Summary
Peter Attia breaks down the distinction between HIIT (high-intensity interval training) and SIT (sprint interval training) in this AMA episode preview. He defines HIIT as sub-maximal efforts at 80-95% max heart rate designed for the maximum aerobic training zone, while SIT is supra-maximal — exceeding VO2 max output — typically done as 10-20 second all-out efforts. Attia makes a key practical point: the only effort where you can truly go all-out is up to about 10 seconds, citing Alex Hutchinson's work in Endure. Anything beyond that involves subconscious pacing. He stresses that when someone mentions HIIT, you should ask specifically about effort level, work duration, rest duration, and number of repetitions rather than relying on the vague HIIT label.
Key Points
- HIIT is sub-maximal (80-95% max HR) for aerobic training; SIT is supra-maximal, exceeding VO2 max power output
- True all-out effort can only be sustained for about 10 seconds; anything longer involves subconscious pacing
- SIT is typically 10-20 second sprints pegged to power or speed exceeding VO2 max, not necessarily higher heart rate
- Tabata protocol (20s on, 10s off, 8 rounds) is a subset of SIT, first described in the mid-1990s
- HIIT has a "brand issue" — no uniform definition, so always ask about specific effort, duration, and rest parameters
- Max heart rate formula (220 minus age) is insufficient; actual testing via VO2 max or stress test is better
- Different interval protocols produce very different metabolic effects despite all being called "HIIT"
- Attia personally trains based on watts and RPE rather than heart rate for interval work
Key Moments
HIIT vs SIT — the critical distinction
Peter Attia defines SIT as supra-maximal efforts exceeding VO2 max output, typically 10-20 second all-out efforts, versus HIIT which is sub-maximal at 80-95% max heart rate. SIT doesn't necessarily produce higher heart rates because the short duration doesn't allow HR to peak.
"So sprint interval training in some ways I think is easier to understand because it is supra-maximal. So these are efforts that would exceed the output of your VO2 max."
You can only truly go all-out for about 10 seconds
Citing Alex Hutchinson's Endure, Attia explains that true all-out effort can only be sustained for about 10 seconds. Anything longer involves a subconscious governor. This matters when evaluating "all out" sprint protocols.
"I find this to be completely true for me, which is the only effort in which you can truly go all out is an effort up to about 10 seconds. And that might sound crazy because I'm sure there are people listening to this. You say, what are you talking about, man? I do 60 second all out bursts."
