What are my rights if the job I was asked to do is not part of my job description?

I was hired as a Branch Manager for a construction and renovation company. My job description is to travel to the properties, evaluate the work, bid, estimate and schedule and supervise maintenance techs who actually do the repair work. I have recently been expected to actually do the manual work even though I was not hired to do the work, nor am I physically able to due to a severe back condition. They are aware of my limitations, but still expect it of me. I was recently asked to provide a doctor's note to prove I have said back condition and limitations which I'm not sure if they have a right to request or get. I work in one state, but the company is based in another. I would like to point out that I was never told I would be doing any physical labor, nor is it in my job description.