When to Use Light Weights and Heavy Weights During Your Lifting Sessions

You've likely heard that when it comes to building muscle mass, heavy weights are preferred over lighter weights. And if you're just trying to improve general muscular fitness, you're better off doing a higher number of reps using lighter weights, right?

As it turns out, what to lift when and for what purpose is a hotly debated topic in the fitness industry. Here, experts break down the science and reveal when, exactly, you should use light weights or heavy weights in your strength training routine.

Should You Use Light Weights or Heavy Weights?

Despite the simplicity of the question, the jury is still out on whether you should choose light weights or heavy weights for your strength training workouts. Currently, the research available on the topic is conflicting. A 2016 study, published in the Journal of Sports Science & Medicine, found that when the number of sets is equal, training with heavy weights may be best to maximize muscular strength (re: the greatest amount of force a muscle can produce), while training with moderate weights may be best to increase muscle size.

However, findings published in the Journal of Strength & Conditioning Research in 2015 diverge from those conclusions. In this study, a small group of men who were experienced in resistance training were tasked with performing different exercises using either a lower weight for 25 to 35 reps per set or a higher weight for just eight to 12 reps per set. At the end of the study, the participants who used heavier weights for fewer reps had significantly greater improvements in back squat strength and one-rep max for the bench press, but the folks who used lighter weights for a high number of reps had greater improvements in upper-body muscular endurance. The kicker: Both groups had similar increases in muscle size, according to the researchers.

What's more, it may not be just about choosing a light weight or a heavy weight but also working the muscle to total failure or exhaustion, says Michele Olson, Ph.D., F.A.C.S.M., C.S.C.S., a senior clinical professor at Huntingdon College in Montgomery, Alabama. "If you work your muscles on each set until you cannot execute another repetition with appropriate form, the heaviness of the weight becomes less important."

Take one study published in the Journal of Applied Physiology. It found that regardless of how heavy (or light) people lifted, they increased strength and decreased body fat in similar amounts when they lifted to near failure. This concept of working to failure — regardless of how much weight you're lifting — is not new, adds Olson. And the American College of Sports Medicine advises that people choose a weight that feels like an 8 out of 10 in terms of difficulty, doing three sets of eight to 12 reps with good form, and ensuring the last rep of each set feels challenging to complete.

TL;DR: Choosing a heavy weight may help you improve muscular strength, while a light weight could build your muscular endurance, research suggests. The best load to amp up muscle size, though, is uncertain. Still, lifting until near failure, no matter what weight you're using, could play a key role in helping you increase muscle strength and size, according to experts and research.

4 Points to Consider When Lifting Weights

The light weights vs. heavy weights debate isn't necessarily settled, and load isn't the only factor you should consider. "Everything matters: Sets, reps, tempo, rest, and exercise selection will all determine what kind of results you get from your weight lifting," says Lawrence Betz, C.S.C.S., the former director of the Brooklyn Athletic Club. So what other points should you keep in mind while lifting instead of hyper-focusing solely on light weights or heavy weights?

Know What 'Heavy Enough' Feels Like

As a general rule of thumb, if you're using adequate load during an exercise, ideally, the last one or two repetitions of a set should feel seriously challenging to complete with proper form, says Jessica Matthews, D.B.H., M.S., an associate professor and program director in the College of Health Sciences at Point Loma Nazarene University. If you power through your last rep without breaking a sweat, take it as a sign to up your load.

Reconsider What 'Light' Weight Means

Most folks could probably benefit from reconsidering what they count as "light," says Olson. For the average 145-pound woman, a maximal squat is about 130 to 135 pounds. Therefore, lifting a "light" amount in this instance would equate to doing about 25 to 30 reps holding 15- to 20-pound dumbbells. "Most women consider a dumbbell of about 10 pounds to be heavy," she says.

Focus On Volume

"The general consensus has always been that volume (how often you lift and how many reps you do) is the most important factor in seeing results from lifting weights," explains Dan Roberts, a celebrity strength and conditioning coach, trainer, and creator of the online fitness plan Methodology X. "No one thinks you need to lift incredibly heavy weights to get stronger."

So rather than focusing on lifting heavy all the time, it's also important to focus on lifting enough, he says. For example, instead of back squatting as heavy of a weight as you can handle for one rep every time you work out, try lightening your load to something you can do for 8 to 12 reps a few times a week. Essentially, you're decreasing the load and increasing volume so you can train your muscles to work for an extended amount of time (re: building muscular endurance, which helps delay fatigue).

Keep an Eye On Your Form

How you're actually lifting the weights is also important, says Betz. Make sure you're not trying to lift so heavy that you have bad form, which can potentially lead to strains and injury and make the exercise less effective. No matter how heavy you're lifting, include appropriate rest periods, too. Otherwise, you're just headed for added injury instead of added muscle. Ouch.