Browse evidence-based analysis of health-related claims and assertions
The way your muscles fire when you do leg presses vs. leg extensions matches up with which muscles end up getting bigger after training.
Descriptive
Leg presses make your butt and inner thigh muscles bigger, but leg extensions don’t affect those muscles at all.
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.
Correlational
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.
When measuring overall body muscle growth by weight (not direct scans), shorter rest periods don’t seem worse — and might even be slightly better — because the tools used aren’t precise enough to see real muscle changes.
Quantitative
Whether you push your muscles to complete exhaustion or stop a few reps short doesn’t change whether longer breaks between sets help you build more muscle.
Taking a bit more than a minute between sets when lifting weights might help your arms and legs grow slightly more muscle, but the difference is so small it might not matter much in real life.
When you eat more protein after losing weight, your body uses more of it and holds onto more muscle instead of breaking it down.
Causal
After losing weight, your body tries to burn fewer calories than it should — but eating more protein helps your metabolism return to normal, while eating less protein keeps it slowed down.
Eating more protein after losing weight makes your body burn more fat for fuel instead of carbs, which shows up as a lower respiratory quotient.
When you train your quads, the big front muscle (rectus femoris) grows more than the side muscles (vastii), even when you do the same total amount of work.
Even though the deep thigh muscle (vastus intermedius) isn’t the biggest or most active, changes in how its fibers are angled are surprisingly linked to how much stronger people get during dynamic leg movements.
After 10 weeks of lifting weights, whether you rest 20 seconds or 2 minutes between sets, your thigh muscle gets about 15% bigger and you can lift about 40% more weight.
The more your body slows down its calorie burning after weight loss, the more likely you are to eat more than you burn — which makes it easier to gain weight back.
People who naturally have a longer lever arm between their kneecap and knee joint tend to get stronger in eccentric movements (like lowering a weight) after training, likely because their anatomy gives them a mechanical advantage.
You can do more sets with shorter rests to match the total work of fewer sets with longer rests—and you’ll still get the same muscle growth and strength gains.