Do each exercise for 10 to 15 repetitions, using weights that produce fatigue but not failure, and go from one move to the next without stopping. Willardson suggests repeating the circuit one to three times, every other day. The best workout ever, according to science. This total-body routine is your research-proven formula to building muscle and torching fat. It seems like every other week there’s a new study touting the best way to work out. And, really, how you do it depends a lot on your goal—someone looking to gain muscle mass may hit the gym differently than someone training for a triathlon.