HIIT versus SIT

 Interesting Outside article by Alex Hutchinson about the differences between HIIT and SIT training and how they differently affect improvements in "peripheral adaptation" and "central adaptation."

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.

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.

How should that change your workout?

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.”