Bone can withstand force from either end but doesn't like being twisted.
I fell at school camp and broke my arm. It wasn't a big fall, I just tripped and landed on grass. Both bones broke. How come sometimes when you fall nothing happens, and other times the bone breaks? - E
The simple answer is that bone isn't as strong as
you think it is. Bone is made of calcium, just like chalk. Like chalk, it is extremely hard to break it by squeezing the ends together, but very easy to snap by flexing or twisting it. So for the actions of normal living, like standing, walking, and carrying things, our bones do fine. But when our long bones get planted in the ground at one end and our bodies keep moving at the other, the shafts of bone in the middle sometimes snap.
Given how weak calcium is, bone remodeling is going on at all times, with new calcium being laid down along lines of stress. You can see these fan-like lines on x-rays, the fine lines of calcium looking just like the cables on a suspension bridge, with essentially the same effect of distributing loads and forces over a wide area.
Some people, especially the elderly and those with osteoporosis or kidney disease, have weak bones that can snap even under normal use. An example is the loss of height many of us suffer in old age -- largely a result of our spine bones crunching down and compacting under our own weight. What appears to help the most aren't calcium supplements but weightlifting. The added stress of the weight ramps up bone remodeling and causes bone to be laid down more densely, and along the lines of greatest stress -- unlike most osteoporosis pills which merely harden bone by interfering with bone breakdown, something that's actually essential for healthy bone turnover).
But for a young person, a fracture is more likely just bad luck. A fall in just the wrong direction, with the right amount of force, showed you the limitations of having a skeleton made of chalk.
