If you have been injured or have a disability you are protected at work by the Americans With Disabilities Act ("ADA").  The ADA is a very robust act that gives a wide range of protections.  Under the ADA you can't be fired for having a disability or other injury.  And under the ADA if you ask for a reasonable accommodation, your employer must grant it to you unless doing so is not reasonable and unless the business just can't do it for business reasons.  Many times employers just don't feel like putting forth the effort.  It is really remarkable that this type of thing occurs but that seems to be the case.  The ADA is a federal law and therefore you can file a claim with the Equal Employment Opportunity Commission ("EEOC").   After filing with the EEOC you can then ask for a right to sue letter and file a lawsuit in federal court.  It is very important for you to have the proper documentation and to follow the strict time limits that apply. 

It is important that you don't notify the company verbally but rather put your request in writing and have your request backed up with medical documentation from your physician.  As an employee you just can't tell your boss you have a medical condition and expect him to take your word for it.  This means when you go to see your physician make sure he documents your condition and writes it in such a way that it will help you at work.  If you can't lift over 40 pounds make sure it is written in your medical file.  Don't expect your boss to be happy with your request but as long as it is reasonable, it must be granted.  Make sure you put the request in writing to human resources and follow-up in writing too.