Browse evidence-based analysis of health-related claims and assertions
After 8 weeks of either heavy or light lifting, the middle part of the outer thigh didn’t get any thicker—no matter which way they trained.
Descriptive
When you lift weights, how much total work you do matters more for getting stronger and bigger than whether you use heavy weights with few reps or light weights with many reps.
Neither heavy nor light lifting made people better at doing more reps in a row—except maybe a small, not certain improvement with lighter lifts in squats.
Whether people lift heavy weights with few reps or lighter weights with more reps—while doing the same total amount of work—both ways make their muscles grow about the same size in the chest and thighs after 8 weeks.
When people who already lift weights do either mostly heavy lifts or mostly lighter lifts—but keep the total amount of work the same—both ways make them just as strong after 8 weeks.
Doing more weight training raises levels of a protein called Frizzled-1 in older people’s muscles, no matter whether they gain muscle or not.
Causal
Some older people don’t build muscle no matter how much they train—and their muscles don’t show any detectable biological changes, even when they do more exercise.
People who build some muscle with more training (but not a lot) show reduced activity in systems that break down muscle and increased activity in systems that help move proteins around and respond to hormones.
People who build the most muscle from weight training show increased activity in their muscle cells’ systems that rebuild proteins, use energy during exercise, and fix damaged proteins.
For older people, doing four sets of weight exercises instead of just one can help build more muscle—but only if their body is already somewhat responsive to exercise; if it’s not responsive at all, more sets don’t help.
If you lift light weights but push them until you can't do another rep, you can grow your muscles just as much as someone lifting heavy weights—so long as you're putting in the same total effort.
Pushing your muscles to complete exhaustion feels way harder than stopping just before you can't do another rep—this extra effort might make you more tired and less likely to stick with your workout routine.
To get stronger, it matters more how heavy the weight is than whether you push to your absolute limit—lifting heavier weights makes you stronger, no matter if you stop short of failure.
If you're lifting heavy weights, you don't need to push to absolute exhaustion to grow your muscles—stopping just before failure works just as well, as long as you're lifting heavy enough.
If you're new to lifting and using light weights, you need to push your muscles until they can't do another rep to grow them bigger—otherwise, even doing more reps won't help.
Both heavy and light weight training, done until exhaustion, didn’t change how hard participants could push or pull in a static muscle test after nine weeks.
Lifting light weights for nine weeks didn't make people stronger on a machine that tests muscle power at different speeds, and in fact, their performance slightly dropped — while heavy lifting kept their strength steady.
Lifting heavier weights for nine weeks made young men stronger in a max lift test than lifting lighter weights, even when both groups worked until they couldn't do another rep.
After training one arm, both arms become easier to turn on—your brain can activate muscles with less effort, even in the arm you didn’t train.
Only the arm you actually train gets a boost in its muscle cells' ability to sustain signals from the brain—your other arm doesn’t get this change.
After training one arm, the other arm gets stronger because its motor neurons become easier to activate and receive more synchronized signals from the brain.
When your brain sends more coordinated signals to your muscles and less random noise, your movements become smoother and more controlled—even in muscles you didn’t train.
Working out one arm can make the other arm stronger and more steady, even if you didn't touch it—your brain is learning to control it better.
When people lift weights until they can’t do another rep, both slow-twitch and fast-twitch muscle fibers grow at about the same rate — so fast-twitch fibers aren’t naturally better at getting bigger.