When you see the biggest guy in the gym pull up to the weight room, you might assume he’ll be reaching for the heaviest weights. You’ve gotta pump massive iron to build massive muscles, right? Well, not really.

“There’s a lot of lore around this routine or that routine, and a lot of it comes from former Soviet bloc country training regimens where most people were taking steroids,” says Stuart Phillips, PhD, a kinesiology professor and research director at McMaster University. Some of it also comes from a misunderstood study from 1946: While rehabilitating soldiers from World War II, army physician Thomas DeLorme argued that heavy resistance training was better at building muscle than, say, repetitive activities like walking or biking, and for decades, many took that to mean only heavy weights were helpful, says Dr. Phillips.

Yet the more scientists look into it, the more they’re finding that heavy lifting isn’t a prerequisite for growing muscle, or as the experts call it, “hypertrophy.” Recently, Dr. Phillips led a network meta-analysis published in the British Journal of Sports Medicine that looked at 192 randomized, controlled studies with a total sample size of more than 5,000 people to find the “optimal prescription for hypertrophy.” What his team discovered shocked many. “You can lift lighter weights, and as long as you lift them with a high degree of effort, they’re as good as heavier weights in making you bigger,” he says. Even just using your own body weight, like with push-ups or lunges, works. The key is simply to get pretty close to what personal trainers call “failure,” or the point where you feel like you can’t keep going any longer. That could take up to 25 to 30 reps, and you’ll still build muscle, says Dr. Phillips.

To understand the physiology at play, it helps to know the difference between our two types of muscle fibers: fast twitch, or type II, produces force but fatigues quickly (think sprinting), while slow twitch, or type I, gives us endurance but aren’t super powerful (think marathon running). When you want to get bigger, it’s the fast twitch you mostly want to target since those have between 30 to 50 percent more growth potential than their slow counterparts, says Bradley Schoenfeld, PhD, graduate director of the Human Performance and Fitness program at Lehman College, who wrote the book Science and Development of Muscle Hypertrophy.

While it used to be thought that only heavy loads—weights you can only lift about three to five times—could activate the fast-twitch fibers, we now know that’s not the case, Dr. Schoenfeld says. “Provided that you train with a lot of effort where the last reps are difficult to complete, you will recruit the majority of the fast-twitch muscle fibers,” he says. “Muscle growth tends to be the same.”

That said, if your goals are more about strength than size, you’ll want to keep reaching for the largest dumbbells. Our bodies get better at what we practice, so if you want to be strong enough to lift heavy things, you have to practice lifting heavy things, Dr. Phillips says.

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