Health Claims

If young, healthy guys lift weights for 12 weeks, their muscles get stronger at moderate speeds — like kicking or swinging — and they can generate 13% to 32% more force, no matter how they did the lifting.

Causal

If young, healthy guys do strength training for 12 weeks using different types of muscle contractions—whether slow or fast—they’ll get significantly stronger in their thigh muscles, no matter which style they pick.

Causal

How fast or slow you lift and lower weights probably doesn’t make much difference in how much muscle you build—but it might matter a little depending on whether you’re pushing down, pulling up, working a specific body part, or going all the way to exhaustion.

Descriptive

Lifting weights fast or slow doesn’t really make a difference in how much your muscles grow — the tiny difference you might see is so small it probably doesn’t matter.

Quantitative

Whether you lift weights slowly or quickly, you’re likely to gain about the same amount of muscle — the speed doesn’t seem to make a big difference.

Correlational

To figure out whether speeding up or slowing down the lifting vs. lowering part of a weight workout makes your muscles grow more, scientists need to test each part separately—not at the same time.

Mechanistic

When you first start lifting weights, you get stronger not because your muscles grow bigger right away, but because your brain gets better at telling your muscles when and how to contract.

Mechanistic

You can build muscle just as well lifting light weights as heavy ones — as long as you push yourself until you can’t do another rep.

Causal

When people lift weights without going all the way to exhaustion, using machines that force a set speed might make fast lifting look better—but that’s because those machines don’t act like real free weights or gym machines we usually use.

Descriptive

If you lift weights until you're really tired, it doesn't matter if you lift slowly or quickly—you'll still build about the same amount of muscle.

Descriptive

When people lift weights regularly, their muscles usually get about 5% bigger on average, no matter what kind of weight routine they follow.

Descriptive

Lifting weights really slowly with lighter weights doesn’t build as much muscle as lifting heavier weights at a normal speed—even if you push yourself to the limit in both cases.

Causal

If you lift and lower weights quickly or slowly, as long as you do both phases the same way, your muscles grow about the same amount—speed doesn’t make a big difference.

Descriptive

How fast or slow you lift and lower weights doesn’t matter as much for building muscle as how much weight you lift, how many reps you do, or how close you push yourself to failure.

Quantitative

Lifting weights fast or slow doesn’t make much difference in how much your muscles grow — the extra gain from going slow is practically nothing.

Quantitative

When you first start lifting weights, your muscles get stronger not just because they’re growing, but because your brain and nerves get better at telling your muscles when and how hard to contract.

Mechanistic

When people of any age do strength exercises multiple times in a row, their muscles fire more intensely for a short while—and that’s when they get a quick boost in strength.

Correlational

As people get older, their muscles don't send signals as quickly when they try to move, which might be why they lose strength over time.

Correlational

When male mice of a common lab breed ate food with artificial sweeteners instead of plain water for a long time, they didn’t gain more or less weight, fat, or muscle, and they ate about the same amount of food as mice that only drank water.

Correlational

Mice that drank aspartame or sucralose had more liver inflammation than mice drinking plain water, but mice drinking Reb M had the least inflammation.

Correlational

When mice on a fatty diet drank sucralose instead of fructose, they became more sensitive to insulin and gained less weight—but they didn’t do any better than mice that just drank water.

Correlational

When male mice of a common lab breed drank aspartame instead of water for a long time, their blood sugar took longer to go back down after eating, meaning their bodies didn’t handle sugar as well.

Correlational

When male mice eat a fatty diet and drink something called Reb M instead of water or sugar water, their gut bacteria called Lachnospiraceae grow more, and they have more butyrate in their blood — a chemical that’s good for gut health.

Correlational

When mice on a fatty diet drank a sweetener called Reb M instead of water or sugar water, their bodies handled insulin better—meaning their blood sugar stayed more under control without making more insulin.

Correlational