system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does it "hurt" when you swing at a baseball but miss?
gpt
Basically your brain analyses from past experience how much force you need to swing with to hit the ball, and usually goes overboard because you want to hit it hard. If you're just swinging in the air without an aim to hit anything, you're using less force. 

Not connecting with the ball means the bat has excess force from the energy you put into it, making it swing further than expected because the force hasn't been transferred to the ball, and overextending or otherwise stretching our muscles which causes the pain.