I should have clarified - the piece of practical body armor that seems most useful based on injuries I’ve seen is knee pads.
If you crash hard enough where you separate a shoulder or break a rib, the kind of body armor required to protect against that is a whole other level above knee pads.
Where I’ve seen knee injuries is on rocky terrain where it doesn’t take much. A slow speed lose-your-balance kind of fall where your knee hits a pointy rock hurts!