Now, my old goal keeping coach (field hockey) was probably just trying to scare me with this, but he claimed that if someone stepped (or fell) on your legs whilst you were laying on the ground, (legs on top of each other) they would snap instantly. I am presuming you would have to be wearing goal keeping gear that doesn’t give, but is it possible?
After all, they are basically 4 flat boards strapped to either side of your legs that end above your knee cap.
I don’t know about field hockey but this actually happened to a friend of mine playing ice hockey. He made a save and was laying there covering the puck positioned much like you described. Someone was pushed on top of him and his femur (thigh bone) snapped in half. The person that landed on him weighed 250+ pounds so I’m sure that had something to do with it. I do not think this is an automatic though. I have seen this same thing happen with no injury at all. It is definitly possible though.
I’d be much more worried about breaking something beyond my legs when I play hockey. I’ve been a goaltender for over 10 years, and I’ve managed to avoid signifiant injury sofar. While I cannot specifically recall the situation you have described, I am sure it has happened to me. The femur is one of the strongest bones in your body, and is very difficult to break. I’m much more concerned about blowing out a knee than I am breaking a bone.
Also, the leg pads do not lend themselves to standing up easily in the position you’ve described. If anyone falls on me, I generally wind up either on my back or stomach, not compressed on my side.