Osteoporosis increases with age. It is a major cause of mortality, morbidity, and medical expense worldwide . It has been shown that increased activity level is associated with decreased osteoporosis [1–4]. These activities include walking, running, weight lifting, or Tai Chi [2–5]. However, it is not clearly known yet what characteristics of these exercises would best stimulate the growth of bone mineral density (BMD). Some believe that it is the fast rate of loading to the lower extremity during heel strike that best stimulates growth of BMD , some believe it is the peak loading to the lower extremity during stance , and some believe it is the level of muscle activation during weight bearing .