According to the mechanostat theory, the mechanical loads on bones help to determine bone strength, and the largest loads come from muscle forces. Indeed, recent studies found that muscle strength and the bending strength of bone are highly correlated (r>0.93), and bone ,,mass" and muscle mass likewise (r>0.93). Two strain thresholds seem to help to control bone strength. If the mean voluntary loads on bone do not exceed the minimum threshold for remodeling (MESr), remodeling removes bone until this threshold is exceeded. Between 800 and 1600 uStrain bone is preserved. If mean loads on bone regularly exceed 1600 µtrain Strain (minimum threshold for modeling MESm) bone is added to make it stronger. This suggests that any physical training that does not exceed 1600 µtrain will not increase bone strength. Growing muscle strength and body weight in children can cause bone strains that exceed the modeling threshold. This could help to explain the active bone modeling and increases in bone ,,mass" and strength that occur in growing children. Data in an Argentine absorptiometric study show that in children bone and muscle mass both increase linearly until puberty, but in girls at 12 years of age bone ,,mass" begins increasing faster than muscle mass. A similar but smaller increase occurs in boys at age 15. This suggests, estrogen may make girls store more bone than needed for strictly mechanical reasons, possibly to provide calcium stores during lactation after pregnancy.