My Apple watch often thinks I might have fallen when I haven't. Last week, I took a bad fall and was incapacitated with a broken tibular plateau. Watch did not detect fall. What happened? I was very lucky that a nearby neighbor was at home.
Apple Watch Series 6, watchOS 9
As you probably know, Apple Watch cannot detect all falls. Your watch might detect high-impact activity as a fall and trigger Fall Detection. In your case, in order for the watch to detect the fall, a lot would depend on how your wrist reacts to the fall, and unfortunately, not your knee. For more info, see:
Use Fall Detection with Apple Watch - Apple Support
Sorry to hear you was injured, i had a similar experience. Unfortunately it appears that there's a lot of people experiencing the same problem. The response on here to anyone that has had this problem is the common response of ''Apple Watch cannot detect all falls'' . But hard falls where you fall hard enough as you have to break your tibular or in my case i was unconscious, and the fall detection doesn't register when someone isn't moving. Surely shows that there is a major issue with one of the major advertised features of these watches !! As per usual you will see someone will reply trying to mask the problem, which many like yourself are experiencing.
Fall not detected
