The present text deals with the relationship of muscle force and mass to bone mass and geometry in the developing skeleton of children and adolescents. Recent results of the last ten years are discussed with reflection on Harold Frost's 'mechanostat hypothesis'. Bone mass and geometry follow the development of body size and muscle force in children and adolescents. Thereby, bone is adapted to the tissue strain due to biomechanical forces. This process is modified by hormonal signals (i.e., estrogens and androgens). Therefore, the quantified relationship of muscle force to bone stability is a reasonable approach to distinguish between primary and secondary bone diseases. Primary bone diseases are characterized by a disturbed adaptation of bone to biomechanical forces. In contrast, secondary bone diseases show a correct adaptation of bone to loaded forces in combination with a decline of muscle force. Therefore, the 'Functional Muscle-Bone Unit' was introduced into the diagnostics of pediatric bone diseases. The ratio of two parameters--referred to bone strength on the one and to biomechanical forces on the other side--is a reasonable diagnostic approach to distinguish between primary and secondary bone diseases.