Employer will not accept physician release

My employer has asked me to take a fitness exam after my physician released me back to my job without limitation. I had a partial shoulder replacement on January 26th. I did not ask my employer for medical leave, I requested vacation pay and made myself available for light duty return two weeks post surgery. I was initially told that this exam would be just an evaluation of my abilities should I get hurt in the future. While I was on light duty assignment I had a disagreement with my supervisor about the safety of transporting a wheelchair patient. After this disgreement I was told that this medical exam was a condition of reinstatement. My employer is asking me perform certain exercises that are not possible for me to do. My employer states these exercises mimic the required job functions of my job. I wouldn't have been able to perform these exercises even prior to having surgery due to my advanced arthritic shoulder! My last employee performance evaluation in December was perfect.My physician was angered by my employers stance and has written them a letter stating his opinion on my ability to perform my job. My physician also advised me to seek legal counsel.My employer just sent me a certified letter stating they receiver my doctors letter but still require my compliance in regards to the physical. This IS NOT a workers comp. claim. My employee rule book only mentions fitness evaluations when returning from a workers comp. injury.