Found this in Arena mode, not sure if it also applies in Adventure and Fortress mode:
There's no message when you control a unit in Arena mode and attempt to break a joint. The joint gets bruised or broken, but there's no message. Grabbing and locking are fine.
This is a copy of the relevant battle from gamelog.txt, with the exception of the upper case "
BREAK ATTEMPT" entries, those were inserted by me, to show where I attempted a joint break (and, by extension, where the message is missing).
Elf 1 misses You!
Elf 1 misses You!
You grab Elf 1 by the right upper arm with your right upper arm!
Elf 1 misses You!
You lock Elf 1's right shoulder with Your right upper arm!
BREAK ATTEMPT
Elf 1 misses You!
Elf 1 misses You!
You grab Elf 1 by the left upper arm with your left upper arm!
Elf 1 misses You!
You adjust the grip of Your left upper arm on Elf 1's left upper arm.
You lock Elf 1's left shoulder with Your left upper arm!
BREAK ATTEMPT
Elf 1 misses You!
The Elf 1 gives in to pain.
The Elf 1 falls over.
You grab Elf 1 by the right lower arm with your right lower arm!
You lock Elf 1's right elbow with Your right lower arm!
BREAK ATTEMPT
You grab Elf 1 by the left lower arm with your left lower arm!
You lock Elf 1's left elbow with Your left lower arm!
BREAK ATTEMPT
This is the elf's would description after that particular encounter, to show that all four joints were either broken or injured by my attacks:
Her left elbow is broken. Her left elbow is oozing elf blood. Her right shoulder is broken. Her right elbow is broken. Her left shoulder is dented.