Osteoporosis is a common disease in Norway, and is a skeletal disorder characterized by low bone mass and micro-architectural deterioration of bone tissue with a consequent increase in bone fragility and susceptibility to fracture. Physical activity is essential for bone remodeling and prevention of osteoporosis. Studies report that mechanical loading of the skeleton is especially important for achieving higher BMD among children before entering puberty. The peak bone mass plays an important role for further BMD during life, which indicates that prevention should start early. The objectives of this literature study were to review present knowledge of physical activity’s influence as prevention and reduction of osteoporosis among humans according to women, men and children. I also included animal and cell studies which investigated the impact of mechanical strain.