It is increasingly accepted that osteoporosis is a paediatric issue. The prepubertal human skeleton is quite sensitive to the mechanical stimulation elicited by physical activity. To achieve the benefits for bone deriving from physical activity, it is not necessary to perform high volumes of exercise, since a notable osteogenic effect may be achieved with just 3 hours of participation in sports. Physical activity or participation in sport should start at prepubertal ages and should be maintained through the pubertal development to obtain the maximal peak bone mass potentially achievable. Starting physical activity prior to the pubertal growth spurt stimulates both bone and skeletal muscle hypertrophy to a greater degree than observed with normal growth in non-physically active children. High strain-eliciting sport like gymnastics, or participation in sports or weight-bearing physical activities like football or handball, are strongly recommended to increase the peak bone mass. Moreover, the increase in lean mass is the most important predictor for bone mineral mass accrual during prepubertal growth throughout the population. Since skeletal muscle is the primary component of lean mass, participation in sport could have not only a direct osteogenic effect, but also an indirect effect by increasing muscle mass and hence the tensions generated on bones during prepubertal years.