Mountain bikers can experience a whole range of injuries and it depends on the difficulty level of the course and the experience of the rider. I saw a video where the rider fell of the side of a cliff into the mountain valley. He survived but he had two fractures and many bruises to show for his efforts. You could just as easily break your neck if you don't know what you are doing. Mountain biking is a good sport but it is a dangerous one. Always wear the appropriate protection when you are in the mountains with your bike.