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'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.

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.

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.