The baseline characteristics of the study population are displayed in Table 1; the mean age was 68.9 and 69.2 years in men and women, respectively. All women were postmenopausal. The mean body mass index score for women was 27.4 kg/m2, compared to 27.0 kg/m2 for men. Among men, 60.4% were either current or ex-smokers, compared with only 34.8% of women (p < 0.001). Men consumed more alcohol compared to women (p < 0.001). At baseline, men had larger muscles in both their upper and lower limbs (p < 0.001), stronger grip strength (p < 0.001) and walked faster than women (p = 0.010). In contrast, women had greater amounts of fat in their upper and lower limbs than men (p < 0.001). Additionally, at baseline, men had greater bone mass, density, size and strength in the radius, than women (p < 0.001). Similar sex-differences were found in the tibia bone parameters (p < 0.001).
Longitudinal changes in muscle and bone parameters are summarised in Table 2, with change in muscle mass, strength and function also displayed in Fig. 1. Muscle strength and function decreased faster than muscle mass, as displayed in the figure. As expected, over time, men and women’s forearm and calf mCSA reduced. However, the rate of loss did not differ significantly by sex. In both sexes, the rate of loss of grip strength and gait speed was over 2% per year, whereas muscle size reduced at a slower rate. Figure 1 provides a graphical representation of the proportionate change in men and women of muscle area at the forearm and calf, grip strength and gait speed. The rate of increase in forearm and calf fCSA differed by sex, affecting men more than women [forearm; men: mean (SD) 1.25 (4.84) %/year, women: mean (SD) 0.17 (2.15) %/year, calf: men: mean (SD) 3.62 (20.5) %/year, women: mean (SD) 0.57 (2.65) %/year]. However, only the rate of increase in forearm fCSA reached statistical significance (p = 0.010).
In both the radius and tibia, total bone mass and trabecular density tended to reduce at a greater rate in women than men (p < 0.10). Proximally, total area had a tendency to increase over time, showing positive values in both sexes. The rate of increase was significantly higher in men [mean (SD) 1.78 (1.64) %/year] than women [mean (SD) 1.03 (1.69) %/year] in the radius (p < 0.001). The rate of loss of cortical area was significantly greater in women than men in both the radius (p = 0.006) and tibia (p < 0.001). In the radius, cortical density decreased at a greater rate for men than for women (p = 0.001).
In general, changes in cortical bone geometry, not cortical BMD, were more strongly associated with changes in muscle mass and function, particularly at the radius (Table 3). In men, positive associations were seen between change in forearm muscle area and changes in radial total mass z-score (regression coefficient 0.30, 95% CI 0.14–0.47, p < 0.001) distally, radial Ct.Ar z-score (regression coefficient 0.36, 95% CI 0.20–0.52, p < 0.001) and cBMD z-score (regression coefficient 0.18, 95% CI 0.01–0.35, p = 0.034) in the mid-shaft, although the latter was not maintained after adjustment for demographic and lifestyle factors. No associations were seen in women. Change in calf muscle area was associated with changes in tibial Ct.Ar z-score, in both men (regression coefficient 0.14, 95% CI 0.00–0.27, p = 0.043) and women (regression coefficient 0.16, 95% CI 0.01–0.30, p = 0.032), though the association did not remain significant in men after adjustment for demographic and lifestyle factors. In women, there were also positive associations between change in calf muscle area and total mass z-score (regression coefficient 0.19, 95% CI 0.05–0.33, p = 0.007) and tBMD z-score (regression coefficient 0.25, 95% CI 0.12–0.39, p < 0.001).
Positive associations were found between change in grip strength and change in forearm mCSA (z-score) in both sexes (men: regression coefficient 0.11, 95% CI 0.03–0.18, p = 0.005, women: regression coefficient: 0.06, 95% CI 0.01–0.11, p = 0.015). In women, change in gait speed was also associated with change in calf mCSA z-score (regression coefficient 0.07, 95% CI 0.01–0.12, p = 0.012). In both men and women, a change in gait speed was associated with a change in calf fCSA z-score (men: regression coefficient − 0.06, 95% CI − 0.12 to − 0.01, p = 0.029; women: regression coefficient − 0.05, 95% CI − 0.11 to − 0.00, p = 0.045), though this association did not remain in women after adjustment for demographic and lifestyle factors.
Change in grip strength was associated with change in total radial mass z-score in men (regression coefficient 0.09, 95% CI 0.01–0.16, p = 0.025) but not when demographic and lifestyle factors were adjusted for. Change in grip strength was not associated with any other changes in radial size or bone mineral density in either sex. In men, change in grip strength was associated with change in tibia diaphysis total area z-score (regression coefficient 0.10, 95% CI 0.02–0.17, p = 0.016) and tibial mass z-score but only when demographic and lifestyle factors were adjusted for (regression coefficient 0.10, 95% CI 0.02–0.19, p = 0.022). In contrast, no relationship was identified between change in grip strength and change in tibial size or density in women.