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.

The more you lean back when doing pressing exercises, the more the front of your shoulder muscles do the work instead of your chest — and when you're standing straight up, your front shoulders are working the hardest.

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 people do bench press on an inclined bench, the upper part of their chest muscle swells more than the lower part, showing different responses within the same muscle.

When people do flat bench press, the lower part of their chest muscle grows more than the upper part - and this is the opposite of what happens when they do incline bench press.

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.