Light or Heavy Weights for Muscle Growth and Strength

Should you use light or heavy weights if you want to build muscle and strength? Perhaps moderate weights trump them both? For years or even decades, the question of strength training load has been contested. Many studies and scientific articles examine the subject, not to mention the heated discussions in gyms between proponents of one approach or the other.

A new review and meta-analysis weighs in with the latest, and perhaps the most significant, research update.¹ In this article, we analyze the analysis and see how the current scientific winds blow. Science isn’t static. New findings and new angles of approach merit further studies and expand our knowledge.

The new meta-analysis on the topic is likely the most comprehensive one yet. Also, it examines some parameters in a fresh light. In other words, it’s time to dive head-first into the training-load pool once again.

Light or Heavy Weights? What About Moderate?

Engaging in strength training offers tremendous positive effects. Not only do you strengthen your body and your muscles, allowing you to lift impressive poundages and build bulging biceps. In addition, your health makes serious gains as well, and you lay the foundation for keeping age-related issues at bay.

You have many tools at your disposal for manipulating the results of your efforts in the gym. Some of these include your training volume, your training frequency, and last but not least, your training intensity.

When you hear “training intensity,” you might get the mental picture of keeping your set rest to a minimum and working out at a rapid pace. However, that’s not what high intensity means in this context. Instead, high-intensity strength training means using heavy weights. The closer to your one-repetition maximum, or 1RM, you lift, the higher your training intensity.

Your muscles grow bigger and stronger independent of load, as long as you train to or close to failure. That’s pretty well-established. However, the optimal approach to strength-training intensity for strength and hypertrophy is still an unknown entity.

If your goal is to get stronger, training heavy is more effective than using light weights. That is because you recruit most, if not all, the motor units in the working muscles right off the bat. So even though doing many reps with a lighter weight is at least as tiring, there is little doubt that using a heavier weight makes you gain more strength.

As for building muscle, science doesn’t offer nearly as clear-cut an answer. The “repetition continuum” theory suggests that using heavy weights and low reps is the strength zone. Performing many reps with light weights is the muscle endurance zone. Therefore, using moderate weights with a moderate number of reps is the hypertrophy zone.

Recent research challenges that classic model. We now know that many other factors than motor unit recruitment play essential roles for muscle growth. Regardless of which mechanisms affect hypertrophy, most current studies can’t find any difference in muscle growth depending on your training load. Heavy, moderate, or light weights lead to similar muscle mass gains as long as you reach failure.

The New Meta-Analysis

After 80 years of research, we have plenty of studies looking at the effects of training load to go through. In particular, studies comparing the results of heavy and light-load training on strength and hypertrophy are, if not a dime a dozen, at least reasonably common during the last two decades. The purpose of the new review is to analyze the effects of strength training to failure using light, moderate, or heavy loads on muscle growth and strength gains in healthy adults. In addition, it examines if and how differences in study protocols, participant status, and other training-related factors affect the outcomes.

Low-load training or light weights: below 60% of 1RM or more than 15 reps.
Moderate-load training: between 60 and 79% of 1RM or 9–15 reps.
Heavy-load training or heavy weights: at least 80% of 1RM or eight reps or fewer.

The meta-analysis defines muscle hypertrophy as an increase in muscle mass or size—nothing unexpected there. Of course, we can measure strength in many different ways, but here the researchers only looked at 1RM-tests in various lifts.

The first search through the available research revealed close to 6,000 potentially relevant studies. The researchers discarded duplicates and studies in languages other than English. They also removed studies without enough information about the training protocols or loads used, studies that didn’t properly assess the training and participants before and after the training, studies shorter than six weeks in duration, and studies using blood flow restriction training.

By the way, there is nothing wrong with blood flow restriction training. On the contrary, it’s quite a promising field of research, but it deserves an analysis of its own. Here, the researchers focused on traditional strength training.

After eliminating thousands of studies that failed to meet the inclusion criteria, the researchers ended up with 28 studies in a digital pile.

The Studies, the Participants, and the Research Protocols

Combined, the participant count of the 28 studies reached 747 healthy men and women with an average age of 23.4 years. However, most of the studies, 19 out of 28 to be exact, featured male subjects. In other words, only about 30% of the available research features female subjects.
Three-fourths of all participants had no prior experience of strength training. The details of the participant characteristics also reveal that not a single study compares the effects of different training loads on athletes or well-trained subjects. For example, to qualify as a strength athlete, you need to squat double your bodyweight and bench press 1.5 times your body weight. Yet, no studies reported participants able to perform those feats.
Seventeen of the included studies compared light-load strength training to high-load strength training. Four studies examined the effects of light-load training compared to moderate-load training, and five looked at moderate-load compared to heavy-load. Finally, two studies combined all three loading protocols: low-load vs. moderate-load vs. heavy-load.
On average, the studies lasted nine weeks, during which the subjects performed 25 strength-training workouts.
Fifteen studies measured lower-body muscle growth. Eight studies looked at upper-body hypertrophy. Five studies measured whole-body hypertrophy rather than individual muscles, for example, by using DXA (dual-energy X-ray absorptiometry)
Twenty studies documented lower-body strength gains, while 12 looked at upper-body strength. But that’s 32, not 28, you might say. Yes, a couple of studies measured both upper- and lower-body strength. Regardless of muscles or exercises, the researchers measured the strength gains of the participants using 1RM-tests.
Eighteen studies reported total training volume.

Light or Heavy Weights: Results

Let’s move on to what you’re probably here for, the results! Which is the best option for strength and muscle mass: low-rep training using heavy weights or pumping it out using lighter loads? Or, perhaps a compromise in the form of moderate weights and a moderate number of reps gives you the edge? What if none of the above turns out to be superior to the other options? Let’s check it out!

Muscle Growth

Thirty-five comparisons compiled from 24 studies found no load-dependent effects on muscle growth. The participants experienced similar gains in muscle mass from light, moderate, and heavy lifting.

Heavy-load training had a tiny edge over light-load training. However, the researchers concluded that the difference is most likely irrelevant, the effect size being so small.

Also, they performed a subgroup analysis, using only the studies with the best methodological quality. Those comparisons didn’t reveal any differences in muscle growth training with low, moderate, or heavy loads, either.

However, untrained participants gained more muscle than those with strength-training experience– nothing unexpected there. Also, more training, meaning a higher number of training sessions, resulted in more significant gains.

Other parameters, like participant age, study protocol, or sex, did not affect the outcomes or explain individual differences in muscle growth.

Strength

If it doesn’t matter if you train heavy or light for muscle mass, you can’t say the same when it comes to strength.

Thirty-nine comparisons, also from 24 studies, all featuring 1RM-tests, show that both heavy- and moderate-load training increase strength more than light-load training, even light training to muscular failure.

Also, the analysis of the available research revealed that heavy-load training might be a little more effective for strength gains than moderate-load training. However, the difference wasn’t significant, meaning it could just have been the result of chance.

Older studies and studies featuring male participants showed the most significant strength gains by training heavy compared to using light weights. However, other factors, like study protocol, training status, and the number of training sessions, did not explain any recorded differences in strength gains.

Summary – Should You Use Light or Heavy Weights?

So, what did the new meta-analysis reveal? Summarized in key points, this is the takeaway:

Are you new to the gym or a recreational lifter? If that’s the case, you can expect similar muscle growth regardless of how heavy you lift. You build just as much muscle using light weights as you do with moderate or heavy weights. At least if we’re talking about a couple of months. Unfortunately, there are no real long-term studies on the subject. Maybe a difference in muscle growth from training with different loading schemes appears after, say, a year or longer? That remains to be seen.
Another observation reveals that untrained subjects experience a more significant increase in muscle mass than recreationally trained lifters. Not much of a revelation, of course, but it is a prime example of diminishing returns. The returns you get from your strength-training efforts decrease pretty rapidly as you get more experienced. Recent research shows that actual muscular hypertrophy, meaning the addition of new muscle protein, might not be a reality until you’ve spent several weeks lifting.² The initial gains from taking up weight training might not be “real muscle” at all, but edema or water retention. Whatever makes up your first gains, absolute beginners gain the most of it.
Let’s say you’ve passed the beginner’s stage. If that’s the case, you’ll experience more significant gains in muscle mass by increasing the number of workouts you perform during a given time, say, a week or a month. Beginners do not get the same dose-related muscle growth by working out more frequently.
Male or female, and regardless if we´re talking upper or lower body: light, moderate, and heavy training produce similar gains in muscle mass.
As for strength, training heavy is superior to using light weights. That’s probably the least surprising result from the entire meta-analysis. A quick peek at previous research would have revealed the same thing without any advanced statistical calculations.
The results did not change after the researchers performed a sub-analysis using only the best studies of the bunch, meaning those of the highest methodological quality.
The fact that men gain more strength than women from heavy training compared to light training is an exciting find. On the other hand, women gained more strength from moderate-load training than men. Why the difference? That is unclear at the moment, but the researchers speculate that differences in perceived exertion and study methodology might have something to do with it. Also, studies with male participants outnumber studies with female subjects several times over, making it hard to interpret the results.

In summary: if you’re new to the lifting game or have some strength training experience, you’ll likely gain the same amounts of muscle regardless of the training load. However, adding plates to the bar and going heavier is superior for strength gains.

What About Highly Trained Athletes?

Another takeaway from this meta-analysis is that you might not be able to rely on the results once you’ve left the status of recreational lifter behind. You’re not guaranteed to gain similar amounts of muscle from light-load and heavy-load training as an athlete.

No studies compare the effects of light, moderate and heavy strength training on muscle hypertrophy and strength in highly trained subjects. Even when the participants in the available studies had trained for 2–7 years, they had failed to gain enough strength and muscle mass to quality for athlete status.

Beginners experience greater gains than recreationally trained lifters, but what if you’re already an experienced lifter with some serious strength and muscle to show for your years of dedication? Unfortunately, you can only rely on speculations.

At least if you’re training only for muscle mass. If strength is your game, choosing between light and heavy weights is easy. Go heavy, at least for the majority of your training. You get good at what you do, and if you regularly lift close to your 1RM, you get good at lifting close to your 1RM. Light-load training to failure might lead to similar gains in muscle mass through other compensatory mechanisms, but those aren’t relevant for gaining as much strength as possible. If you want strength, you have to move some serious weight. The neurological and muscular adaptations you get from heavy-load training are superior to those you get from light-load training, regardless of your training experience.

Strengths and Limitations

The new meta-analysis is likely the most comprehensive one on the topic yet. Having a large number of studies with a large number of participants is an evident strength. Also, unlike previous reviews, it adds other factors like sex, training status, and training protocol to the mix, giving extra spice to the analysis.

The main limitation is the quality of the included studies. It is, with few exceptions, low. Too often, the studies don’t randomize participants into groups, don’t blind the researchers measuring stuff, and don’t comprehensively present the study results. Those kinds of issues aren’t uncommon in strength training research. That is more an acknowledgment of the sad state of strength-training research over the years, not something the meta-analysis has any say over. Things are getting better, but it’s not an overnight thing and makes it hard to determine the reliability of the results.

The researchers mitigated the problem by performing a subgroup analysis using only the most high-quality studies. The conclusion of that analysis did not differ from the overall results.

Another limitation is the complete absence of well-trained athletes. That is also something the researchers behind the meta-analysis have no control over. Still, it is impossible to extrapolate the results to strength athletes like aspiring bodybuilders, powerlifters, and strongmen. Even when the participants had been strength-training for years, they still weren’t strong.

Several of the included studies performed 1RM-tests many times, up to 5 times throughout a couple of months. That might also be considered a weakness. While it might sound good in theory, it means that participants in the light-load groups actually lifted heavy weights reasonably often. One can speculate that the results would have been different if they had used light weights and light weights only, instead of adding several heavy-load workouts in the form of 1RM-tests during the study.

Training to Failure or Not

Training to failure is not a requirement for building muscle and strength. According to some studies, doing so might offer some benefits, but others fail to find any advantages. As previously mentioned, the new meta-analysis only included studies where the subjects trained to failure.

In a controlled study, failure is one way to ensure that all participants experience at least somewhat similar levels of muscular stress. Thus, if the meta-analysis had mixed and matched failure studies with non-failure studies, it would be hard to compare them fairly. But, of course, in most studies on the subject, the participants train to failure.

Also, you avoid some of the individual differences in the capacity to do many reps with a certain weight. For example, one lifter might be able to do many more reps at a certain 1RM-percentage than another lifter. When everyone trains to failure, you eliminate the problem of having the subjects terminating their sets with different numbers of reps “left in the tank.”

Light or Heavy Weights: Practical Applications

The researchers suggest some ways you can implement the results of the meta-analysis into your training. However, those are limited to concluding that 80% of your 1RM might be a good intensity for gaining both muscle and strength if you, for some reason, can’t or won’t go heavier.

The specificity principle tells us that we have to perform the task we want to excel at. If your main goal is getting as strong as possible, most of your training needs to be heavy-load training, with maybe some light work for variety’s sake. You will build muscle as well. Strength training is conceptually easier than training for muscle mass. There is little doubt that heavy-load training is superior. Therefore, you should focus on heavy training if you’re looking to gain maximum strength.

Using the specificity principle to guide your hypertrophy training isn’t as easy. Building muscle is not a concrete skill you perform.

To build muscle, I firmly believe variety is a crucial factor.

Likely, you build at least comparable amounts of muscle with light-load training, medium-load training, or heavy-load training, as long as you challenge yourself in the gym by training to failure or close to failure. However, using heavy or light weights exclusively introduces other problems.

Always lifting heavy builds a lot of muscle and strength. At least for as long as your body can take it. Constant heavy low-rep training taxes the joints and increases the risk of muscular injuries. If your main goal is building muscle, with getting strong being an afterthought, it sounds downright stupid to lift as heavy as you can all the time. Nothing suggests that you’ll get better results from this practice, but you will increase the risk of injury over time.

Utilizing light-load training with lots of reps to failure all the time might build the same amount of muscle as using heavy weights. However, not many can handle performing 20–30 reps all the time, especially in multi-joint exercises. High-rep training hurts. Not in a way that signals injury, but the pump and lactic acid in your muscles will soon make you want to cry. And knowing that every workout involves dozens of sets of 20–30 reps is enough to give anyone pre-workout anxiety. Training sessions fueled by anxiety rarely lead to productive results in the long run.

Say you had to choose between heavy-load, light-load, and moderate-load training and stick to that choice. I’d recommend taking the road of moderation. It’s the best of all three worlds. You get bigger and stronger, but without the same risk of injury as constantly training heavy. Also, you avoid the mental stress of always training light to failure.

But why not vary both your training load and the number of reps you perform?

You could split your training schedule into blocks of weeks or months over the course of a year, each block consisting of heavy, light, or moderate-load training depending on your current goals.

Another way to introduce variety into your training is to train both heavy and light within the same workout. For example, start your chest workout with heavy bench presses. Next, switch to moderate-weight incline dumbbell presses. Finish off with some lighter work using cables or machines where you go for the pump. A complete chest workout and you incorporate all three loading schemes.

Training to failure can be an effective method to gain muscle mass and strength, but far from the only one. Even if this meta-analysis only includes studies in which the subjects go for failure, we don’t know if failure is the long-term solution for great gains. However, one thing is clear. Constantly training to failure is more stressful than terminating your sets a rep or two before reaching failure. If you enjoy training to failure, go for it. But, be careful not to ignore when and if your muscles and nervous system tell your body that you might need to cut back on the intensity for a while.

Also, remember that current research on the topic only examines the effects on untrained and recreationally trained individuals. Almost no research on strength athletes like bodybuilders and powerlifters exists. In other words, the meta-analysis suggests that you build as much muscle training light as by training heavy. However, that conclusion might not apply if you have been training for years and are already strong and muscular.

Conclusions

Untrained and recreational lifters build similar amounts of muscle using light-load training, moderate-load training, and heavy-load training, as long as they train to failure.
You get stronger by lifting heavy weights compared to lifting light weights.
Once you’re past the beginner’s stage, you need to train more to keep gaining as much as possible, for example, by performing more training sessions per week.
The available scientific research is lacking. The results aren’t necessarily applicable to strength athletes and bodybuilders. Neither do we know if they extend beyond longer periods than a couple of months. However, the conclusion that you need to train heavy to get strong feels like a safe bet.

I recommend that you don’t focus exclusively on certain types of training or specific repetition ranges. Sure, if you want to get strong, your training must include a lot of heavy lifting. However, even then, it’s probably wise to give your body a break from the heavy weights now and then. For example, if building muscle is your primary goal and you don’t care too much about getting as strong as possible, you probably benefit from skipping super-heavy low-rep training. Instead, go for moderate to heavy loads mixed with lighter sessions where you chase the pump.

If you train for health and well-being, with hypertrophy and strength gains being more of a bonus, feel free to use whatever loads you enjoy lifting the most. Vary between light, moderate and heavy training to your heart’s content. Regardless of how you prefer to engage in strength training, it’s one of the best activities you can engage in, both for body and mind.