As the demand on your muscles increases beyond what those type I muscle fibers can handle—by increasing the weight or the number of reps, for example—your body will recruit type II muscle fibers, too, he explains. That’s what you’re aiming for if you want to maximize muscle growth, because those type II fibers have more potential for growth than type I fibers.
“People say that lifting heavier loads is the only way you can recruit type II fibers, but that’s just not true,” Phillips says. “You can recruit type II muscle fibers by induction of fatigue.”
There is a downside to lighter weights, though: They’re probably not as good at building strength in the long run, Phillips says.
Strength isn’t just a function of muscle size—it’s also a function of practice, he says. So guys who practice lifting heavy weights four times per week are going to be better at lifting heavy weights than guys who lift only light weights.
Even so, the two groups in this study saw similar strength gains. But that’s probably because all the subjects re-tested their one-rep maxes every three weeks, Phillips says, so even the light-weight group got to practice lifting heavy.
Bottom line: Lighter weights give you more choices, says Phillips.
No one’s saying you should start curling 10-pound dumbbells exclusively. (Although these 100 Exercises You Can Do With a 10-Pound Dumbbell are worth trying.)
But if you want to give your muscles or your joints an occasional break—or if you tweak a shoulder or a knee and need to give it a break—you can switch to lighter weights to reduce the stress on your joints, tendons, and ligaments for a period of time without sacrificing gains, Phillips says.