Browse evidence-based analysis of health-related claims and assertions
When you lift weights and your muscles grow, all the tiny parts inside the muscle cells grow together in balance — so the overall mix inside the cell stays the same.
Feeling sore after a workout doesn’t mean you’re building muscle or that your workout was effective — DOMS isn’t a good way to measure progress.
The more total weight you lift over time—how heavy it is, how many times, and how many sets—the more your muscles grow, and even small differences in total lifting add up to small differences in growth.
That muscle 'pump' you feel when lifting weights? It might feel like growth, but this claim says it doesn't actually cause your muscles to get bigger over time.
When you lift weights, your muscles get bigger not because you're growing new muscle fibers, but because the ones you already have are getting thicker. Scientists don't think you actually create new muscle fibers from training.
Muscle growth mainly happens because the tiny fibers that make your muscles contract get bigger and stronger; the fluid part of the muscle increases too, but only as a side effect and not on its own.
You can grow muscle not just by lifting weights, but also by stretching—just as long as the muscle feels enough tension, no matter how it's created.
The main reason muscles grow bigger when you work them is because of the force they create when they squeeze — and right now, science says that’s the only proven main trigger.
Lifting weights makes your muscles grow because the longer and harder you work them, the more they get the signal to build new muscle fibers.
Right now, the way scientists guess how many muscle fibers someone has isn't very precise — the numbers jump around a lot, so it's hard to spot small changes in muscle fibers.
When people lift weights, their arm and leg muscles show similar changes in muscle fiber numbers — and in both, we don’t see clear signs of new muscle fibers forming.
How long you lift weights—up to 6 months—doesn’t seem to change the number of muscle fibers you have, according to this claim.
Whether you're new to lifting weights or already experienced, your muscles seem to add new fibers at about the same rate when you start resistance training — at least in the first 6 months.
Lifting weights for up to 6 months probably doesn’t increase the number of muscle fibers in your arms or legs — instead, your muscles grow because the existing fibers get bigger, not because you grow new ones.
If healthy young guys take L-carnitine with carbs twice a day for 12 weeks, they don’t gain the extra body fat that usually comes from just taking carbs — even though both groups are eating more calories than they need.
Taking L-carnitine with carbs for 12 weeks might turn up the activity of 73 genes that help the body burn fuel better — especially those involved in how insulin works and how fats are used — in healthy young guys, compared to just taking carbs.
If young, healthy guys boost their muscle carnitine by 20% with supplements, their muscles might be moving fat into energy factories way more—like four times more at rest.
If healthy young guys take L-carnitine and carbs for 12 weeks, they might burn 6% more energy during light workouts — mostly by burning more fat, not carbs.
If healthy young guys take a certain amount of L-carnitine with carbs every day for 12 weeks, they might store more carnitine in their muscles, burn more energy during light exercise, and avoid gaining fat compared to just taking carbs.
When inactive young people do light cycling, how hard they feel they're working is closely linked to their body switching from burning fat to burning carbs.
When inactive young people do light cycling, their bodies burn the most fat at very easy effort levels — but as it starts feeling harder than 'light,' fat burning drops off, showing there's a sweet spot for burning fat during exercise.
If you're out of shape and find light cycling easy, your body burns more fat. But if the same workout feels hard, your body switches to burning more sugar instead.
When inactive young adults do light cycling, their bodies burn 2 to 4 times more calories than when resting, and they keep burning fat at a high rate even as the effort increases.
Even super light leg pedaling while sitting can help your body burn more fat than just sitting still — and you don’t need to break a sweat for it to work.