Unless you're a pro athlete, "VO2 max" probably sounds like a formula you forgot from high school chemistry. Here's a cheat sheet: The number is the amount of oxygen your body can use during physical activity. The higher it is, the fitter you are—and the easier a longer workout will feel. These three tips will take yours up a notch.
1. Push harder. Intense exercise trains your body to use oxygen more efficiently. At least once a week, make your cardio a sprint-interval session: After a 3-minute warm-up, move as fast as you can for 60 seconds, recover at a slower pace for 60 seconds, and repeat for 16 to 20 minutes.
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2. Take a supplement. Human growth hormone, a substance secreted by the pituitary gland, has been shown to improve everything from your skin to metabolism. And it might help you go the distance too: A clinical trial found that taking the supplement SeroVital-hgh ($99; serovital.com) daily for two weeks increased participants' VO2 max by 6 percent.
3. Strike a pose. Intervals are tops for boosting VO2 max, but practicing yoga six times a week for three months significantly increased exercisers' scores, according to a recent Indian study.