2. You're Not Working Your Muscles to Fatigue

Using light weights for 30 repetitions isn’t nearly as effective at sculpting muscles as lifting heavier weights for fewer reps, says Lauren Simmons, R.D., C.S.C.S., of Core Dynamics. That said, you don’t have to lift like a bodybuilder to get results, either, as long as you work to exhaustion. β€œIt’s gotta be challenging,” says Contreras. β€œYou can do sets of 20 repetitions, as long as you’re taking at least some sets to momentary muscular failure, where you can’t do any more reps with good form.” Simmons recommends the sweet spot is eight to 10 repetitions for four to five sets, gradually increasing the weight throughout until you reach muscle fatigue by the end of the exercise.