One of the rules we have on the MTB's is that if you fall- you get up quickly. Then if you don't get up-the others know you are in trouble. Problem comes about- as has happened- is if you get up and then find it hurts- then you have to sit down again to get assistance. Only time I didn't get up-I was tangled up in the frame- and yhe next rider down the hill panicked when he saw me still down- put the brakes on and went into a skid and ran over my rear wheel. He hurt after that.