The biggest reason people get stronger during dynamic leg movements after training isn’t just muscle growth—it’s how much their brain tells their thigh muscles to fire harder.

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.

Even though the deep thigh muscle (vastus intermedius) isn’t the biggest or most active, changes in how its fibers are angled are surprisingly linked to how much stronger people get during dynamic leg movements.

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.

When you get stronger holding a position (like pushing against a wall), your muscle size matters more — but when you’re moving (like kicking a ball), how hard you activate your muscles matters more than how big they are.