When your thigh muscles fire more strongly during a max effort leg push, you tend to get stronger faster after doing 12 weeks of holding a knee extension position — and this muscle firing explains about 30% of why some people get stronger than others.

Men who started with a longer tendon lever arm and could already activate their thigh muscles better tended to get stronger in eccentric movements after training — their starting point mattered.

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.

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 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 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.