Fights on the job. You can finish it, just don't start it.
If you get injured at the work place you have to show that it arose out of your employment (e.g. it was part of your work duties) and in the course of your employment (you were working at the time). Showing that a fight is part of your work duties is usually the reason you win or lose a case.
If you and a co-worker get in to a fight over a girl, a sports team or some other random nonsense, it will not be considered to have arisen out of your job duties. Even if you try to back out of the fight, if the fight itself isn't about work you will probably have a difficult time winning the case.
On the other hand, if you are fighting about who needs to do what with a job or something else that is work related, you will win your case as long as you are not the aggressor. Who the aggressor is can be open to interpretation, but if you throw the first punch you are probably going to lose unless you were somehow backed in to a corner and given no choice.
If you don't start a fight that is about work, but beat the other person up after they hit you, while you might have some criminal problems, under the IL workers comp act you will likely win your case.
This is really a judgment call situation. We never want anyone to make a conclusion from just reading our blog, but you especially shouldn't make a decision without having someone go over the facts of a fight. Often each side will tell a different story so while normally witnesses aren't a huge deal in work comp, it is extremely important in fights.
We are workers' compensation attorneys that help people with Illinois work injuries anywhere in IL via our statewide network of attorneys. Contact us and we will answer your questions or find the right lawyer for your situation.