Browse evidence-based analysis of health-related claims and assertions
Some people have much more active brown fat than others—up to 10 times more—based on how much glucose their fat tissue takes up after cold exposure.
Descriptive
Just because CrossFit and weightlifters jumped about the same height doesn’t mean they’re equally powerful — the test might not have been sensitive enough to spot a real difference.
People who do CrossFit and people who lift weights end up with about the same size leg muscles after years of training — the type of workout doesn’t seem to change muscle growth much.
Correlational
People who do CrossFit, lift weights, or just stay active all generate about the same peak power when jumping — no one group is clearly stronger or more powerful than the others.
The way your muscles fire when you do leg presses vs. leg extensions matches up with which muscles end up getting bigger after training.
People who do CrossFit-style workouts can jump higher than people who just exercise casually, but they don’t jump noticeably higher than people who lift weights regularly.
Leg presses make your butt and inner thigh muscles bigger, but leg extensions don’t affect those muscles at all.
Muscles don’t grow evenly — some parts get bigger than others, and different muscles in the thigh respond differently to the same exercise.
Causal
Drop sets make you lift more total weight, but that doesn’t mean you get stronger or bigger all over — just in one specific part of one muscle.
Drop sets don’t make the side muscle of your thigh bigger than regular sets — no matter where you measure it.
Even though drop sets make you do more total work, they don’t make you stronger than regular sets when you’re just starting out with leg extensions.
Doing leg extensions with drop sets (going to failure, then lowering the weight and going again) makes the front part of your thigh muscle bigger in the upper and middle areas, but not the bottom part or the side muscle.
The tiny harmful molecules made when your muscles burn during exercise might actually help them grow by sending growth signals — but too many can hurt instead.
Mechanistic
When your muscles swell up from the burn during a workout, that puffiness might trick your cells into thinking they’re being stretched, turning on growth signals — like a fake workout signal.
When your muscles get tired and burn during exercise, they release signaling chemicals (like IL-6) that might help repair and grow muscle by activating nearby stem cells — but we’re still figuring out if this really happens in people.
Even though lifting weights makes your body release growth hormone and testosterone, studies show that these short-term hormone surges don’t actually make your muscles grow bigger over time.
When you wrap your arm tightly and lift light weights, your muscles still grow — and your body turns on the same growth signals as when you lift heavy weights, which hints that the burning feeling might be key.
When you lift weights until your muscles burn, the buildup of waste products might help your muscles grow bigger, but scientists aren't sure yet if it's actually causing the growth or just happening at the same time.
To test if rest time matters, researchers made sure both groups did the exact same amount of total lifting—so any differences (or lack of them) must be due to rest time, not how much they lifted.
By having each person train one leg with short rests and the other with long rests, researchers can be more sure that any differences (or lack of differences) are really due to rest time, not individual differences.
If you lift the same total weight, your quads grow just as much whether you rest 20 seconds or 2 minutes between sets—so you don’t need to feel the burn to build muscle.
Even if you rest only 20 seconds between sets, you can still get just as strong as someone who rests 2 minutes—as long as you do the same total amount of lifting.