Get a full cardio workout in 15 minutes

If you're in a rush, consider interval training. Just 15 minutes of intense interval training can replace 45 minutes of low-level cardio, like walking.

The difference? Instead of maintaining a moderate pace throughout your exercise session, you alternate all-out sprints with rest periods.

This technique burns calories and can be potent for "everyday people, even those with heart conditions or previously inactive people," says Martin Gibala, a kinesiologist at McMaster University in Ontario.

As for timing, alternating 1 minute of sprinting with 1 minute of rest is a good plan for most people, Gibala says, but, "anything from 30 seconds to 2 minutes of sprinting with rest periods from 1 to 4 minutes in between will work."
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Option: If your running sprint can't be an all-out effort (85% of your max), don't hit the track. Instead, "sprint" on a stationary bike to cut risk of injury; check with your doctor.
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Fitsmart
Jorge Cruise
USA Today Weekend