Two formats worth knowing
AMRAP (As Many Rounds/Reps As Possible): set a fixed time, 10 to 20 minutes, and complete as many full rounds of a given exercise sequence as you can in that window, resting only when needed. Progress is measured in rounds completed, which makes it easy to track improvement session to session without changing the workout itself.
EMOM (Every Minute On the Minute): perform a set number of reps at the start of every minute, then rest for whatever time remains in that minute. As you get fitter, the rest portion grows since the work portion takes less time, which is the built-in progression mechanism.
Both formats are a direct application of progressive overload, increasing demand over time, without needing to invent a harder exercise every week. The demand increases because you're either doing more work in the same window or the same work faster.
An AMRAP built from the existing programs
12-minute AMRAP, one round of: 10 burpees, 15 bodyweight squats, 10 push-ups, 20 mountain climbers (10 each side). Count total rounds completed. This reuses exercises from the beginner and HIIT circuits, just in a format that rewards pacing and work capacity instead of a fixed round count.
Adding load without losing the circuit format
- Weighted vest for bodyweight moves. Squats, lunges, and push-ups all get harder without changing the movement pattern, see the equipment guide for what to buy first.
- Kettlebell swings or goblet squats swapped in for a bodyweight station. Replace one circuit station rather than adding a separate lifting block, keeps the session's circuit character intact.
- Tempo changes. A 3-second lowering phase on push-ups or squats increases time under tension without any equipment at all, useful if you want more difficulty with zero added cost.
What the interval research actually specifies
ACSM's published HIIT guidance defines the work interval as lasting 15 seconds to 4 minutes at 80 to 95% of max heart rate, with recovery equal to or slightly longer than the work interval at 40 to 50% of max heart rate, the pair repeated 6 to 10 times per session. That's a range, not a fixed protocol, which is why a 20-second Tabata-style block and a 60-second-on, 60-second-off EMOM both count as legitimate HIIT despite looking different on paper: both keep the work interval in the 80 to 95% zone and the rest short enough that the next interval starts before full recovery.
Total HIIT session time in the research, work and rest combined, typically runs 10 to 40 minutes. The 12-minute AMRAP above sits at the shorter, higher-intensity end of that range, which is consistent with using it as a finisher rather than a full session on its own.