Bones may seem like an inanimate object that just exists to hold our bodies together. But in reality our bones are living organisms that change shapes and grows throughout the course of our lives. Most of this shaping is the result of forces which pull, press and twist the skeleton as we move.

The biggest of these forces is caused by our muscles. During movement, bones experience a huge amounts of force. For example, when a triple jumper’s heels hit the ground, the force is approximately 15 times their actual body weight — or the weight of a small sedan. In fact, because muscles attach very close to the joints normally, muscular forces are much greater than these impact forces. Similar to the way that you have to push harder to lift someone on a see-saw the closer you get to the middle of it.

As a result of this, bones also experience a great impact and muscle force during daily mundane tasks, totaling more than five times body weight. These forces bend, squash and twist bones. The shin bone, for example, briefly becomes nearly a millimeter shorter as your foot hits the ground when running. The bone is able to sense small changes, and can grow astronomically — in the months after starting daily exercise — in order to reduce the risk of breaking. Another example, the racket arm bones of a tennis player can be 20 percent thicker and contain 40 percent more bone mineral than their other arm. And sprint runners have up to a third more bone in their shin bone than people who don’t regularly exercise.

And while exercise is great, not all exercise gives us strong bones. There seems to be a need for high impacts (striking a tennis ball, or hitting the floor from a jump) in order to produce a big enough muscle and impact forces to make our bones change. Swimmers and cyclists may have very healthy muscles, lungs, and hearts but their bones are not much different from people who don’t exercise. The response bone’s have to these forces greatly vary along its length. Near the joints, bones get denser and bigger, whereas bone shafts tend to get thicker and bigger with very little change in bone density. Bones can also change in shape. The shin bone shaft begins as a circular tube, but tends to get wider from front to back as we develop and start to move until the shin forms a tear-drop shape.

Beginning exercise early on in life can ensure strong bones for the rest of your life. Children who start to walk early have up to 40 percent more bone in their shins than children who start later; these effects usually last until their late teen years. This is very important as stronger, bigger bones are less likely to break as we age. Of course, exercise trials can be effective in making children’s bones stronger, and also in reducing severe bone loss due to bed rest or spinal cord injuries. Continuous work is giving us a much better picture of how forces transform our bones during various movements. This will enable us to design more effective exercises for strong bones in different age groups. And healthy, strong bones are vital for a healthy, strong life.

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