Weak rears in English Cocker Spaniels

This is a condition of unknown origin that affects English Cocker Spaniels. An affected dog usually has an affected parent. It shows in the rear legs first, and gradually works it's way up the spine until the front legs are affected. You begin to see signs of it between 7-9 years old. You may see slight signs of it at a younger age.

Observations that indicate the beginning stages are

Unable to jump on the couch
Goes up stairs with difficulty

Here are links to two videos of affected dogs:

Red Bitch, advanced stage
Black Bitch standing, middle stage
Black Bitch walking, middle stage