"So ramp up slowly over two to three weeks," says Breakthrough founder Jonathan Roche. "Guys, in particular, will go all-out and waste themselves." Properly done, a high-intensity work interval should be followed by a low-intensity rest interval that allows your heart rate to recover, or come down to a level where you're breathing comfortably. Generally, the more intense the work interval, the longer the rest interval.
From a psychological perspective, intervals are tricky.
"They're more fun because we like to be challenged to do better, but they are more gut-wrenching and grueling," says Ron Jones, an Atlanta- and L.A.-based corporate wellness coach. "Although we know that lukewarm goals don't work very well, too-hard ones can frighten you away."