Even though the vastus intermedius is a deep thigh muscle you can't see, its fiber angle changes were linked to better leg strength during both pushing and lowering movements — it might be more important than it looks.

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 in a static leg press, the main reason is that their thigh muscle near the hip gets bigger and its fibers angle more, which helps push harder.

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.

If you're a young adult who doesn't lift regularly and you do leg extensions slowly (4 seconds down), your inner thigh muscle gets a little thicker than if you do it faster (2 seconds down)—but the outer and front thigh muscles don't change much.