A medical profession can correct me if I'm wrong but my understanding of bones is that they are very strong in one direction and very weak in others. They are strong where, evolutionarily, they needed to be. Leg bones, for example, may be able to bear a lot of downward force as that where humans and earlier animals placed the most pressure. Jumping, landing on the feet, supporting loads. They are, I have been told, weaker when hit from the side as that is not a direction that our evolutionary history decided needed to be protected against. So, a modest hit from side may cause a break while the same hit down the length of the bone may be easily absorbed.