In the world of exercise, variety abounds. You can lift light weights, or heavy weights, or no weights at all. You can run fast, or run slow, or strap on a pair of roller skates. If you don’t exercise at all right now, it doesn’t matter much where you start. But that doesn’t mean that all of these options are equivalent.
Unfortunately, all the nuance gets collapsed when people say, as this Washington Post article recently did, that light weights are “as good as” heavy weights for exercise. Or that, “it doesn’t matter whether the weights are heavy or light,” as this UPI article put it.
Friends, it does matter. You might decide that lighter weights are best for you, or you might want the benefits that heavy weights provide. But you deserve to know the difference. Because there is definitely a difference. I’ll point out that the two studies cited in that Washington Post article both noted greater strength gains in people who trained with heavier weights. One was a meta-analysis gathering the results of 178 different studies, and here’s what it found:
Higher-load (>80% of single repetition maximum) prescriptions maximised strength gains, and all prescriptions comparably promoted muscle hypertrophy [growth].
In short: light weights can help you gain muscle (with caveats, which we’ll discuss) but they aren’t a good way to build your strength, in the sense of your real-world ability to lift heavy things.
When you should use light weights
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You can do a lot with light weights, and I’m not knocking them. They’re especially great for beginners, since they’re easy to handle and not too intimidating. If you’ve never lifted a weight before, it makes sense to grab the 5-pound or 10-pound dumbbells rather than heading straight for the squat rack.
If you’d like to continue with light weights past the beginner stage, that can work too—depending on your goals. It is true that, as we’ve reported before, you don’t need to lift heavy weights to build muscle. Light weights, which I’ll define here as anything you need to lift for 12+ reps before you start to feel any burn or fatigue, can signal your body to build more muscle tissue as long as you keep lifting until you literally cannot lift them any more. That’s called lifting “to failure.”
The problem is, lifting light weights to the point of failure is boring as hell. It’s also easy to stop short of failure, because you’re tired and you want to quit. If you’re capable of lifting 20 reps with a certain weight, but you stop around 12, you’re going to miss out on most of those muscle-building benefits.
We also know that most people underestimate themselves, doing fewer reps or choosing a too-light weight for their intended exercise. If you aren’t constantly asking yourself “can I do more?”, you might be missing out on the muscle growth (or “toning”) that you’re hoping to get.
When you should use heavy weights
A muscle’s size and its strength are two different but overlapping concepts. A bigger muscle does tend to be a stronger one, and vice versa. But if you want to be able to lift something heavy in real life—like a 50 pound bag of cement at Home Depot—a person who trains with 50-pound weights is going to have an easier time of it than a person who never has never picked up a dumbbell bigger than 10 pounds.
Remember how light weights need to be lifted to failure to stimulate muscle growth? That’s because our bodies can choose to only “recruit” a few muscle fibers at a time to do a job. If you pick up a 2-pound dumbbell, your nervous system says “ehh, we only need a few motor units to do this job” and doesn’t bother activating the rest. But as you reach your 18th, 19th, 20th rep, it has to recruit more and more of those fibers as the ones you used at first begin to tire out.
With heavy weights, though, you end up recruiting large numbers of muscle fibers right from the start. I’m glossing over a lot of the science here, but this article from the National Strength and Conditioning Association lays it out in more detail if you’d like to dig deeper.
Heavier weights tell your body to grow the muscle and they teach your muscle fibers and your brain how to work together. The first time you try a new exercise, it will seem hard. After a few weeks, or maybe even a few days, you’ll be able to move a lot more weight more smoothly—even if you haven’t grown any extra muscle tissue by that time.
You need to work with heavy weights if you want to learn to move heavy weights. Many people also prefer heavy weights because each set of an exercise is over in just a few reps—maybe eight or 10, or in some cases as little as a single rep.
You also don’t need to go all the way to failure when you’re training with heavy weights, which is something that lifters often appreciate. If I have 200 pounds on the barbell for squats, I might technically be able to squeeze out seven reps. But my workout for the day might only call for four or five. I find that a lot more enjoyable than doing 20 goblet squats with a dumbbell. The dumbbell is lighter, but using a heavy barbell means I get to stop before I’m exhausted.
How to combine the benefits of heavy and light weights
As with many things in life, the “why not both?” approach is best for most people. Strength athletes (including powerlifters, Olympic weightlifters, and Crossfitters, to name a few) will typically center their routines around a few heavy lifts, and then get in some extra work with lighter or medium-ish “accessory” lifts. That’s still a solid approach for the average person who just wants to lift for fun or for health.
It’s also important to remember the “light” weights need to get heavier over time, as you get stronger. Even Jane Fonda, remembered for eight-count exercises with small dumbbells, told viewers in the introduction to her famous video that they’ll want to switch out for heavier weights as they gain experience. This is, in a nutshell, the concept of progressive overload.
Meanwhile, “heavy” just means any weight that doesn’t take dozens of reps to get results. If you can only do about 10 pushups, then pushups count as a “heavy” exercise for you. Going for something heavy on a compound exercise (one that involves multiple body parts) gives your body a loud signal to increase your strength, while leaving you plenty of time for lighter exercises if those are how you prefer to spend your gym time.