Shorter cranks!
It worked like a miracle for me. (One knee doesn't have full range of motion)
175mm Constant knee pain. Cadence wouldn't go over 62 RPM
170mm Occasional knee pain. Cadence 80 RPM
165mm No pain. Cadence 85 RPM
160mm. No pain. Cadence 82 RPM. That was getting too short.
IF you are 70, your knees probably just don't bend like they used to.