Joey Thurman, an ACE-certified personal trainer and host of the Fad or Future podcast, explained that muscle building isn't dependent on a high rep count. "People focus on how many reps they need to do to add muscle, but reps don't matter as much as the stress you put on the muscle tissue itself," Thurman told POPSUGAR. He suggests working until you can no longer keep proper form for a particular exercise, whether it's done with or without weights.
Take your time with your reps, too. Thurman explained that muscle fibres need to be torn down to grow back stronger, and the most muscle damage occurs during the negative — or eccentric — part of the motion. "If you only focus on the way up and don't control the way down, you are missing a large part of the growth potential," he said. So, for example, when doing a bicep curl, you should pause at the top of the move, then slowly lower back down.