Dr. Gibala: Lots of different ways to define things. I think there can be general agreement that when we say high-intensity, we're talking a relative heart rate of about 80% of your maximum. But there's no universal definition for high-intensity training or high-intensity interval training. But that would be a typical or average threshold.

So about 80% of max. You're working pretty hard, but it's not an all-out sprint. Again, the other "I" in there is just intermittent. So this idea of going relatively hard, backing off, and repeating that pattern a couple of times. And interval training, it's this thing that we seem to rediscover every decade or so.

You know, athletes have used it since the turn of the century, but it's certainly become popular the last decade or so or renewed popularity because many people cite time as a major barrier, some more legitimate than others, of course, if it's a real time-press issue or not.

Rhonda: Is interval training... Like when you say interval training, because you said high-intensity, you're going like 80% of your max heart rate. If you're doing... Is interval training also that as well? It's just kind of interchanged? Or is it like, do you go a little bit less than that? Yeah.

Dr. Gibala: Increasingly, I like the term interval training as a more general catch-all term. And again, that's just relatively harder effort and backing off, but it provides, I think, more a broader way of doing that.

So if you're just starting out getting into exercise, maybe you just want to start out by walking fast for a few lightposts and then backing it off. That's a form of interval training. Don't worry at all about whether you're at 80%. So I think of HIIT, high-intensity interval training, as one type or one facet of interval training as opposed to being the be-all and end-all.

Rhonda: Got it. So with high-intensity interval training, HIIT, there's... I've heard you talk about, and you've published research, you've written about it in a popular book, the health benefits of high-intensity interval training on VO2max, which I'm sure you're gonna talk all about. But recently, there's been a very...

Another form that's become very popular of training for improving VO2max, which is zone two training, as defined by a lower-intensity sort of lactate threshold training. However, that type of training does require a pretty big time commitment. I mean, anywhere between three to six hours a week. So can someone achieve similar improvements or really good improvements in VO2max from doing, let's say, 20 to 25 minutes of high-intensity interval training three to four times a week?

And if so, what... Are we leaving anything on the table if we're not doing that long-duration sort of zone two type of training?

Dr. Gibala: Yeah, so in short, I think you can do more vigorous or high-intensity exercise for shorter periods of time and at least see similar improvements in VO2max. So for the individual who is time-pressed, I don't think there's a need to do three to four hours of what I understand to be zone two training weekly in order to maximize improvements in VO2max.

In fact, there's evidence that would suggest that more vigorous intensity exercise, higher-intensity exercise can potentiate or lead to greater improvements in VO2max or eliminate what's known as non-response. So some people engage in training and it's very frustrating because their VO2max doesn't change at all.

And there's various reasons for that that we didn't talk about. But for example, some evidence has shown that moderate-intensity continuous exercise, even for six months or so, doing guideline-based evidence, roughly 40% of people don't see a measurable improvement in their VO2max. Now some of that non-response was eliminated in a group that was doing the same total amount of exercise but engaging in a more vigorous manner.

So that would seem to argue against zone two somewhat, but I think, you know, all roads lead to Rome. You know, there's many different strategies that you can engage in successfully. And I think a big thing is what do you like, right? Do you like and enjoy vigorous intermittent-type exercise? Then maybe it's for you.

If you prefer continuous lower-intensity moderate-exercise training and that's just what you like and you absolutely hate intervals, that's okay too.