Had a really quick read of all the other comments. I felt like I wasn’t getting a bio mechanical reason as to why ACL tissue fiber can’t slowly be damaged which leads to what seems like “sudden failure” (leg is useable no longer). It seems incredibly counter intuitive to describe all ACL failures as sudden. Stress on the ligament over time should lead to it weakening to the point of what looks like sudden failure to the observing sports fan or coaching staff. With this being the case, for the injured athlete perhaps they have felt strains and tweaks in this ligament throughout their careers until the day it goes pop on them.

Anecdotally: I was a cross country and track runner, and now rock climber, I feel as though my own ACL has told me something weird is going on while it has never failed on me. Can’t imagine that pain sensation isn’t coming with some level of microscopic damage to the tissue.