How to Test And Demonstrate Inpairment.

I'll skip the specifics of my back problems. Many of you have worse physical conditions and I wish you the best.
In addition to trying to get better physically, I am having problems with doctors and how they cooperate or not in my collecting my private disability insurance.
I am in pain every day resulting from a herniation and ddd. But, the level of pain and the degree to which it affects my ability to conduct activities without greatly increasing pain and stiffness is very variable and mostly depends on what activities I did the day before.
My disability insurance pays when I cannot perform my regular occupation. It's one of the better policies out there, or at least you used to be able to get such policies.
My regular occupation was leading seminars which requires nearly daily travel by plane or driving, standing all day, carting around 3 - 6 heavy boxes from location to location,and other physical activities.
The reality is that I could probably make it through one or two days with increasing pain, but by the third or fourth day I would be in such pain, have stiffness and be "generally messed up" to the extent that I would not be able to continue.
But the doctors, who have to sign my insurance form monthly, want to see some "objective" test. Well, on any given day when I have not been active for several previous days, they are not really testing my ability to perform the job.
The question:
does anyone have experience in this area? How do you demonstrate objectively what the 3+ day impacts of activity would be?
Thank you.