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The usual way of explaining VO2 max, the canonical measure of aerobic fitness, is that it’s a function of how quickly you can pump oxygen-rich blood to your muscles. You need lots of blood to carry the oxygen, and a big strong heart to pump it out. And it’s true that endurance training, over time, tends to increase the total amount of blood circulating in your body, and the amount that your heart can pump with each stroke.
That’s only half the story, though. Back in 1870, a German physician named Adolf Fick explained what became known as the Fick principle, which basically says that the amount of oxygen your body uses is the amount your heart pumps out minus the amount that returns to the heart unused. Your muscles may be screaming for oxygen, but if they can’t extract and metabolize it before the blood rushes past, then pumping faster won’t help. That means there’s a whole other set of adaptations that determine your fitness, like the density of the network of capillaries that seep blood into your muscles and the quantity and efficiency of the mitochondria that fuel contractions in your muscle cells.
A new review in Sports Medicine, from Michael Rosenblat of Simon Fraser University and colleagues at the University of Toronto and Monash University, digs into this distinction as it applies to interval training. Does every training regimen that boosts your VO2 max trigger roughly the same changes in your body, or are there different—and perhaps complementary—mechanisms with different workout styles?
Rosenblat’s review pools the results of 32 studies that compare two particular styles of interval training. One is sprint interval training, or SIT, which involves all-out sprints typically lasting 30 seconds or less, with several minutes recovery. The other is high-intensity interval training, or HIIT, which typically involves intervals lasting one to five minutes at an intensity that is hard but below your maximal aerobic power (the highest speed you hit in a VO2 max test before giving up). If that sounds familiar, it might be because Rosenblat did a similar comparison last year that compared the effects of SIT and HIIT on time-trial performance, concluding that they’re similarly effective.
This time, they were interested in how the different training approaches work. All the studies they included look at measures that can be grouped as central adaptations (e.g. how much blood you’ve got, and how much your heart can pump with each stroke or per unit of time), which determine how much oxygen gets to your muscles; or peripheral adaptations (e.g. capillary density, various markers of mitochondrial function), which determine how much oxygen gets extracted from the blood before it returns to the lungs to stock up again.
As you’d expect, there was a lot of overlap in the results. After all, 30-second intervals and one-minute intervals are more of a Gala-to-Honeycrisp than an apples-to-oranges comparison. But there was a trend. The HIIT workouts tended to produce bigger changes in the central variables: only HIIT changed the amount of blood pumped per heart beat or per unit time. And the SIT workouts seemed to trigger bigger peripheral changes in capillary density and mitochondrial function.
It’s important to acknowledge the limitations of pooling the results of a whole bunch of small studies with diverse and mostly untrained subject populations. These results should be considered tentative for now. But they do line up with another interesting study from researchers at Western University a decade ago, which compared SIT workouts (4 x 30 seconds of sprinting, with 4:00 rest) to longer continuous training (30 to 60 minutes of running at a moderate effort). After six weeks, both groups saw similar improvements in VO2 max (11.5 vs. 12.5 percent) and 2,000-meter time trial performance (4.6 and 5.9 percent). But only the continuous group increased maximum cardiac output, which is the amount of blood per minute pumped by the heart at VO2 max.
Together, these two results suggest that longer, slower efforts with less rest increase fitness through central adaptations, while shorter, faster efforts with more rest trigger peripheral adaptations. Of course, it’s not all or nothing. All types of training will produce both central and peripheral adaptations.
In fact, a study last year from Martin Gibala’s group at McMaster University tested that claim by putting volunteers through an ultra-minimalist SIT program three times a week: just three 20-second all-out sprints with 2:00 recovery, preceded by an easy 2:00 warm-up and followed by a 3:00 cool-down. Since previous research has found no central adaptations after six weeks of SIT training, Gibala’s team extended the study duration out to 12 weeks—and this time they did find a six percent increase in maximum cardiac output, which presumably contributed to the 21 percent increase in VO2 max. So SIT may offer a smaller central stimulus than long runs, but it’s not zero.
The takeaway from the new meta-analysis, according to Rosenblat, is that “you should likely include both interval types, but cycle through the two types.” His advice is alternating a two-week SIT cycle and a four-week HIIT cycle. The sample workouts he gave based on his previous article were 4 x 30 seconds with 4:00 recovery and 5 x 5:00 with 2:30 recovery. My own takeaway is a little broader. Just because two workouts produce the same external results—a similar improvement in race time or VO2 max, say—doesn’t mean they’re doing the same thing inside your body. That means the workouts are not interchangeable. In the real world, if you’re choosing between short sprints, longer intervals, and continuous runs, my bet is that the best choice is “all of the above.”
Alex Hutchinson writes Sweat Science for Outside; for more, check out his Twitter and Facebook, sign up for the email newsletter, and read his book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance.