Deliberately slow concentric and eccentric tempos (10 seconds lift, 4 seconds lower) reduce muscle hypertrophy compared to standard tempos (1–2 seconds lift, 1–2 seconds lower) when training volume and effort are matched.

Study: Effects of resistance training on hypertrophy, strength and tensiomyography parameters of elbow flexors: role of eccentric phase duration

This study found that lifting weights slowly on the way down didn’t make muscles grow any less than lifting quickly—even though the claim says slow lifting makes muscles grow less. But it didn’t test slow lifting on the way up, so it’s not a perfect test.

Study: Does Varying Repetition Tempo in a Single-Joint Lower Body Exercise Augment Muscle Size and Strength in Resistance-Trained Men?

This study found that lifting weights slowly (3 seconds down) didn’t make your muscles grow less than lifting quickly (1 second down)—so the idea that super-slow lifts (like 10 seconds up) make you grow less isn’t supported by this research.

Study: Neither repetition duration nor number of muscle actions affect strength increases, body composition, muscle size, or fasted blood glucose in trained males and females.

When people lifted weights slowly (10 seconds up, 10 seconds down) versus normally (2 seconds up, 4 seconds down), both groups grew just as much muscle—so slow lifting doesn’t make you grow less muscle if you do the same total work.