Weight-bearing exercises, started in your youth and continued throughout your life, can help prevent osteoporosis. These exercises, such as walking, jogging, climbing, dancing, or lifting weights, help you build strong bones as a young person, and then help you maintain your bone thickness (density) as an adult. However, if you stop exercising, your bones will begin to thin. Starting these exercises at any age will help prevent bone loss. It is best to do weight-bearing exercise for 45 to 60 minutes at least 4 days a week.
Exercises that are not weight-bearing, such as swimming, are good for your general health. However, they do not work your muscles and bones against gravity and therefore do not stimulate new bone growth.
WebMD Medical Reference from Healthwise