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.

This study changed only how slowly people lowered the weight in squats, while keeping how fast they lifted it the same—and found that slowing the lowering part made muscles grow more. That proves you need to test each part of the movement separately to see what it does.

The study looked at slow and fast lifting speeds separately for pushing and pulling phases of exercise, which is exactly what the claim says you need to do to figure out which one builds muscle better. Even though neither was much better, the way they tested it proves the method in the claim is the right one to use.

The study changed how fast or slow people lifted and lowered weights separately and found that each part affected muscles differently — proving you can't just change both at once and know what each one does.