Osteoporosis is a prevalent health concern among older adults and is associated with an increased risk of falls that can cause fracture, injury and even mortality. Identifying the factors related to falls occur within this population is essential for the development of effective regimes for fall prevention. The long-term objectives of this work are to ascertain the mechanisms for effectively controlling balance in seniors with osteoporosis and to provide a basis for developing fall prevention programs. The entire body's center of mass (CoM) is a critical indicator for balance control and the coordination among joints and muscles to control the CoM is still unclear. The aim1 of this study is to develop biomechanical models that control the entire body's center of mass (CoM) during upright quiet stance in seniors with osteoporosis. The aim2 of this study is to identify the mechanism to control the entire body's CoM after receiving a balance perturbation in seniors with osteoporosis. The findings of this research can provide needed information regarding the processes of balance control in this vulnerable osteoporosis population, and has the potential to be applied to individuals with other neuromuscular and orthopedic deficits.