I see, you are talking about the long bar that acts as a seesaw when pushed by the knee lifter.
I did loosen the spring on the vertical shaft that gets pushed up by the knee lifter, but didn't think to look for a spring on the flat bar itself.
That spring and screw affect the inside alternating/vibrating foot.
Oops, I should have left it alone. How do I know how far to screw it down? I haven't seen it mentioned in the service manual I found online.