Bone density as we get older can suffer and we are told to lift weights to increase density, Tai Chi longevity exercise has an exercise that is fun that is often called the Bear exercise and can be done more often because it requires no equipment. Standing start turning the waist and let the arms follow, as the arms swing to and fro increase the waist movement until your hands start to slap on the body around the midsection and hips/sacrum. After awhile bend the knees and waist slapping down the legs and rising up the legs. Now let one hand slap on the other arm and vice versa to and fro, now the shoulders and scapula and side of neck. Finish by tapping on the skull and jaw. They call this Patting and in Shiatsu they call it Do-In. The vibrations are said to cause the bone to strengthen in to what they call Tiger Bone, you also see it in iron body exercise.