Strength training is having a moment—and, whether on #GymTok or the real gym, the emphasis is largely on lifting heavy. “That’s one key attribute for not just bigger muscles, but stronger muscles and stronger bones,” says Abbie Smith-Ryan, a professor of exercise physiology and nutrition at the University of North Carolina at Chapel Hill.
Yet “heavy” is a relative term. The same barbell that feels nearly impossible for a novice to pick up might be a cinch for an experienced lifter. The American College of Sports Medicine (ACSM) defines “heavy” as 80% or more of what’s called your “one rep maximum,” meaning the heaviest possible weight you could lift once. Exercise physiologist Rachelle Acitelli Reed says this translates to something that’s very, very hard to do four to six repetitions with. “You’re squeaking out that last rep, and probably exhaling audibly,” she says. It’s so challenging that many of us avoid it—particularly women who might have been warned that it would create bulky muscles.
Yet many fitness trainers encourage heavy lifting because it makes training more efficient. Doing a biceps curl with 50-pound dumbbells gives the muscles a much bigger stimulus than 5-pounders, so you can tire them out—and therefore see results—faster.
Heavy weights can also uniquely combat some of the changes to our muscles that naturally come with aging. Smith-Ryan explains that as we get older, we tend to lose some of the strength-oriented, “fast-twitch” muscle fibers that are responsible for producing force. “We can prevent some of that with heavier lifting,” she says. Our bone mineral density also benefits, since hoisting heavy things against the pull of gravity stimulates the bone tissue to rebuild, according to James McKendry, an assistant professor of nutrition and healthy aging at the University of British Columbia.
But, he says, “I think the push for heavier loads being the only mechanism to preserve bone is a little misplaced. Lifting weights of any resistance can be effective.”
Many exercise scientists today agree that heavier isn’t always better. “If you actually look at the science, there’s so many instances where the more moderate dose is very appropriate,” Reed says. Earlier this year, the ACSM published a position stand based on the findings of 137 reviews emphasizing that many kinds of resistance training—including many kinds of weights, or “loads”—can generate results.
The ideal weight depends on your goals
Not everyone heads to the gym for the same reason. The guy next to you might be grunting under that massive weight plate simply because he’s working toward a new bench press personal record.
“If you are trying to maximally build the most amount of strength, using a heavy load is pretty much the only way that you’re going to [do it],” McKendry says. This is thanks to the exercise-science principle of specificity: You’ll get better at whatever you do over and over. So if you want to win a Strongman competition, you’ll need to regularly practice pumping the heaviest iron you can.
But if you’re more focused on building bigger (not the strongest) muscles, ACSM says effort is more important than load. You just need to keep working close to the point—within one to three reps, McKendry says—that your muscles are no longer able to pull off another lift with good form. Nearly any weights that get you to this point can trigger muscle growth; using lighter ones will simply take longer.
Maybe your goal is to build more muscular power with moves like kettlebell swings and power cleans. (While this might seem like a niche concern exclusively for athletes, McKendry argues it’s essential for anyone who wants to be able to catch themselves from a trip or fall.) For this, the ACSM recommends a more moderate weight, between 30 and 70% of your one-rep max. Since the benefit comes from moving as quickly as possible during the lifting phase, you need a weight that’s light enough to let you get some speed.
Then there’s muscular endurance. “Performing one-repetition maximums doesn’t translate to better performance particularly well for marathon runners or triathletes,” McKendry says. Thanks to that principle of specificity, a better bet for these folks, he says, is to aim for lots of reps with lighter weights to train the muscles to keep contracting repeatedly over time.
When lighter works better
Fitness goals aside, there are plenty of people for whom heavy weights simply aren’t as appropriate. Maybe they’ve never stepped in a weight room before and need to learn good form first. Maybe they have joint issues or osteoarthritis or an injury that would make a heavy weight inadvisable. “I’m never going to get my mom in her 60s started lifting at 90% of her one-rep max,” Reed says. “Instead, I’d start with a lower load to build technique and the neuromuscular connection, improve confidence, and then progress up to a more moderate load and keep her close to that.”
For many people, heavy weights can simply be so intimidating that we avoid lifting at all. Less than a third of Americans say they do strength training twice a week—the government’s recommended minimum—and nearly 60% percent do no muscle-strengthening activities regularly. “The best weight you can choose is the weight that you’re going to lift consistently, safely, and keep coming back to,” McKendry says.
He points out that most people go to the gym just to get a little fitter and healthier—and for that, nearly any weight will do, as long as you regularly give your muscles a solid challenge. “There’s been years of arguing about the right rep and set scheme or the perfect weight,” Reed says. “But what the evidence consistently shows is that we just need people to do something.”