Now that I’ve picked up the habit of high intensity interval training, I’ve developed a belief about it that explains why it increases your cardio-respiratory system so well—that minutes of HIIT exercise can give you the benefit of hours of moderate exercise.

Muscles develop when you stress them. You build skeletal muscles by stressing them, such as by lifting as much as you can. If you can left, say, 25 pounds with some muscle, then lifting 20 to 25 pounds will cause it to develop. Lifting a 5-pound weight with that muscle all day won’t stress it to rebuild.

If the heart and diaphragm are muscles, they’ll probably develop when you stress them, which happens when you push them as hard as you can. Just as lifting a light weight won’t develop a skeletal muscle, neither will lightly stressing your heart and diaphragm, which happens when you jog or do other light exercise.

Push your heart to its limit and it will grow.

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I’m not a medical doctor, so I don’t know if this belief is consistent with how the heart and diaphragm develop, but it helps me make sense of HIIT.

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