Browse evidence-based analysis of health-related claims and assertions
Some people build more muscle from weight training than others, and it might be because of tiny molecules in their bodies that control muscle growth genes. One study found these molecules act differently in big vs. small muscle gainers, but we’re not totally sure yet.
Mechanistic
People whose muscles grow a lot with training might get that boost because of special cells that help muscle fibers expand, while those who don’t gain as much might not see the same cell activity — but scientists aren’t fully agreed on this yet.
People who build more muscle from weight training tend to make more ribosomes—the tiny machines in cells that build proteins—compared to those who don’t gain as much muscle. One study found big gains in these protein-makers in strong responders, and lab studies back up that this process helps muscle grow.
Correlational
If you're new to working out, doing leg exercises for 10 weeks can make one of your big thigh muscles grow bigger, even if the individual muscle fibers don't get thicker.
Quantitative
Doing 10 weeks of leg exercises that strengthen your knee muscles boosts one specific hormone receptor in your muscles (ERα), but doesn’t change two others (AR and ERβ).
Causal
If you're new to strength training, having more tiny blood vessels around your fast-twitch muscle fibers might help those muscles grow more when you start working out.
People new to weight training who have more of a certain protein in their muscles (called ERα) tend to gain more muscle size after 10 weeks of leg exercises — so this protein might help explain why some people build muscle faster than others.
Doing full-body strength training three times a week for three months helps older adults, even those over 85, gain about 2% more muscle — and it works just as well for the very elderly as it does for the 'younger' elderly.
Older adults, even those over 85, can gain just as much leg strength as younger seniors from 12 weeks of regular weight training — and some even gain a bit more!
Doing full-body strength training three times a week for three months can boost thigh muscle size by about 10–11% in older adults, even for those over 85 — showing that very old age doesn’t stop muscles from growing with exercise.
Women after menopause who have higher levels of a protein called GDF-15 in their blood tend to be weaker in their legs and walk slower, especially if they're healthy and haven't had cancer.
Doing strength training for 12 weeks may help postmenopausal women — both healthy ones and breast cancer survivors — move better, walk farther, and get up and go faster.
Doing strength training for 12 weeks can boost muscle strength by about a quarter to nearly half in women after menopause — whether they're healthy or have survived breast cancer.
If postmenopausal women do strength training three times a week for three months, their thigh muscles get about 5 to 8% thicker — whether or not they’ve had breast cancer.
Doing strength training three times a week for three months helps both healthy postmenopausal women and breast cancer survivors gain muscle and get stronger — and both groups benefit about the same.
Lifting weights, using machines, or doing static holds all seem to build leg and arm muscles just as well — there's no proof that one way is better than the others for getting bigger muscles.
Descriptive
Doing more strength training might help your muscles grow bigger, especially in your thighs and arms, but we're not exactly sure how much is best because there hasn't been enough research comparing different workout levels.
All three types of muscle movements—squeezing, lengthening, and holding—can help build muscle in your thighs and arms just as well, as long as you train hard and often enough.
If middle-aged Japanese women who are doing strength training start eating more protein at breakfast—especially if they weren’t eating much before—they’re likely to gain more muscle.
Working out with weights twice a week for four months can help middle-aged Japanese women build a little more muscle, even as they get older.
If middle-aged Japanese women eat more protein at breakfast over time while strength training, they may gain more muscle — but if they already eat a lot of protein at breakfast to start with, they might not gain as much.
Lifting weights helps your muscles grow, especially when you eat after exercising — together, they give your muscles a big boost and help prevent muscle loss from aging or not moving enough.
As we get older or if we're inactive or sick, our muscles don't build new protein as well after eating — and that's a big reason why we lose muscle, not because of changes in how our muscles work when we're not eating.
Your muscles stay the same size over time because your body constantly builds and breaks them down. Eating tips the balance toward building, while not eating tips it toward breaking down — and over time, these ups and downs balance out.