Here is something new I
learned about my own experience.
I agree that if I could hike downhill as much as I cycle, I would not experience this pain.
But, for me, that is not practical since I ride 12 to 13 hours per week (commuting), and I work full time most of the year, etc...
Based on my recent experience having more than a week off the bike on travel and holiday events,
and based on the idea that the bundles of muscles fibers when rubbed together under force in one direction (cycling)
build up a grain on the muscle bundle edges, I decided to try something new to reduce the pain.
After a few days inside with no exercise and needing to return home, I also had to go down many flights of stairs to move belongings.
The same pain happened acutely during this activity, so I stopped and for 10 minutes where I sat on a high chair and swung my legs freely front and back so that the knees bent and straightened and the quad muscles would be flexed and rub together, but not be under tension.
I thought this action might slowly reduce the scarred edges to the muscle fibers (assuming that's what was there).
I then resumed walking down stairs.
The quad pain was greatly reduced and almost eliminated, and from experience, this was a huge improvement.
So, although this result does not prove the muscle fiber theory, it at least supports it.
And it gives me a way to reduce the pain when it happens.
I might work these leg swinging actions into my daily routine.