Browse evidence-based analysis of health-related claims and assertions
All the studies looked only at leg muscles in young people — we don’t know if the same results apply to arms, women, or older adults.
Descriptive
Whether you lift light weights or heavy weights until you can’t do another rep, your muscle fibers don’t seem to grow significantly more with one than the other — but we can’t be sure because the results are too uncertain.
Quantitative
Just because one exercise makes your muscles 'fire' more during the first workout doesn’t mean it’ll make them grow more over time—muscle activation during a single set doesn’t tell you what will work best for building muscle.
Causal
Whether you train with squats or hip thrusts, you’ll get just as better at deadlifts and pushing against a wall—both exercises improve real-world leg strength equally.
If you want to get stronger at squats, you need to squat; if you want to get stronger at hip thrusts, you need to do hip thrusts—training makes you better at the exact movement you practice.
Squats make your thighs bigger—especially the front and inner muscles—while hip thrusts don’t really change your thigh size, making squats better if you want to build overall leg muscle.
Doing hip thrusts or squats for nine weeks, with the same total amount of work, makes your butt muscles grow just as much—so neither exercise is clearly better for building glutes if you're new to lifting.
When you lift weights, your body’s protein-making response doesn’t change much from week to week — but it’s very different from person to person.
Just because your muscles get bigger from lifting weights doesn't mean you'll get much stronger — the two don't really go hand in hand.
Correlational
People grow muscle at very different rates — but each person tends to grow their arms and legs similarly, no matter what weight they use. This means your genes and biology matter more than how heavy your weights are.
Your muscles make more protein when you start lifting weights, but after a few weeks, that boost fades — even if you keep lifting harder — no matter if you use heavy or light weights.
When people lift weights until they can't do another rep, their muscles grow about the same amount whether they use heavy or light weights — and how much they grow depends more on their own body than on how heavy the weights are.
Doing bicep curls with one arm makes that arm stronger for one-arm curls, but doesn’t make you stronger for two-arm curls or the other arm.
Training your right arm alone makes your right arm stronger, but doesn’t help your left arm get stronger — the benefit stays on the side you trained.
Training one arm at a time doesn’t make you stronger when lifting with both arms together — your total strength gain is the same as training both arms at once.
Whether you lift weights with one arm or both at the same time, your biceps grow about the same amount after 8 weeks of training.
Doing bicep curls one arm at a time makes your right arm stronger than doing them with both arms together, but doesn't make your biceps bigger than using both arms.
Older women who lift weights—even just one arm or leg at a time—gain muscle and fight off the natural muscle loss that comes with aging.
Training one arm or leg at a time helps older women build muscle just as well as training both at once—but it doesn’t help them use both sides together any better.
When older women try to push with both legs at once on a leg press or pull with both arms on a lat machine, they’re weaker than when using each side separately—but this doesn’t happen when they extend their knees.
Whether older women train one arm or leg at a time or both together, they both gain muscle—just as much as each other—and both are better than not training at all.
When older women train by pushing or pulling with both arms or legs at the same time, they get better at using both sides together, making their strength more efficient.
Training with squats or leg extensions didn’t make the upper or middle parts of your outer thigh muscle grow much — only the bottom part did, and only with squats.
Squats make the lower part of your outer thigh bigger, while leg extensions make your front thigh muscle bigger — your muscles grow in different spots depending on how you train them.