The top part of your chest muscle is really good at helping your arm move forward, while the middle part keeps your shoulder steady and the bottom part helps pull your arm backward — based on how these parts are shaped in dead bodies studied by scientists.

When you lift your arm out in front of you, certain shoulder muscles are better at helping the movement because of how they're positioned, while others are stronger at pulling the arm back down.

Different parts of big shoulder muscles actually work in their own unique ways because they pull with different strength and direction — kind of like how different sections of a fan blade might push air differently.

In dead human bodies, the big chest muscle has a regular upper part and a lower part made of 6 to 7 separate pieces, where the lowest piece bends forward to hold the one above it — a pattern that might affect how the muscle works or heals after surgery.

The chest muscle in dead bodies has a top part that's smooth and a bottom part made of 6 to 7 separate chunks, where the lowest chunk wraps around the one above it like a cradle — this unique shape might change how the muscle works and how doctors fix it in surgery.

The upper part of your chest muscle doesn't help much when you push something overhead out to the side, because it's in a bad position to pull. But it works a lot better when you push something overhead in front of you, because then it's in a good position to help.