Orangetheory encourages the use of a heart rate monitor to track cardio fitness in its workouts.
4) Why does HIIT improve cardio health?
Researchers still haven’t figured out exactly why HIIT works to improve aerobic fitness more than continuous types of exercise. But one key hypothesis, Gibala explained, has to do with the heart’s ability to pump blood.
One measure for blood pumping is something called stroke volume, or the volume of blood that comes out when the heart contracts. And a major determinant of VO2 max is stroke volume.
“The maximum amount of blood that comes out of the heart is improved by exercise training,” said Gibala, “and there’s evidence that when you do interval exercise training, the stroke volume increases even more.”
5) Is HIIT the best exercise regimen for weight loss?
There’s no doubt that interval training can be a time-efficient way to burn calories. Researchers have repeatedly shown that people can burn comparable amounts of calories in HIIT routines lasting, say, 20 minutes, compared to longer continuous exercise routines lasting, say, 50 minutes. The reason for that, Gibala said, is that higher-intensity exercise, like intervals, results in a greater disturbance of the body’s homeostasis, “and it literally takes more energy and oxygen to return it to normal basal levels.” (We’ll get to the related “afterburn” effect in a moment.)
But the question is whether that calorie burn translates into weight loss, and that’s where HIIT falls short. A 2019 systematic review of the trials comparing HIIT and SIT with moderate-intensity continuous training found all workouts performed about the same on fat loss. (Side note: the journal hyped the review’s findings, leading one of the study author’s to put them in context in this Twitter thread.)
“Many people overstate the potential for interval training to cause you to lose weight,” said Gibala. But that’s a problem with exercise in general, not HIIT specifically. As we’ve explained, it’s much easier to lose weight by cutting calories in your diet than trying to burn excess calories.
That’s especially true if your workout is only 20 minutes long, said Jeffrey Horowitz, a kinesiology professor at the University of Michigan. To burn a lot of calories, “you need to exercise [for] a more prolonged period of time. HIIT routines, by definition, tend to be shorter. So if your goal is weight loss, you might consider a longer interval routine, and you definitely want to look at your diet.”
Gibala summed up, “In terms of the overall magnitude of calorie burning, it tends to be small relative to what you can achieve by dietary changes.”
6) What about the “afterburn” effect?
Many HIIT gyms tout exercise programs that will lead to an “afterburn” or “excess post-exercise oxygen consumption” — a period of elevated calorie burn after you exercise.
“This revs your metabolism and makes you burn calories long after your workout is over,” Orangetheory claims.
“The afterburn effect is real — but it’s often overstated,” Gibala said. “When we’ve measured it in a lab, we’ve shown that a 20-minute session of intervals can result in same calorie burn over 24 hours as a 50-minute bout of continuous exercise. So that means the afterburn effect is greater after the intervals — but it peters out after a while.”
It’s also marginal, he added, not the kind of calorie loss that would lead to lasting weight loss. (I saw the same effect when I entered a metabolic chamber to measure my metabolism. In the periods after I hit the exercise bike, my metabolic rate ramped up — but only by a few more calories each minute, and the effect wore off within half an hour of exercising.)
Building more muscles, however, can be a little more helpful for the afterburn. Here’s why: One of the variables that affects your resting metabolic rate is the amount of lean muscle you have. At any given weight, the more muscle on your body, and the less fat, the higher your metabolic rate. That’s because muscle uses a lot more energy than fat while at rest.
So the logic is if you can build up your muscle and reduce your body fat, you’ll have a higher resting metabolism and more quickly burn the fuel in your body. But that takes work — a lot more work than a short aerobic HIIT workout. And even a short HIIT workout may not be for everyone.
“Intervals can be demanding mentally and physically, so some steady-state continuous is nice once in a while,” Gibala said. “[But] for those who truly are super time-pressed and can tolerate intervals almost exclusively, it’s the most efficient way to train.”