SPRINT 8 IS A 20-MINUTE WORKOUT you can do with any aerobic activity: swimming, running, rowing, cycling—you name it. After a three-minute warm-up, start a series of eight 30-second intervals. Prep your muscles for the pace with the first three, then push yourself as hard as possible through the rest. "If you can keep charging past 30 seconds, you aren't trying hard enough," says Sprint 8 creator Phil Campbell.

Between each sprint, slow to an easy pace for 90 seconds, to fully recover for the next one. Finish with a couple minutes of easy work.

For the best results, do a series every other day so your body has time off. If you don't currently have an exercise routine, Campbell suggests you start with at least three weeks of moderate sprints to strengthen your joints' connective tissue and to prep the muscles for full-bore exertion.