I don't know if it pertains in your case, but walking downhill uses different muscles, or in a different way. "Eccentric loading", as the external load (body weight downhill) is opposed by the muscle contraction. I found this out myself a couple of years ago hiking up and then down Stone Mountain, a local attraction. Though not as acute as you describe, those muscles became overwhelmed and walking was more difficult. Balance suffered. It's due to prolonged disuse and caught me by surprise because I was generally physically active.
The bright side is that using and strengthening those muscles can bring the functionality back, or at least mitigate the problem. My own strategy has been to take up walking a bit more, some moderate running and treadmill, and stairs whenever possible. I believe that it has helped, probably the running more than anything although I realize that's not for everyone. I can't tell you for sure until I go back to Stone Mountain and compare, but just using those muscles in that way is bound to help.