By Ali Eaves
Bro wisdom says that if you want to get big, you have to lift heavy. But emerging science begs to differ.
In a new study from McMaster University, researchers put experienced lifters through a resistance training regimen for 12 weeks. They all did the same workouts: a typical muscle-building program that included barbell bench presses, biceps curls, leg presses, and knee extensions, among other exercises.
Half of the subjects lifted heavy weights—75 to 90 percent of their one-rep maximums for each exercise, lifting to failure, which usually meant they performed 8 to 12 reps per set.
The other half, however, lifted only 30 to 50 percent of their one-rep maxes. But they also lifted to failure, which typically worked out to 20 to 25 reps per set.
The men in both groups put on the same amount of muscle over the 12-week period on average: 2.4 pounds, to be exact.
The researchers also biopsied the subjects’ muscles and found that there was no significant difference between the two groups’ growth in the size of their muscle fibers—both type I and type II fibers.
The key driver of muscle growth is activating as many of your muscle fibers as possible, says study author Stuart Phillips, Ph.D. You can do that by lifting heavy or by lifting to failure, Phillips says.
Here’s how it works: When you exercise or even go about daily life, you recruit your smallest (type I) muscle fibers first, for the easiest activities, Phillips says.
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.
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.