If you're moving your index finger gently and keep your thumb pointing down, the main hand muscle that helps do that gets a lot more mechanical advantage—like getting better leverage—than if your thumb is pointing up.

Moving your thumb around doesn’t really change how strong a certain hand muscle is when it helps bend your index finger — that muscle’s job stays pretty much the same no matter how your thumb is positioned.

When people move their index finger with a little force, the thumb's position changes how hard the brain has to work to control a specific hand muscle — it works harder when the thumb is down.

When you bend your index finger, your brain doesn’t change how hard it tells the other finger-bending muscles to work when you change your thumb position—even though it does adjust the muscle that moves your thumb sideways. This suggests your brain fine-tunes certain hand muscles depending on how your hand is shaped.

If you point your thumb down, your hand muscle can squeeze 60% harder when zapped by electricity — even if you're not trying to move it yourself.