Age-related differences in physical activity level and bone density in humans

Currently age-related bone loss is not fully understood. Factors affecting bone density include: hormonal changes in women, physical activity, genetic determination, and peak level of bone mass attained in youth. Loss of bone mass with bed rest, spaceflight, and paralysis all indicate a significant contribution from external forces on bone mass. Several theories relating mechanical forces to bone density have been proposed. Validation of these theories has been limited by the inability to accurately measure cumulative loads imposed during normal daily activity. The calcaneus (heel) has recently received increased research focus as an alternative site for non-invasive measurement of bone density. Situated closest to externally applied loads (ground reaction force, GRFz), being responsive to changes in external loading, and its ability to be radiographically isolated all make the calcaneus an attractive site for bone density measurement. The objectives of this work were to examine the influence of age and activity on bone density in the calcaneus within the context of bone adaptation theories and to establish new methods of quantifying daily physical activity.;The first studies report calibration and performance testing of the NASA GRF Activity Monitor and baseline (non-exercising) activity in terms of 'relative' GRFz (relative to subject body weight) for a group of men and women. Reasonably accurate long-term activity monitoring was shown using our calibration, data collection and monitoring equipment (RMSE ∼0.185 Body Weights, BW). Objective quantification of human activity was shown with clear differences between loading histories.;The next study on female subjects investigated differences in physical activity. Lowloaders (LL) were lower in all activity parameters compared to Highloaders (HL) and Elderly (E) women had a lower mean cycle load compared to Young (Y) women. A lower number of high magnitude cycles generated by E women may be the likely cause for a lower mean cycle load compared to Y women. High BW individuals were objectively shown to have a lower activity parameter.;The next study on the same female subjects investigated the relationship between physical activity and bone density. Using two-way ANOVA, Calcaneal Bone Mineral Density (CBMD) was not related to self-selected activity group (HL/LL) but was related to age group (E<Y). Nevertheless, when CBMD was scaled to account for differences in BW it was significant to activity group. Time spent at highload and number of highloading cycles measured using the NASA GRF Activity Monitor did not correlate to CBMD. The stress exponent used in our bone remodeling theory was determined from this study to be 6.07, falling within previously predicted values (i.e. 3--8). Refined data on distribution of calcaneal density may be needed to find a correlation between activity group and CBMD. Importantly, this study showed that a 'daily stimulus' is predictive of CBMD, although with a low correlation.;The final study reported here investigates gait, physical activity and age differences for the same female subjects. HL walked and ran faster, suggesting higher muscle mass than LL. CBMD was positively correlated to mean peak GRF measured off a force plate and to maximum running speed. Results indicate a significant trend for lower gait speed and associated peak vertical GRF with E female subjects, as expected although not confirmed with activity data. These findings support the idea that age related bone loss is associated with slower gait speed and perhaps reduced muscle mass.