The reality: Stopping to rest for a minute or so after every exercise can sabotage results. "Too much rest lets lactic acid dissipate,” said Rubenstein. And muscle growth (and tone and definition) requires lactic acid. Instead, keep up the intensity by moving quickly from one exercise to the next. "You should be somewhat recovered but not completely," says Rubenstein. If you can do 15 reps the first set, and one or two fewer on the next, you're on the right track.