I have problems with both
The bad back is from when I used to do demolition when I was about 20 by trying to show off and picking big sandstone blocks up while everyone else was picking them up in pairs. I picked up a monster and felt something happen in my back. It did not hurt when I felt it go but I could not work for a week and have problems ever since.
The knees are shafted because I never wore knee pads on different jobs including one job where I was leaning over metal girders for most of the day.
So my stupid fault really!
If I am doing stairs it's a nightmare because if I bend over my back starts killing me so I will kneel down for a bit but they start so I swap to bending over so sometimes I will sit down on the edge of a step.
To top it off my wrist gives me loads of jip if I use the hand tool for much time.
If i'm on a job i just get one of my workers to do the stairs or upholstery now.
I'm just falling to bits really