Browse evidence-based analysis of health-related claims and assertions
In young men who don't exercise, fast-twitch muscle fibers are more tightly packed with muscle-building parts than slow-twitch fibers. When they start lifting weights, this difference stays the same, but all muscle fibers get bigger.
Descriptive
When young guys who don't exercise start lifting weights for 10 weeks, the energy-producing parts inside their muscles grow faster than the muscles themselves.
Correlational
When young guys who haven't lifted weights before do strength training for 10 weeks, their muscle fibers grow in a way that makes them stronger, not just bigger with extra fluid. It's like building real muscle instead of just puffing it up.
Mechanistic
Slow-twitch muscle fibers need longer workout times to grow because they tire less easily. This can be done by lifting lighter weights until you can't lift anymore.
Slow-twitch muscle fibers (type I) can work longer without getting tired compared to fast-twitch fibers (type II), which might help you lift weights for more time before exhaustion.
Lifting lighter weights might make your slow-twitch muscles grow more than your fast-twitch muscles, according to some new ideas.
Causal
Lifting heavy weights might make your fast-twitch muscle fibers grow bigger than your slow-twitch ones, according to some new research.
When a nerve is cut and then reconnects to the same or similar muscles, the body can relearn how to use those muscles in the right order again.
When a nerve in the wrist is cut and then heals, making nearby muscles work together better, it seems like the body starts using those muscles in a more organized way again.
When certain nerves in the arm are completely cut, the muscles don't work in the right order because the nerve fibers grow back in the wrong directions.
When certain nerves in the arm are completely cut, the way muscles respond doesn't follow the usual pattern where smaller movements happen before bigger ones.
When young men do a specific type of hard strength training, their bodies send strong signals to build muscle for hours afterward.
When you lift weights, how hard you push or pull (the force) matters more for muscle growth signals than how long you hold the weight, based on comparing workouts that take the same time but use different amounts of force.
When young men do one set of really tiring weightlifting, it gives the smallest boost to muscle-building signals right after exercise, compared to doing max-stretch moves or multiple sets that take the same total time.
When young men do a certain type of tough exercise, their fast-twitch muscle fibers show more chemical changes than other fibers, which means different muscle types react differently to hard workouts.
When young men do a specific type of hard weight training that focuses on lowering weights slowly, it triggers more muscle-building signals in their bodies compared to other common workout styles, even if the workout time is the same.
This research suggests that in both younger and older people, the way muscles activate their fibers follows the same basic rule: smaller muscle fibers get called into action before bigger ones. This pattern stays consistent as we age.
As people get older, their muscles change in a way that makes certain muscle parts bigger and faster-acting, which is like rewiring how the muscles work.
When muscles grow from lifting weights, some types of muscle fibers might not get as strong or reliable as others, according to the numbers.
When muscles grow bigger overall, it's because both slow-twitch and fast-twitch muscle fibers get bigger. But if one type grows more than the other, it means the other type grows less in comparison.
People aren't very good at guessing how close they are to muscle failure when doing many reps of weightlifting.
Lifting lighter weights many times in a row might help grow certain slow-twitch muscles better because they stay working longer.
When you lift light weights until you can't anymore, it fully uses your fast-twitch muscle fibers even though the weight isn't heavy.
Your muscles use smaller, less powerful fibers first when you start moving, and only bring in the bigger, stronger fibers when you need more power or get tired.