Science Reveals Best Way to Build Size and Strength, Settling the High Reps vs Heavy Weight Debate

Conventional gym wisdom has long offered a basic rule of thumb for novice and intermediate weightlifters on how to build muscle: If you want to increase size, lift relatively light weights for a lot of reps. If you want to get strong, lift heavy weights for fewer reps.

Simple, right? Not exactly. Recent research is illuminating some misconceptions.

A study from McMaster University also challenges the old gym law and instead states that lifting relatively light weights (about 50 percent of your one-rep max) for about 20 to 25 reps is just as efficient at building both strength and muscle size as lifting heavier weights (up to 90 percent of one-rep max) for eight to 12 reps.

"Fatigue is the great equalizer here," Stuart Phillips, Ph.D., a kinesiology professor at McMaster and the senior author of the study, wrote about the research. "Lift to the point of exhaustion and it doesn't matter whether the weights are heavy or light."

Phillips and his colleagues asked 49 men, each about 23 years old, to do a 12-week program of total-body resistance training. The lifters were split into two groups: a high-rep group, which lifted at 30 to 50 percent of their one-rep max for 20 to 25 reps a set, and a low-rep group, which lifted at 75 to 90 percent of their one-rep max for 8 to 12 reps a set. Both groups lifted to failure, and did four exercises: inclined leg press, barbell bench press, machine-guided knee extension, and machine-guided shoulder press.

At the end of 12 weeks, the authors tested the participants’ muscle mass and found that both groups had made essentially equal gains in strength and size—except for the bench press, which was higher among the low-rep group.

Why the equal gains? Total work volume—that is, reps times weight—is a good way to force muscle growth.

A 2023 meta-analysis published in the British Journal of Sports Medicine supports these findings. Researchers found those who lift lighter weights to the same degree as those who train heavier weights (read: training to failure) gained comparable results. The key is training to failure, meaning you can't do one more rep.

Related: 50 Best Back Exercises to Build Serious Mass in 2025

How to Grow Muscle

“As long as you’re doing enough volume, you’ll positively adapt to the training,” says Sean Collins, C.S.C.S., a USA Powerlifting-certified trainer and powerlifting coach at Murder of Crows Barbell Club in Brooklyn, NY. "Volume acts as a driver that overloads the body to make an adaptation, also known as super-compensation.”

That’s good news, especially for those who are hesitant to hit the heavy weights but still want to make gains.

“Studies like this are a wake-up call to trainers and lifters who thought you exclusively had to lift heavy—at least 75 percent of your max, or about 10 reps and below—to get big,” says Sean Hyson, C.S.C.S. “It’s also encouraging for older, beat-up guys with joint issues and injuries. They sometimes think they can’t train hard anymore, but if they just go lighter and do more reps, they can build muscle too.”

Bottom line: “It’s the effort you put in that matters most,” Hyson says. “Lifting heavier builds more strength, but lifting to failure with any weight can build bigger, more aesthetic muscles.”

Related: 50 Best Chest Exercises of 2025 to Build Your Pecs and Pushing Power

Tips for Gaining Muscle Mass

So does this mean for the average lifter? Here are important takeaways you should remember:

You Don’t Always Have to Lift to Failure

“Going to failure might be a major determiner of muscle growth, but it’s dangerous,” Hyson says. “That means going to the point of exhaustion, and that’s when form breaks down. It doesn’t matter if you’re lifting light weights—if you rep out to where you start shaking, straining, or breaking form to get the last couple reps, you risk hurting yourself.”

But It’s Okay to Get Close

So you don’t want to kill yourself, but you still want to make gains. The compromise? “Take your sets to within about two reps shy of failure,” Hyson says. “This might prevent you from achieving the absolute best gains from that workout, but in the long run it’s much safer and lets you build steady progress over time.”

Elite Lifters Still Need to Lift Heavy

Young lifters and beginners can make gains with lightweight, high-rep work. But if you’re an experienced weightlifter with years of focused programming under your belt, “increasing loads—with increasing volume, intensity, and weakness correction through accessory exercises—is really the only way to build strength,” Collins says.

“When getting to a point where people want to focus on strength gains, they'd have to almost certainly use heavier weight. However, hypertrophy elicited through higher reps and lower weight will build new muscle; the more cross-sectional muscle fiber you have, the more you can neurologically recruit, thereby making you stronger.”

Gains Come From Creativity, Not Just Volume

“Muscle growth can be accomplished in a multitude of ways and weights,” Collins says. “Sure, a general everyday gym athlete—like the participants in this study—will see progress by increasing either the intensity of the lift or increasing the volume of the lift.” But you can also make gains by simply programming new exercises or modalities: “Varying your movement and tempo can make higher-rep, lighter-weight sets feel very difficult.”

Don’t just assume you have to constantly do high-volume work. In fact, doing that exclusively can lead to burnout or injury.

Another tactic? “Try a periodized approach,” Collins says. “Start with a few weeks of high-volume workouts with lower weight and higher reps, which will definitely lead to muscular hypertrophy, strengthen your joints, and prepare your mind and your body for higher-intensity work. Then, when you’re ready, start doing higher-intensity workouts with more weight, which will elicit a higher one-rep max than before. Working with a strength coach can ensure your training is organized in such a way where you'll continue to grow strength and size.

Related: The Best Sprint Workouts to Get Faster, Build Muscle, and Drop Fat