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.

When guys who’ve never lifted weights before start training, the more their thigh muscles ‘fire up’ during max efforts, the stronger they tend to get—both in holding a heavy position and in lifting one heavy weight.

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.

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 untrained men get stronger during dynamic leg extensions, the biggest factor is how much they can turn on their thigh muscles — bigger muscles and fiber angles help a little, but turning on the muscles harder matters most.

When untrained men get stronger during dynamic leg movements like standing up, the biggest factor is how much their thigh muscles fire, especially when combined with changes in the deep thigh muscle’s fiber angle and overall muscle size.