When you train smaller muscles with single-joint moves—bicep curls, leg extensions, and lateral raises, to name a few—you should stay in a higher rep range.
There are two reasons for this rule, says David Otey, personal training manager at Equinox Sports Club New York. First, when you do fewer reps you’re likely using heavier weights, which builds power and strength. Because you use smaller muscles (like those in your arms) for everyday movements (like pushing and pulling), training them for endurance rather than power is more important for functional fitness, he explains.
Second, for multi-joint exercises like squats, it’s okay to go heavy for 4 or 6 reps because the load is distributed across the joints in the ankles, knees, hips, and spine. But accessory moves put all the strain on one area. “Working at a high intensity can overpower the integrity of the joint for the benefit of the muscle, risking injury,” Otey says. Increasing your rep range forces you to lift lighter.
For accessory moves that work just one joint or muscle, perform 8 to 15 reps for 3 sets. Otey suggests you choose a weight that almost brings you to failure.