No medical practitioner, in this day and age, is going to sign their name to a document which states you are cleared to do anything. To do so would be going directly against sage advice from our lawyers and indemnity insurers - and we are obliged to adhere to this advice. Doctors can advise you to refrain from something or not to do something, life lift heavy objects, or not work. Outside of advice not to do something, as sensible adults, you are by definition cleared to do whatever you please within the boundaries of common sense. If your employer wants to know specifics about your medical condition and specifics about what you can and cannot do in the workplace, they are welcome to write and ask specific questions. This request must be accompanied by a signed document from you allowing the release of your medical information to third parties.