Well, yeah. But even if you don't know it was your ACL, you might be vaguely curious as to why you heard a loud "pop, felt a searing pain unlike any you've ever known, and experienced near-instantaneous, grapefruit-sized swelling. That, according to orthopedic surgeon Peter Laimins, is the most common sequence of events associated with complete ACL rippage. A subtler, partial tear might not be quite as cinematic, but you'll still know something's wrong. The "not know it till later syndrome occurs when some unwitting athlete figures a working knee couldn't actually be a busted knee. And while it's true that some lucky people don't experience immediate pain and swelling, there's another telltale sign: With a wrecked ACL, you won't be able to ski well. According to Laimins, "People can jog, run, or bike with an ACL tear, but they can't ski at a high level or excel at pivoting sports. So, if you twist your knee, feel no pain, but then suddenly ski like your grandma, it's probably time to see the doctor.
Ask Dr. Flake