Trauma excepted, muscle forces cause the largest loads on bones and the largest bone strains. In children, steadily increasing muscle strength increases bone loads and strains above a modeling threshold, which allows modeling to increase bone strength and "mass" and conservation-mode remodeling to retain existing bone. As a result, bone strength and "mass" both increase. In young adults, muscle strength plateaus, so bone strength can increase enough to reduce strains below the modeling threshold, turn modeling off, and also plateau. Those strains still exceed the lower remodeling threshold, so conservation-mode remodeling retains existing bone. Most aging adults lose momentary muscle strength, so their bone strains fall toward the remodeling threshold. That drop leaves modeling off and switches remodeling to its disuse mode to begin removing bone next to marrow, contributing to the well-known age-related loss of bone. Although "vigorous" exercise by aging adults can raise strains above the remodeling threshold to turn conservation-mode remodeling back on and reduce or stop further bone losses, causing the much larger strains needed to reach or exceed the modeling threshold would require larger increases in momentary muscle strength and muscle mass than most such adults could achieve. Thus, exercises that can readily increase bone strength and "mass" in children and adolescents (in whom modeling is already turned on) only seem to reduce bone loss in aging adults. This difference makes their bones seem partly unresponsive to physical exercise. This effect would occur in addition to possible nonbiomechanical explanations that others have suggested for the phenomenon.