity becomes part of the game, not an island.
If you want to watch an interval class where the work is provably hard and the rest provably easy, speak to an expert.