[PowerManDL]

Originally Posted by Bytsi

I'll give you my 2cents - if you can maintain whatever you are doing for more than 30-60 seconds, it's not a full-out max effort. It may be an intense interval, but when you say you could "sprint" for 20 minutes straight, you're right - that is steady-state, not a sprint. Your definition of sprint is not the same as mine - for me, sprint in the context of HIIT would be running as fast as I possibly could, until I literally could not go another step at that pace. When I do this in my workouts, I can run for 20-30 seconds before I MUST slow down. Same on the stairmaster for me - I set it at a high enough level that I literally would fall off if I didn't slow it down after 30 seconds.

This is another area where the definitions get fuzzy: what are we talking about when we say "maximal effort"?

If you're talking pure speed/power, then you're correct, you can't keep that up for very long at all. Even elite-level 100m sprinters are slowing down by the end of the race, which is <11 seconds.

However there's also maximal anaerobic capacity, which is a little different. This is the activation and usage of glycogen for energy, and that can carry activity for upwards of 90-120 seconds before the lactate burn and lack of energy causes you to stop.

Those definitions tend to get blurred, as "maximal effort" in one is not the same as in the other.

For fat burning reasons, the second type of effort is what you're after, so you'd really want to aim for "maximal effort" in a zone from as short as 30s to as long as maybe 90s. This wouldn't be your top speed, but rather your best speed and effort in that time bracket.

I also don't think you need - or want - a long recovery time in true HIIT. What I understood of the original studies was that you want to stay anaerobic, but you can't stay anaerobic for more than approximately 60 seconds, by definition.

The original Tremblay study from 1994 that got everyone all in a rage (the one showing the 9x greater skinfold losses for less energy expenditure) about HIIT used this protocol:

Short intervals: 10-15 bouts of 15 seconds, progressing to 30s

Long intervals: 4-5 bouts of 60 increasing to 90s

Each bout was separated by a recovery period allowing HR to return to 130bpm, so it was a pretty decent recovery time.

Anaerobic metabolism can also function as purely anaerobic or oxygen-dependent, depending on length of exercise. The longer work intervals tend to be more of the latter.