What can I do if my former employer is claiming I was fired and is now withholding my final paycheck?
Just firing you because you had a hurt knee doesn't seem fair, but I've no way of knowing what kind of history you have with this company and what kinds of adverse comments (if any) may be sitting in your personnel file.
But the short of it is this: termination papers are legal documents. They will likely outline what you're entitled to receive in the way of severance pay (if any), what continuing obligations you have (such as keeping certain company info confidential) and any other company responsibilities.
You have a right to have these documents reviewed by an attorney of your choosing prior to signing them. Have the papers sent to you and review them with a local attorney who is familiar with Minnesota employment law before signing. The attorney may be able to negotiate a better deal for you, depending on the circumstances.