We don’t have enough proof to say whether lifting weights slowly or quickly, or pushing down vs. lifting up, is better for building stronger or bigger thigh muscles in young, healthy guys after 12 weeks of working out.

When young men do isometric leg exercises for 12 weeks, their thigh muscles get bigger—and that bigger muscle size helps explain about 1 in 5 of their strength gains, even if their nerves aren’t getting better at telling muscles to work harder.

If you're new to working out and do leg exercises where you lower the weight slowly and quickly, doing it at a speed of 180 degrees per second might give you the biggest strength gains after 8 weeks — faster eccentric moves might just be the secret sauce for getting stronger.

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.

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.

When guys who’ve never lifted weights before start training, their muscles get stronger partly because their brain gets better at telling the muscles to fire harder — but most of the strength gain still comes from the muscles actually getting bigger.