1 An Efficient Heart Is a Stronger Heart

If you lift weights in a circuit style, go to CrossFit classes, or do high-intensity interval training (HIIT), you may think you’re getting a more efficient, modified form of “cardio” and eliciting the same effects as a long, slow run. Not so fast.

While HIIT is an awesome way to burn through a ton of calories compared to steady-state cardio—and I do love putting together a nasty circuit to knock out in 20 minutes when I’m crunched for time—those short yet intense circuit sessions don’t necessarily train the cardiovascular system in the way we think.


One of the most important adaptations from frequent aerobic exercise is a thing called stroke volume, which is defined by how much blood is pushed out of your left ventricle (one of the heart’s chambers) during cardiovascular work. If your heart can deliver more blood with less work, you’ve got a serious advantage, and you can gain it from steady-state cardio.

In order to improve stroke volume, the left ventricle must grow bigger to allow more blood to fill it and subsequently get pumped out. Long, slow bouts of cardio—like running and using the stair climber or the elliptical—can help. On the other hand, circuit training is less effective for improving stroke volume because the intensity and duration of those sessions generally avoid the aerobic zone, which is necessary for adaptations to take place.

And to be clear, steady-state cardio should be performed in the aerobic zone, where you can easily run and talk at the same time. For most people, this means that their heart rate is going to be somewhere between 120-150 beats per minute.

Remember that this aerobic zone is the sweet spot for bringing about improvements in your cardiovascular system, which can translate into better muscular gains overtime.