I have trained clients with FM

It means being patient, moving slowly and using rate of perceived exertion rather than heart rate zones. many of the meds my clients were on affected HR so using zones was essentially useless.

it also means taking it one day at a time. some days are good and some days they might call an hour before to cancel.

i had one woman actually break down in tears with me because she was so deconditioned. we did basic leg lifts and just simple movements to get things going and she was having a hard time. i told her it was ok, and to take it one day at a time, and not to over do it. this was at a gym, and i kind of wish she would have bought some PT just for the extra coaching and comfort.

anyway that is my experience with FM.