Some people like to say that lifting heavy weights is the only way to build muscle. High reps and light weights might improve your endurance, these people argue, but they’re not going to make your muscles any bigger.
In fact, the latest science shows that training with lighter weights and higher reps is a surprisingly effective way to make your muscles grow. Let’s dig in and take a closer look at what it all means for you.
A few years back, a team of US researchers ran a very simple experiment: They took two groups of guys and got them to lift weights three times a week for eight weeks. Both groups followed the same training program, but with one key difference. For each exercise, the first group did three sets of 8 to 12 reps with a heavy(ish) weight. Group two used a lighter weight and did three sets of 25 to 35 reps. Conventional wisdom has it that the group lifting the heavier weights would gain the most muscle. Those in the light group would see some muscle growth, but to a much smaller degree.
But that isn’t what happened: The researchers found no significant difference in the rate of muscle growth between the two groups. Training with higher reps and lighter weights led to gains in muscle size that were on par with heavier training. And this wasn’t just a one-off. Stuart Phillips, a kinesiology professor at Canada’s McMaster University, has authored several studies that look at the impact of different rep ranges on muscle growth. All show very similar gains in muscle mass whether training is done with light weights and high reps or heavier weights and lower reps.
In one study, Phillips and his team got a group of men to train their legs three times a week for ten weeks, using either high or low reps. The result? The amount of new muscle added to both legs was almost identical. Training with 30-40 reps stimulated just as much muscle growth as sets of 10-12 reps
Of course, these are the results from just a few studies. And drawing conclusions about anything from two or three studies is never a good idea. However, there’s plenty of other research out there showing that higher reps and lighter weights trigger just as much muscle growth as heavier weights and lower reps.
In one 2018 study, eight weeks of training the arms with light weights (20 rep-max) led to gains in muscle size that were no different than those seen with heavier weights (8 rep-max). In another, Japanese researchers found that training with high reps and light weights (30-40 reps per set) builds just as much muscle as low reps (8-12 reps per set) and heavier weights. Most of the studies we’ve looked at show similar rates of muscle growth with both low and high reps. But what happens when you start using ultra-high reps? And by ultra-high, I’m talking about 60-70 reps per set?
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That was the question asked by a team of Brazilian scientists, who took a group of 30 untrained men and got them to lift weights twice a week for 12 weeks. The lifters were split into three groups. All three groups trained the biceps and quads on one side of their body with very light weights and ultra-high reps—20 percent of their one-rep max, and 60-70 reps per set.
On the other side of their body, the men used one of three different rep ranges: around 30 reps per set, 15-20 reps per set, or 10-15 reps per set. At the end of 12 weeks, training with low, moderate, and high reps all led to similar gains in muscle size. But it was a different story for the side of the body that was trained with ultra-high reps, where muscles grew at half the rate they did in the other three protocols. In other words, while sets of 30 reps led to gains in size that were on par with sets in the 10 to 15 rep range, training with just 20 percent of your one-rep max appears to be below the threshold needed to maximize gains in muscle size.
So, what does all of this mean for you and what you do in the gym? What it doesn’t mean is that training with light weights and high reps is now the “best way” to build muscle. The fact that it’s possible to gain muscle using higher reps and lighter weights doesn’t necessarily mean that it’s a good idea to do so.
Remember, the high rep training programs didn’t lead to superior gains in size or strength. But each set took twice as long to complete. Training in a higher rep range is also highly unpleasant and extremely painful—a lot harder than lower reps and heavier weights. Furthermore, as a personal trainer, doing longer, more painful workouts simply to generate the same results doesn’t sound like a great idea to me. What’s more, research shows that lower reps and heavier weights still win the day as far as gains in strength are concerned.
What it does mean is that the range of repetitions you can use to build muscle is a lot wider than previously thought. That gives you a lot more choice about the type of training you do.
For example, you might find that lifting heavy weights causes pain in your shoulders, elbows, knees, or wrists. The solution is very simple: If going heavy on certain exercises causes pain, just go light instead.
Maybe you train at home, or in a gym with a limited range of equipment, and lifting heavy weights on certain exercises isn’t an option. Perhaps you just prefer using lighter weights on some exercises, and heavier weights on others.
In both cases, you can train with lighter weights and higher reps safe in the knowledge that you’re not missing out on any gains. In short, as long as you train hard and push yourself, your muscles can be made to grow with a variety of rep ranges and weights, from light to medium to heavy.
Christian Finn is a UK-based personal trainer who holds a masters degree in exercise science. He writes frequently about fitness and nutrition on his personal site, MuscleEvo.
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