The main findings from this 10-year population-based observational study were that the mean annual percentage rate of loss in forearm BMD were 1.5 to 2.0-fold greater in women compared to men aged over 60 years, while the annual losses in grip strength, balance or gait velocity were similar in both genders. Further comparison of the changes in each of the musculoskeletal and functional parameters within each 10-year age-change category revealed that for both men and women aged <70 years, the mean annual percentage loss in grip strength was on average, 1-3% greater than the mean annual decline in BMD, balance and gait velocity. The greatest deterioration in balance and gait velocity occurred after the age of 60 and 70 years, respectively, in both men and women. Together, these findings argue for the importance of promoting targeted lifestyle and exercise interventions that focus on improving muscle strength, balance and gait in older adults, with a particular focus on optimising muscle strength from the age of 50 years and balance and gait speed from 60 years and older.
The finding that the mean annual rates of loss in forearm BMD were greater in women compared to men aged over 60 years is consistent with the results from several [17, 18] but not all  previous prospective studies examining age-related changes in forearm BMD. For instance, in a 6-year prospective study of Norwegian women and men aged 45–84 years it was reported that the rates of bone loss at the distal forearm increased significantly across the 5-year age groups from 45–49 to 75+ years in men but not women . These findings in women are perhaps not unexpected since bone loss accelerates during the menopause. However, we found that the rate of forearm bone loss was greatest in women from 60 to 70 and 70+ years; there were no significant differences in bone loss across the three age-categories in men. Although others have reported similar findings at the distal radius , it is possible that these mixed results may relate to the skeletal site assessed. Most previous studies have measured distal or ultradistal forearm BMD, which contains predominantly trabecular bone, whereas we assessed BMD at 6 cm proximal to the ulnar styloid process, a site consisting mostly of cortical bone. Indeed, there is evidence for a gender-specific difference in cortical and trabecular bone loss. A cross-sectional study using high-resolution quantitative computed tomography to assess distal radius cortical and trabecular bone microarchitecture in 644 Canadian adults aged 20–99 years reported that women tended to experience a greater decline in cortical thickness and cortical BMD and a greater increase in cortical porosity compared to men; trabecular bone loss was similar between men and women . Since the results from this study were not presented in 5- or 10-year age categories, it is not possible to determine whether there were gender differences in cortical and trabecular bone loss after the age of 50 years. Nevertheless, these findings provide some evidence to support the greater rate of bone loss in the older women in our study.
With regard to the changes in grip strength, we found that there was a consistent rate of loss (2.2- 3.1%/year) in grip strength across the three age-change categories which did not differ by gender. However, there was a trend (P = 0.06) for a greater rate of loss in grip strength in men compared to women in the 50 to 60 and 60 to 70 year age-categories. While this is consistent with the findings from a 10-year prospective study in 120 adults aged 46–78 years which observed a greater loss in elbow extensor and flexor strength in men than women , the general consensus is that men and women experience similar relative losses in muscle strength [8, 9, 23]. In terms of the magnitude of the annual rate of loss in grip strength, many previous studies (particularly cross-sectional) have reported a mean loss ranging from 0.5% to 2.0% per year [9, 10, 24]. In contrast, several prospective studies conducted in both men and women have reported rates of loss ranging from 2.4% to 2.8% per annum, which is consistent with the findings from our study [8, 9]. However, in most of these studies the course of the decline in strength accelerated with advancing age [3, 9, 10], whereas we observed a consistent pattern of loss in grip strength in both men and women from the age of 50 years. While it is difficult to explain this finding, it is possibly related to the characteristics of our cohort. The participants included in the prospective analysis were generally healthy, independent living elderly men and women and thus it is possible that they had ‘more to lose’ than a typical cohort of older adults. Indeed, those lost to follow-up were significantly older and had lower BMD and grip strength, poorer function, and poorer health than those who returned for the follow-up testing (Additional file 2: Table S1). Thus, it is possibly that our findings may be confounded by selection bias.
In terms of balance and gait, there are also inconclusive findings with regard to the age- and gender- specific patterns of loss. For instance, we found that the age (timing) and rate (magnitude) of loss in both balance and gait speed were both similar for men and women, with balance deteriorating at an earlier age than gait speed (60–70 years vs 70+ years) in both sexes. These findings are consistent with the results from a number of previous studies which have reported a curvilinear relationship between measures of gait and balance with age [5, 6, 15], with little evidence of a gender difference , and a more pronounced decline in balance/postural stability after the age of 60 [5, 7] and gait speed after the age of 65 to 70 years [11, 26]. Given that impaired balance and reduced gait speed have been associated with an increased risk of falls, disability and even reduced survival [1, 2, 4], these findings provide useful information for the optimal time to intervene with respect to targeted lifestyle and exercise strategies to optimize gait and balance in the elderly.
Previous research has shown that various measures of BMD, muscle strength, balance and gait (and their changes) are moderately correlated with each other , but no studies appear to have directly compared the relative rates of change (loss) between these different measures at different ages in both men and women. Our finding that the annual rate of the loss in grip strength was significantly greater than the annual decline in BMD, balance and gait velocity for both men and women in the 50 to 60 and 60 to 70 year age-change categories adds further support to current consensus guidelines recommending that middle aged and older adults partake in regular progressive resistance training to optimise muscle strength and muscle mass . Similarly, the finding that the annual rate of loss in all measures tended to be similar after the age of 70 years suggests that a multi-modal exercise program targeting strength, balance and gait is needed to prevent the age-related decline in these measures in the elderly. However, when interpreting these findings it is important to note that a given percentage change in BMD may not be directly comparable (clinically) to similar percentage changes in muscle strength, balance and gait over the same period (the same applies to the changes in strength, balance and gait). As an example, a 5% loss in BMD is unlikely to be clinically comparable to a 5% decline in muscle strength over the same time. Nevertheless, we believe that these findings provide a unique insight into age-specific relative losses amongst different measures of musculoskeletal health and function in older adults.
The strengths of this study lie in its prospective population-based nature; the long-term follow-up; and the assessment of BMD, grip strength, and physical function using the same measurements and apparatus. However, there are also limitations. First, this was an observational study, and other confounders, such as dietary habits, could have influenced the inferences. However, it is worth noting that all results for all outcome variables were similar whether they were analysed unadjusted or adjusted for all potential covariates (data not shown). Second, the findings may not be applicable to other populations, such as those with a chronic disease(s) or other comorbidities, since our cohort consisted of generally healthy, independent living elderly men and women. This is confounded further by the fact that 47% of men and 38% of women from the original cohort did not return for follow-up assessment, and those lost to follow-up were significantly older, had a greater history of disease/medication use, were more likely to self-report disability, and had lower BMD and poorer function and health than those who returned (Additional file 2: Table S1). Third, it is possible that individual physical performance levels and/or health status may have changed at different times throughout the 10-year follow-up period, which may have masked the timing of the age-related changes in the different measures. Finally, the assessment of BMD was performed using SPA and limited to the forearm, and thus our findings cannot be generalized to other skeletal sites.