High reps vs low reps: which actually builds more muscle without wrecking your joints or your motivation? I ran a 60-day, side-by-side test to find out.

High reps vs low reps? Apparently, you have to train with heavy weights for the best muscle growth; at least, that’s what many people say. 

But not everyone agrees. High reps vs low reps (or light weights vs heavy weights) for muscle growth is actually one of the most debated questions in fitness. 

So, I’m putting it to the test MYSELF. For 60 days straight, I’ll split up my body, not literally. 

One side with heavy weights, the other with light weights.

Let’s start with day 1.

Day 1: Set Up

To decide which side of my body I’ll be training heavy and which I’ll be training light during this high reps vs low reps experiment, I’ll be using the most scientific way possible, and it only costs a dollar. 

The coin toss.

The science gods must be looking out for me because my left side is actually my weaker, non-dominant arm and leg, so the heavy weights might actually balance them out. 

But you might be wondering why I’ve decided to risk lopsiding my physique for the sake of this high reps vs low reps experiment. Well, like most people, I grew up thinking you gotta lift big to get big. 

Even when my joints used to bug me, I wouldn’t lift light weights because deep down I felt like “there’s no way these light weights can build the same muscle”. 

But, what if curling these 20-pound dumbbells IS, in fact, the same as curling these 60s?

Recent studies suggest it’s possible, but they rarely test very low rep ranges, most are done on complete beginners, and we still don’t know if certain muscles — like the chest — might respond differently to heavy versus light weights since they haven’t really been tested.

So here’s my 60-day high reps vs low reps plan. Every major muscle will be trained unilaterally, meaning only one arm or leg at a time. 

Each side will receive the exact same number of total sets, and every set will be taken as close to failure as possible. 

But the heavy side must reach failure within 3-6 reps, whereas the light side must reach failure within 15-30 reps. Plus, to make sure there are actual changes in muscle growth and strength, I’ll also be eating in a small calorie surplus.

But First, We Need A Baseline

I got an hour-long full-body MRI scan that not only measures my muscle growth precisely but also reveals individual muscles that ultrasound can’t distinguish.

Studies hardly ever use MRI because it’s expensive ($1000/hr), but thanks to the coin flip I have some change to spare. (Want to get an MRI scan yourself? Click here to get $300 off your whole body scan at Prenuvo.)

But, I’m also using DEXA, ultrasound, and body circumferences as backups and to get a clear picture of the whole high reps vs low reps for muscle growth story. 

Not to my surprise, as a right-hand frequent user, my right side is slightly bigger.

Here’s my full body report.

After 60 days, I’ll come back for another scan to see how much each muscle grows, and if there’s a difference between the two sides.

Now we all know size matters (wink), but strength is just as important.

 I’ll be tracking everything during this high reps vs low reps experiment using the BWS app throughout the experiment. At the end of the 60 days, I’ll be switching sides to see if they can keep up with each other. 

Meet, Dennis

But I have years of training experience, I’m curious if a beginner’s muscles would respond differently during this high reps vs low reps experiment. 

So I found this Dennis guy on the street, the perfect beginner guinea pig.

Somehow, his report shows that he’s also a right-hand frequent user, with a starting body fat of 18.4% with 113.6 lbs of lean mass. This is his full body report.

Week 1

As you can see, Dennis makes lifting just his bodyweight seem like he’s lifting 300 lbs. 

Not to mention, all the machines looked completely foreign to Dennis. He even had trouble inserting himself into some of them. 

But that’s okay. Some things you can’t just wing on your first try. 

But given that the average beginner can gain around 8-10 lbs of muscle in their first year of training, I’m confident Dennis can gain at least a couple of pounds of muscle if we dial in his training during this high reps vs low reps experiment. 

But, it didn’t take long for Dennis to start complaining about the burning sensation on his high-rep, low-weight side. 

Understanding The Burning Sensation

This “burn” sensation comes from metabolic stress during exercise, and it’s common when you climb stairs quickly, bike uphill, or even carry heavy groceries from your car to your door.

Dennis:

“Oh my God. I feel the burn, like, around 16 to 18. Yeah, but before that, I was comfortable. That's the thing with the light side. Oh, first it's like, oh, this is easy. And then it gets very hard, whereas the heavy side is like, it's hard from the beginning, but you never really get the burn.”

When you lift with higher reps, your muscles accumulate more and more hydrogen ions and other metabolites. The nerves in your muscles detect this, and for reasons we’re not yet aware of, signals are then sent to your brain to create a painful “burning sensation” in response.

When it comes to the whole high reps vs low reps debate, some believe the burning sensation from high reps is actually beneficial to growth, but it comes with the cost of pain and often more muscle soreness. 

On the other hand, no pun intended, with heavier weights, the set ends in half the time. You don’t need nearly as many reps to reach failure, so you avoid the burn completely.

Jeremy:

"Definitely noticing the lighter weights for higher reps performance TANKS. Especially if I do, you know, three maybe four sets. Whereas a heavier weight is more consistent."

So why does our light side feel like they’re getting absolutely wrecked every workout, but our heavy side feels like they’re barely getting worked at all? 

Well, it’s not just because of the burn, but ...

High Reps vs Low Reps: The Difference In Volume Load

When I looked closely at my app to see the total volume we lifted from our first few workouts, I found something surprising when it came to high reps vs low reps.

By multiplying the total sets and reps and weight I lifted across my week 1 workouts, on my light side, it came up to a total of over 100  thousand lbs.

Whereas on the heavy side, because I’m doing far fewer reps, it came up to just half of that.

Some people think total volume lifted and time under tension are the keys to growth. If that’s the case, then when it comes to high reps vs low reps, we should see the high reps (light weight) completely dominate. 

So, this begs the question: can my heavy side actually keep up in growth despite lifting less total volume, taking half the time to finish each set, feeling no burn, and having less soreness the next day?

Week 2

We’ll see what actually happens after the next 7 weeks of our high reps vs low reps experiment, but to make sure Dennis gets enough muscle growth, we have to focus on his nutrition.

So, I had Dennis use our app to start tracking his diet, assigning him a daily calorie and protein target to help him slowly gain weight.

But Dennis was still suffering from belly fat trauma, and he seems to be really attached to the idea of “getting abs”. So the idea of eating more scared him. 

Dennis:

“I have a problem of eating enough calories, I always had a skinny fat until early this year, I strictly cut down a lot. It took me 30 years to see a tiny bit of abs, im not ready to say bye yet”

I don’t blame him. For people who worked so hard to lose weight like he has, eating more calories can be intimidating.

But deep down, I want to show Dennis how adding more muscle can also really improve his look. And for someone who doesn’t have much fat on his body, current research suggests that even just a small calorie surplus can help promote more growth.

Luckily, just 2 weeks in and it seems like the diet is already working … at least on one side. 

Why Is It Easier To See Progress On The Heavy Weight Side?

There’s actually an explanation behind this.

When lifting heavy, let’s say you go from 100 lbs to 105 lbs. That’s only a 5% increase in weight.

But if you lift light, going from 50 lbs to 55 lbs is a 10% increase in weight, making it much tougher to add weight when you’re doing light weight for high reps. 

So for our light sides, the smart way to keep forcing growth is to focus on beating our last week’s reps rather than trying to bump up the weight too quickly. 

However, even though we see the heavy side is lifting more and more weight, both of us notice that our light side gets a WAY bigger pump. 

But What Does The Pump Mean Or Do?

The pump is one of the best feelings in the gym — Arnold described it as:

“I’m like getting the feeling of cumming…day and night”

The pump happens when blood rushes into your muscles faster than it can leave. During a set, your muscles contract so hard they squeeze the veins shut, trapping blood inside, while the arteries keep pushing more in. The more reps you do, the more fluid builds up, swelling the muscle, making it bigger.

But does the pump do anything beneficial for growth? Or is it just a short-term ego boost? 

Well, research is mixed. A lot of tricks designed to maximize the pump — like super short rest periods or painfully slow reps — often don’t lead to much long-term growth at all. 

That said, some scientists believe this swelling effect from high reps provides an additional growth signal to your muscles, giving them a growth advantage over lower reps.  

We’ll see what actually happens after the 60 days of our high reps vs low reps experiment are up.

Week 3-6

But with the midway point of our high reps vs low reps experiment approaching, I reviewed Dennis’ food log and noticed he’s been having a hard time eating enough calories because he hasn’t been coming to work with enough food prepped. 

So, I introduced him to the quick and easy solution I used to use.

“The Muscle Monster.”

This is something I made back in university, when I was struggling to eat enough. 

I mixed protein powder with peanut butter to make a spread, and added sliced banana for creaminess.  The whole thing packs 800 calories and 50 grams of protein.

Weight Updates

But, the bananas aren’t the only thing getting Dennis excited; his heavy right side keeps progressing with the weight, while his weights on the light side have barely moved. 

I’m experiencing the same.

My Bulgarian split squat has gone from 205lbs to 215lbs on the heavy side, whereas my light side is still at the same weight. I was starting to wonder if this high reps vs low reps experiment was going to leave me with a giant strength gap by the end of it. 

To make matters worse, I’m assuming because I’m doing so many sets with light weights for high reps, that the extra fatigue is really taking a toll on my recovery and mentally burning me out.

By this point, I was convinced that if the muscle growth is the same, heavy weight for low reps is the way to go…

Until this happened.

Jeremy:

"Oh… It did not feel good. What happened? I don't know. I felt like a little bit of sharp pain right at the top of the knee. Shit.”

Are Heavy Weights Harder On The Joints?

Heavy weights put more stress on your joints, which isn’t bad — it actually makes your tendons stronger and more resilient over time. But if you only ever use heavy weights, especially with exercises that may not be the best fit for them, it can quickly become too much for your joints to handle.

Jeremy:

“I usually use this pad for my normal work out and its totally fine, but I think the heavy weight it was just too much stress."

Because I didn’t want to aggravate it more by lifting heavy, I tested something on my final set. I trained my heavy leg with lighter weights for higher reps. 

And here’s what shocked me: my heavy side burned out WAY faster. By 10 reps, the burn hit hard. By 13, I had to stop, while my light side could’ve easily kept going. 

What’s happening here is called endurance adaptation.

Here's the difference in training with high reps vs low reps. By consistently training with higher reps, your muscles not only adapt quickly by creating more mitochondria to reduce the “burn” build-up, but your brain also increases its tolerance to the pain.

So it seems each side of my body is getting “stronger” in a different way.

I was not alone; Dennis also noticed that his joints on the right side started hurting during this high reps vs low reps experiment.

By the end of this experiment, we’ll actually test both sides head-to-head, and the results were not what I was expecting. 

By Week 6 of the high reps vs low reps experiment, Dennis has become a monster. He’s lifting heavier every week, and his body weight has also been slowly increasing.

From all his moaning echoing through the office, I could tell he was working hard — he’s now lifting more than double what he started with on the heavy side.

But I noticed a problem with his light side. He was training way too far from true failure. This is actually a big problem because of something called the ...

Fiber Recruitment Principle

Your muscles have smaller muscle fibers, but bigger, more powerful ones, which are actually what grow your muscles the most. 

But these bigger fibers are only called into action if they are needed. Here's what happens with high reps vs low reps:

• Low reps (heavy weight): They're activated right from the very first rep.
• High reps (light weight): They sit on the sidelines until the smaller ones fatigue.

The problem is ... by the time you’re 15+ reps deep, most people can’t push through the pain and just stop their sets, failing to ever recruit and grow their bigger muscle fibers.

I decided to pull out my EMG machine, which measures muscle activation, to prove to Dennis the importance of pushing through the pain during this high reps vs low reps experiment.

So, if Dennis wants his light side to keep up with his heavy side, every workout he’s gonna have to continue mentally pushing through that discomfort.

Week 7-8

But I noticed another problem.

So instead of growing his muscles, I think his ego might’ve grown even bigger ...

When people chase numbers and forget about form, this is what happens: even though it looks like you’re lifting more weight, your exercise might actually be way less effective at targeting the right muscle. All you get is an ILLUSION of progress.

Luckily, I’ve been watching Dennis every week during this high reps vs low reps experiment and correcting him whenever I catch him. But if you don’t have someone like me, that’s why I created our Built With Science plus app. It not only creates a workout based on your body, but also shows you in detail exactly how to do every exercise for the best results. 

As for me, the joint issues continued to get worse. 

Jeremy:

“I think I got like four and a half reps. But my main concern is just my elbow. Like I don't know. It's not bothering me too much right now. I feel like a little bit. Something maybe. It's just like I'm not used to going too heavy, especially for isolation, but definitely has me concerned.  We'll see how it goes the next few weeks of this high reps vs low reps experiment, but I'm thinking that I'm going to have to switch it up to something a little bit more elbow friendly, which is going to suck because this is literally my favorite triceps exercise. But go do what I can to keep the joints healthy and try to make it through the full 60 days.”

We’re on week 7 of our high reps vs low reps experiment now, and the heavy side is starting to cause more and more joint pain, especially on isolation exercises. 

So, as much as I’ve been “team heavy” throughout this experiment, I’m starting to notice its downsides.

If light weights do actually provide similar or even more growth, they can actually be a great option to work around injury.

I know there are certain exercises like overhead extensions where, as soon as I go too heavy, it doesn’t feel very good on my joints. Plus, in some situations, like when I’m travelling, all I have access to are lighter weights, which I used to view as a problem, but it might actually turn out they work just as well, as long as I continue pushing hard.

Day 60 Strength Test

But with 60 days of our high reps vs low reps experiment officially over, it’s time to test our strength. Let’s see if the “weak Dennis” from day one is no more.

We started with the biceps.

Dennis

On day 1, Dennis did 35 on his right and 32.5 on his left.

And now, on day 60 of our high reps vs low reps experiment, he absolutely killed it, even with 37.5 lbs, with both arms.

But, when the weight got up to 40 lbs, something strange happened. Dennis failed to lift 40 on his heavy side. And then when we switched to his LIGHT side ... he succeeded!

This had me confused. I was expecting his heavy arm to get much stronger, but Dennis didn't actually do this exercise throughout the 60 days; we just used it to assess his overall change in bicep strength. So I decided to test an actual exercise he practiced, the preacher curls.

And, as you can see, his light side couldn’t keep up this time. Even after the second attempt. 

We even upped the weight, and this happened ... he did 2 reps with 35 lbs on his left side!

Since it was preacher curls that he did for the 60 days, Dennis learned how to lift heavy, specifically for that exercise. 

But when we tested his biceps strength on a different movement, we still saw that the muscle itself got stronger at a similar rate; it’s just that the skill of controlling and lifting heavy weights is tied to the exact exercise you practice.

Now, moving to the chest press machine, we see that Dennis was able to push 55 on his left side, but couldn’t push 50 on his right side. 

So that begs the question... 

What would happen if we took his day 60 weights from both sides and switched them?

Starting with the 52.5 lbs on the flat dumbbell press that he could easily do on his heavy side... 

Dennis’ right arm wasn’t even able to stabilize the weight to BEGIN the dumbbell press. 

Even with my help, he couldn’t get it up. 

Then, after doing 15 reps of 32.5 on his light side, he went to test his heavy side with the same weight.

Despite the imbalances, Dennis was actually really excited about the gains he made. Enough to convince him to continue focusing on building muscle rather than just abs. I’ll keep you guys updated with his journey.

But now, let’s have a look at how I did.

Jeremy

By the end of the 60 days of our high reps vs low reps experiment, I managed to do 6 reps of 92.5 lbs with my heavy side, and 65 lbs for 16 reps on my light side. 

But when I switched arms and tried 65 with my heavy side, this happened …

Jeremy:

"One thing I was not expecting was the burn in my triceps, which is actually what felt like stopped me."

Then, I tried to see how my light side would do with the heavy weight (92.5lbs). It was instantly way harder.

Let’s see if the same imbalance is seen on the preacher curl as well ... When I did the light weight on the heavy side, I could only manage 13, when the number to beat was 17. And I've definitely noticed that the burn on this side is way more than the right side.

Each side got stronger in its own way, which makes sense. But what about high reps vs low reps: growth differences?

High Reps vs Low Reps: Growth Predictions By People In The Office

First, let’s see what people in the office think when it comes to high reps vs low reps for muscle growth.

Max:

"I gotta go with team heavy.

On the light side, there's kind of more of a mental aspect to it. And, you know, you might start to feel tired and then not get to the point where you're actually getting to failure.

So you're calling me a bitch? Exactly.”

Ethan:

"So I think it'll be light. I think that the light side will look a little leaner and more toned, but the heavy side will be bigger and stronger. Take it to the bank.”

Results

To make sure the “pump” and swelling on our light sides didn’t skew the results of our high reps vs low reps experiment, we both stopped training for 3 days before the scans.

Jeremy Results

The MRI confirmed my brain didn’t grow — but thankfully, my muscles did.

Overall, I gained about 2 pounds from the high reps vs low reps experiment. A bit of that was fat, some was water, but just over half a pound of it was actual muscle (around 0.66 lbs).

But here’s the real question: did one side grow more than the other? I.e., which won when it came to high reps vs low reps?

Surprisingly, my lighter-weight side grew just slightly more in almost every muscle. But the difference was tiny — sometimes as little as 15 grams. That’s nowhere near statistically significant, and honestly, you wouldn’t even notice that difference if you trained this way for a full year.

Both the ultrasound and circumference measurements told the same story about high reps vs low reps.

That said, there was one muscle the MRI picked up that actually grew more on the heavy side. You can’t see this muscle, but it did create a pretty big imbalance, one that I think led to some serious problems after the high reps vs low reps experiment ended.

It’s the iliopsoas.

This muscle helps stabilize your hips and back, and it grew on my left side but actually shrank on my right side.

My hypothesis is that because of the heavy one-sided leg exercises I did during the high reps vs low reps, this muscle worked harder on my left side to stabilize the heavy weight. And unfortunately, after the experiment, I went against my own advice and went straight back into my normal heavy deadlifts … and ended up tweaking the right side of my back for the first time in my life.  

I may have to spend some time after the high reps vs low reps experiment correcting these imbalances, but now let’s see how Dennis’ results compare. 

Dennis Results

Turns out, he gained 3 lbs in muscle. And of that 3 lbs, pretty much of all it was muscle.

And his fat, if anything, dropped a little bit. His Day 1 body fat was 18.4%, and now, it's 17.8%. Because he added more muscle, his body fat actually dropped.

But now the question is, of those 3 lbs, where are they going? Or, in other words, when it comes to high reps vs low reps for muscle growth, which turned out better for him?

For his arm circumference measurements, his left arm grew by 0.8 cm, which is really good. And his right arm grew by 1.09 something cm — a tiny bit more. His left leg grew by 0.3 cm, and his right leg by 0.4 cm.

Every muscle on Dennis after the high reps vs low reps experiment trended toward the lighter side growing just slightly more.

But again, the difference was tiny, well within measurement error and nowhere near statistical significance.

So while there was the smallest trend toward the lighter side, probably just because it involved more total volume, as a scientist, I’d have to call it even.

TL;DR

• High reps vs low reps (light weight vs heavy weight) for muscle growth is one of fitness’s most argued topics, and both camps claim hypertrophy wins if you train hard enough.
• A unilateral, 60-day test matched sets and effort on both sides — 3–6 reps to failure vs 15–30 reps to failure — tracked by MRI, DEXA, ultrasound, and circumferences to compare true high reps vs low reps outcomes.
• The light side accumulated more total volume and produced more burn/pump; the heavy side progressed load more easily (smaller % jumps), spotlighting practical differences in high reps vs low reps even with effort matched.
• Heavy loads can irritate joints/tendons if exercise selection or recovery falter; with light loads, the size-principle means big fibers aren’t fully recruited unless you push near true failure, a key context for high reps vs low reps programming.
• Jeremy gained ~0.66 lb of muscle with a tiny, non-significant edge to the light side; Dennis added ~3 lb of lean mass with the same slight trend — net takeaway: high reps vs low reps delivered essentially equal hypertrophy, while strength gains were movement-specific.

Moving Forward

So, going forward, how would I prefer to train? High reps vs low reps?

Do I want to be happier during training? And on my ego, you know on my left side i can go more weights more weights, but left side … I might do the heavy side.

And remember, there is a middle middle that we didn't touch.

You know what I mean? We did like three to six reps, and then we did 15 to 25. 

But the range right in between — around 6 to 15 reps with a moderate weight — is what I believe is the sweet spot.

It’s light enough that your joints don’t get beat up, but heavy enough that every set doesn’t feel like a mental grind. That’s the rep range I’ll use for most of my training. But I’ll still sprinkle in a few very heavy sets and a few lighter, higher-rep sets, since this variation could very well provide an added benefit.

But the big lesson from this whole experiment is simple: how much weight you use matters far less than how much effort you put in and the form you’re using.

It’s how so many of our app members have transformed their bodies, with a completely done-for-them system.

You can try the app out for free for 2 weeks here:

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