Exercise is essential in building and maintaining strong bones for life. The earlier you start in life the better you will be in terms of preventing osteoporosis. When it comes to strengthening your bones, however, all exercise is not created equal.
The best type of exercise to stimulate bone growth is “weight bearing” exercise. "Weight-bearing" does not only mean walking or placing weight on your bones, it also is any activity that stresses your skeleton (more so than it is accustomed to in everyday normal sedentary type activities) or the surrounding muscles that anchor to the bones.
Bone responds to stresses placed upon it. When bone senses stress or impulses from activity, it tends to become stronger and make more bone, something called Wolff's Law. This mechanism is very specific and only those bones that are stressed will get stronger. For example, jogging can strengthen the bones in your feet, legs and even hips but will do nothing for the bones in your wrist or arm area. Also, high level tennis players have much stronger and thicker bones in their dominant playing arm then in the opposite arm.
In terms of Wolff's Law, the opposite is also true, bones not used or stressed will weaken. As orthopedic surgeons we see this all the time when arms or legs are immobilized in a cast. Bones will weaken. They get strong again over time, when the cast is removed, and the limb is used normally again.
I believe the best overall exercise for building bones is weightlifting. A good weightlifting program that includes the upper body, lower body, and core (abdominal, trunk, and spine area) will strengthen bones throughout your body. This is one reason that weight training is so important for females. Exercise that does not stress bones will not grow them. Swimming, for example, is a wonderful exercise in so many ways. It is good for your heart and your lower back and makes you very fit but because you are almost weightless, you will probably not do much to build your bone stock. Think of it like being and astronaut. Astronauts are weightless in space; they lose tremendous amounts of bone and calcium even with the best designed in-flight exercise programs.
Also, a word of caution for those who already have osteopenia or osteoporosis: Your bones are weaker than normal, and care must be taken with starting any exercise program. You should avoid those in which your are more likely to fall, or over exert or strain yourself resulting in fracture. It would be best to talk to your physician and even start your exercise program in a supervised setting if indeed your bones are weak to begin with. Also, there are exercise programs that can actually improve your balance making falls, and thus fractures, less likely.
With bone health, "use it or lose it" clearly applies. It is never too late to build your bones- so get started today.