I once went to an indoor cycling class with a guy friend who took one look at the 2-pound dumbbells under his seat and rolled his eyes at the lightweights, swapping them out for 5-pounders. When it came time for the arms section—a high-rep cycling intermission full of exercises like shoulder presses, triceps extensions, and lots of pulsing—he grabbed those 5-pound weights and punched right along to the first song. My arms hurt just to think about doing that for five minutes straight. Spoiler: He couldn't even make it through half the moves with those weights. By the time we reached the end of the arms section, he was really hurting, and his arms spent a lot more time at his sides than in the air.
Lifting light weights for many reps is deceptively hard. While many people (my friend included) think heavier is always better for resistance training, that’s not necessarily true. And if you only ever focus on lifting one way—light-weight-high-rep or heavy-weight-low-rep—you’re doing yourself a real disservice. The truth is that both heavy and light weights have their place—it depends on your goals.
Lightweights are good for building muscular endurance.
When you train using higher reps, you use aerobic energy more than when training with lower reps. Using higher reps is really good if you train for any kind of endurance sport.
Distance running, cross-country skiing, obstacle races, rowing, and triathlon are all examples of endurance sports. Whether it's a hike in a nearby park or up Mount Kilamanjaro, at no point will your legs have to carry anything close to their maximum load. You’re really just training to get your body weight (and maybe a backpack) uphill.
What does high reps mean? In general it’s a weight you can lift for 15 reps or more. But if you can knock out 30 reps and aren't tired at all, it’s just too light. You still need to train the muscle to some sort of fatigue, or failure, and maybe not completely where you can’t get the last rep up, but you should feel pretty close to that.
Heavyweights are good for developing strength and targeting specific muscles.
If you need power—for a bench press personal record, CrossFit, or to squat your body weight—you need to train with heavier weights. Similarly, if you want to work on one specific part of your body, for instance, your butt, lifting heavier weights can get you the results you're looking for. Depending on your goal, you should choose a heavier load on exercises that target the body part you want to strengthen or sculpt.
Lifting for pure strength is best partnered with heavy weights. If you're trying for strength, or your max force output, the heavier the weight, the more strength gains you'll have, along with size gains. It's also super time efficient. You simply don't need to do as many reps when you're lifting heavier weights.
Really, any form of strength training can be beneficial—the key is to challenge yourself.
You can gain muscle and change the shape of your body by lifting heavier weights for fewer reps, or lighter weights for more reps. Both are equal when it comes to gaining muscle. The key is challenging your body with progressive overload.
Progressive overload means challenging a muscle to continue seeing results. If you never change the weight or reps, the muscle will no longer adapt. To keep seeing results, you have to keep forcing your muscles to adapt, and one way to do that is with a principle called progressive overload, where, over time, you increase the weight and/or reps for a given move. And that could be just one rep or 1 pound more.
No matter what, having good form is super important.
Lifting heavy can cause injuries when your form isn't right, when you push yourself too hard and exceed your limits, or when you don't have a spotter. Lifting lighter weights over and over with improper form can also do harm.
The one thing to really watch out for is your form. When you lift lighter weights, you can focus more on perfect posture and alignment for each move. The catch is, you still need to focus on your form when lifting heavy. If your form gets really messy on your last few reps, you should probably lower the weight.
When you’re first learning an exercise, lighter weights can help you fine-tune your form. If you’re unfamiliar with a certain exercise and you’re just learning it, higher reps gives you more time to learn the exercise. As a trainer, I start with that. It makes it easier to teach, so there is more time to practice the movement. The focus is not on the load, but on learning it properly.
Confused? Here’s exactly what to do.
Should you lift light weights or go heavy? Or incorporate both into your training? Here are the general guidelines from the National Strength and Conditioning Association, which should be customized based on your goals and background:
If you’re training for muscular endurance: Do 12 or more reps per set.
If you’re training to gain muscle size: Do 6 to 12 reps per set.
If your goal is to increase strength: Do 6 or fewer reps per set.
If you’re training for general health, you can mix it up if you want to. If you're going to go to the gym only two times a week, which is not uncommon, then I would advise you to do a total-body workout both times.
The most important thing is to keep changing either reps or weight to continue to see results. You can build your body with lighter loads and with higher reps. It really comes down to a personal preference. There's a lot that works, so work out regularly and at an intense, challenging level, do what you prefer, and do something you're going to stick with.
If you have joint issues, like arthritis, or are obese, you should consult your doctor first.
Before starting any fitness routine, it’s always a good idea to check in with your doctor first. Heavy lifting can put pressure on your joints, so if you have any pre-existing medical conditions, you’ll want to consult a fitness professional. Talk to your doctor, who may refer you to a personal trainer or a physical therapist who can prescribe the right program for you.