Purpose: To evaluate the independent and combined associations of muscular strength (MS) and cardiorespiratory fitness (CRF) with bone mineral density (BMD) in older adults.

Methods: This cross-sectional study included 353 older adults (189 women and 164 men) aged 65 years or older (mean ages 72 and 71 in women and men, respectively). MS was assessed by grip strength (average of the highest force values after three trials on each hand, Jamar+ digital hand dynamometer, Patterson Medical) and CRF was assessed by the 400-meter walk test (minutes to complete 400-meter walking). Whole-body BMD was measured by dual-energy X-ray absorptiometry (DXA, Hologic Horizon W, Hologic). Low BMD was defined as t-score below -1.0, the value that was calculated with BMD (g/cm2). Linear regression was used to find the associations of MS and CRF with BMD in women and men, separately. Logistic regression was used to calculate odds ratios (ORs) and 95% confidence intervals (CIs) of having low BMD in BMI-specific tertiles of MS and CRF in women and men, separately. To investigate the combined association of MS and CRF with low BMD, joint analyses were conducted after dichotomizing MS and CRF (upper two thirds and lower one third). Then, study participants were assigned to one of weak and unfit, weak and fit, strong and unfit, and strong and fit groups.

Results: In women, only MS was positively associated with t-score after adjusting for age from linear regression. In men, only MS was positively associated with t-score after adjusting for age, current smoking status, heavy alcohol consumption, physical inactivity, supplement/medication for bone health, body mass index, and CRF. In women, MS and CRF appeared to be inversely associated with the prevalence of low BMD, although not significant (all p-trends>0.05) from logistic regression. In men, only MS appeared to be inversely associated with the prevalence of low BMD, although not significant (all p-trends>0.05). In the joint analyses of MS and CRF with low BMD, although not significant, being strong and fit appeared to be more strongly associated with lower prevalence of low BMD in women, but not in men possibly due to the very small number of cases of low BMD in each group (n=1-3).

Conclusion: Our study suggests that higher levels of MS and CRF may be associated with better bone health in older adults although most results did not reach statistical significance in this relatively healthy and fit population. Further prospective studies from large representative older adult populations are clearly warranted.