Are Heavy Weights Better For Building Muscle? The Surprising Results Of a New Study

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.

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