Genetic lineage, bone mass, and physical activity in mice.

Five strains of inbred mice were found to have variations in bone mass although they were similar in body weight. Two of these strains, C57BL/6J and A/J, were studied in greater detail and the former had more bone in both femur and tibia. The increased bone mass was associated with larger quadriceps muscles in the C57BL/6J animals when measured at 18 weeks of age. Activities of animals from these two strains were studied over 24 h periods using a cage with an ultrasonic movement detector and automatic counter. The C57BL/6J animals were more active than the A/J mice. The male C57BL/6J mice tended to have larger testicles and higher testosterone levels than the A/J animals, whereas the female A/J animals had larger ovaries and higher oestradiol levels. As both male and female C57BL/6J animals were more active, it was concluded that the sex hormone differences between the two strains was not responsible for either the changes in bone mass or physical activity. These findings indicate that in the mouse, activity is in part genetically determined. We have hypothesized that this, in turn, could affect muscle mass and secondarily, bone size and strength. If these results can be applied to humans, it would suggest that differences at birth between individuals are important for bone mass in later life and that muscle mass and activity are in part genetically influenced. If this was the case, then muscle mass and strength could be a factor in bone mass and one of the goals in prevention and treatment of osteoporosis should be directed toward preservation and/or augmentation of muscle strength.