Muscle strength and areal bone mineral density at the hip in women: a cross-sectional study

BACKGROUND: Muscle strengthening exercises are promoted for building and maintaining a healthy skeleton. We aimed to investigate the relationship between muscle strength and areal bone mineral density (BMD) at the hip in women aged 26-97 years.

METHODS: This cross-sectional study utilises data from 863 women assessed for the Geelong Osteoporosis Study. Measures of hip flexor and abductor strength were made using a hand-held dynamometer (Nicholas Manual Muscle Tester). The maximal measure from three trials on each leg was used for analyses. BMD was measured at the hip using dual energy x-ray absorptiometry (DXA; Lunar DPX-L). Total lean mass, body fat mass and appendicular lean mass were determined from whole body DXA scans. Linear regression techniques were used with muscle strength as the independent variable and BMD as the dependent variable. Models were adjusted for age and indices of body composition.

RESULTS: Measures of age-adjusted hip flexor strength and hip abductor strength were positively associated with total hip BMD. For each standard deviation (SD) increase in hip flexor strength, the increase in mean total hip BMD (SD) was 10.4 % (p = 0.009). A similar pattern was observed for hip abductor strength, with an increase in mean total hip BMD of 22.8 % (p = 0.025). All associations between hip muscle strength and total hip BMD were independent of height, but were nullified after adjusting for appendicular lean mass or total lean mass.

CONCLUSIONS: There was a positive association observed between muscle strength and BMD at the hip. However, this association was explained by measures of lean mass.

Persisting side-to-side differences in bone mineral content, but not in muscle strength and tendon stiffness after anterior cruciate ligament reconstruction

Tendon stiffness may be involved in limiting peak musculoskeletal forces and thus may constitute an upper limit for bone strength. The patellar tendon bone (PTB) graft, which is harvested from the patellar tendon during surgical reconstruction of the anterior cruciate ligament (ACL), is an ideal scenario to test this hypothesis. Eleven participants were recruited who had undergone surgical reconstruction of the ACL with a PTB graft 1-10 years prior to study inclusion. As previously reported, there was no side-to-side difference in thigh muscle cross-sectional area, in maximum voluntary knee extension torque, or in patellar tendon stiffness, suggesting full recovery of musculature and tendon. However, in the present study bone mineral content (BMC), assessed by peripheral quantitative computed tomography, was lower on the operated side than on the control side in four regions studied (P = 0·0019). Differences were less pronounced in the two sites directly affected by the operation (patella and tibia epiphysis) when compared to the more remote sites. Moreover, significant side-to-side differences were found in BMC in the trabecular compartment in the femoral and tibial epiphysis (P = 0·004 and P = 0·047, respectively) with reductions on the operated side, but increased in the patella (P = 0·00016). Cortical BMC, by contrast, was lower on the operated side at all sites except the tibia epiphysis (P = 0·09). These findings suggest that impaired recovery of BMC following ACL reconstruction is not because of lack of recovery of knee extensor strength or patellar tendon stiffness. The responsible mechanisms still remain to be determined.

WISE-2005: bed-rest induced changes in bone mineral density in women during 60 days simulated microgravity

To better understand the effects of prolonged bed-rest in women, 24 healthy women aged 25 to 40 years participated in 60-days of strict 6° head-down tilt bed-rest (WISE-2005). Subjects were assigned to either a control group (CON, n=8) which performed no countermeasure, an exercise group (EXE, n=8) undertaking a combination of resistive and endurance training or a nutrition group (NUT, n=8), which received a high protein diet. Using peripheral quantitative computed tomography (pQCT) and dual X-ray absorptiometry (DXA), bone mineral density (BMD) changes at various sites, body-composition and lower-leg and forearm muscle cross-sectional area were measured up to 1-year after bed-rest. Bone loss was greatest at the distal tibia and proximal femur, though losses in trabecular density at the distal radius were also seen. Some of these bone losses remained statistically significant one-year after bed-rest. There was no statistically significant impediment of bone loss by either countermeasure in comparison to the control-group. The exercise countermeasure did, however, reduce muscle cross-sectional area and lean mass loss in the lower-limb and also resulted in a greater loss of fat mass whereas the nutrition countermeasure had no impact on these parameters. The findings suggest that regional differences in bone loss occur in women during prolonged bed-rest with incomplete recovery of this loss one-year after bed-rest. The countermeasures as implemented were not optimal in preventing bone loss during bed-rest and further development is required.