3.2. Bone Mineral Density and Physical Performance
It is well established that PP is an independent predictor of the risk for falls and fractures [71
]. However, the relationship between PP and BMD is unclear, with conflicting conclusions from various research studies. With aging, loss of muscle and bone mass has significant implications on both physical function and health [18
Some studies support the notion that PP has weak or no association with BMD of various skeletal sites. A study with 47 older women (80.3 ± 7.0 years) who had previous hip fracture showed no association between walking speed and nonfractured leg and lumbar spine BMD [76
]. Another study [40
] with 104 community dwelling older adults investigated the relationship between femur BMD and handgrip strength, where a positive correlation was only observed in women (r
= 0.4, P
< .001), and not in men (r
= 0.27, P
= .149). There was also a significant correlation between body weight and handgrip strength in women (r
= 0.32, P
= .006); therefore, the latter was allometrically scaled with body weight to reduce the influence from other variables related to body dimension. Linear regression analyses showed that handgrip strength divided by body weight only explained 6% of variation in femur BMD, which was again shown only in the women. The Osteoporotic Fractures in Men Study [14
] including 5,995 men aged 73.7 ± 5.9 years showed that only handgrip strength had a positive association with femoral and lumbar BMD measured by DXA. Accordingly, one standard deviation increase in handgrip strength (8.2
kg) showed significant increases in femoral and lumbar BMD (OR = 1.75[1.3,2.2](%) and 1.74[1.3,2.2](%), resp.) after adjustment for age, but these associations disappeared after adjusting for both age and weight (OR = 0.73[0.3,1.2](%) and 1.05[0.6,1.5](%), resp.). On the other hand, slower STS (3.3 seconds) was not associated with lower femoral and lumbar BMD (OR = −0.05[−0.4, −0.5](%), −0.06[−0.5, −0.4](%), resp.) in age-adjusted models, which appeared to be significant after adjusting for both age and weight (−1.06[−1.5, −0.7](%), −0.85[−1.3, −0.4](%), resp.). Walking speed however showed no association with BMD.
Conversely, other studies have shown positive associations between BMD of various skeletal sites and lower extremity PP measures [41
]. Results of the study in 368 older men aged 78.8 ± 4.1 years living in a veterans' home in Taiwan [78
] found that the calcaneus bone mass measured by Soundscan (quantitative ultrasound) expressed as broadband ultrasound attenuation was positively correlated with walking speed measured by a 6-minute walking distance. After adjusting for BMI, waist circumference, and hemoglobin level as a marker of nutritional adequacy and status, mean broadband ultrasound attenuation (BUA) values were significantly different among the groups, classified according to the quartiles of 6-minute walking distance, showing that the lowest quartile had the shortest 6-minute walking distance. Similar trends were shown in the study with female subjects in Japan [77
]. In this study which was conducted with 200 Japanese women 65 years and older, the relationship between walking parameters and bone health as measured by quantitative ultrasound bone densitometer expressed as stiffness index was examined. The results showed that higher stiffness index (SI) was positively correlated with faster usual and maximum walking speed. In multiple regression analysis with BMI, handgrip strength, and walking parameters, maximum walking speed had the strongest association with SI among women aged between 65 and 74 years. In the study with 606 Finnish older women aged 68.0 ± 1.8 years [41
], hip BMD including femoral neck, trochanter, and total proximal femur and lumbar spine BMD were correlated with PP. After adjusting for age, BMI, hormone therapy use, years since menopause, smoking status, and use of oral glucocorticoids, multivariate linear regression analysis showed that the strongest relationship was between hip BMD and OLS, followed by handgrip strength and OLS. The relationship of spine BMD and both OLS and handgrip strength was weak but remained significant, however, not significant with walking speed or STS, separately.
The findings in previous studies that the PP related to a specific extremity is associated with bone mass of that same extremity are well supported. The results from the Study of Osteoporotic Fractures conducted in 5,405 older women of non-African origins aged 73.8 ± 5.3 years also showed that handgrip strength was positively associated with higher BMD of various skeletal sites, but particularly that of the forearm [44
]. With an increase of 5
kg of handgrip strength, distal radius and femoral neck BMD increased by 3% and 1% after controlling for age and weight, respectively. Additionally, faster walking speed (0.2
m/s) was associated with an increase in 1% BMD of femoral neck, but not at other skeletal sites. Similarly, Lindsey et al. [43
] conducted a study in 116 postmenopausal Caucasian women aged 68.3 ± 6.8 years examining the relationship between different skeletal sites and measures of functionality, including normal and brisk walking speed, step length, OLS, timed chair sit-to-stand test, and handgrip strength.
Results from multiple regression controlled for BMI, age of onset of menarche, total calcium intake, and total hours of physical activity showed that higher femoral neck, hip, and total BMD were significantly associated with faster walking speed, longer step length, and longer OLS time. On the other hand, stronger handgrip strength was significantly associated with all forearm BMD sites, but not with higher femoral neck BMD.
Notably, the relationship between handgrip strength and BMD of forearm was stronger than BMD of other skeletal sites in several studies. Another study with 2,619 community-dwelling older adults participating in the Health ABC study also investigated the correlation between handgrip strength and BMD of various skeletal sites [13
]. Handgrip strength had the strongest positive correlation with upper limb BMD, followed by lower limb BMD or total and femoral neck BMD. Similarly, in another study [36
] where bone mass was measured using an OsteoAnalyzer in 9,704 older women recruited from four clinical centers in the US, an increase in 5
kg of handgrip strength is correlated with a 3.5% increase in radius BMD (CI: 2.8, 4.3).
Other studies have examined handgrip strength from both hands, suggesting that nondominant handgrip strength could be a useful indicator to assess BMD. With 91 healthy active men and women living in Palo Alto, California, aged 70.0 ± 0.7 years [37
], midradius and lumbar spine BMD were measured. In women, grip strength from both hands showed a significant relationship with spine and midradius BMD, especially BMD of the dominant hand. In men, only midradius BMD showed a significantly positive correlation with handgrip strength from both hands. On the other hand, in the study with 649 postmenopausal Caucasian women aged >65 years living in Southern California [42
], handgrip strength from both the dominant and nondominant arm was examined in relation to BMD of upper limb, as measured by single-photon absorptiometry, and of spine and hip as measured by DXA. With dominant handgrip strength, BMD of the hip showed the strongest positive association followed by wrist and spine BMD after adjusting for potential covariates including age, BMI, thiazide use, smoking, regular exercise, arthritis, years since menopause, and estrogen use. With radius BMD, the positive trend was observed, but significance was not reached. However, with the nondominant handgrip strength, a positive association with BMD was observed for all skeletal sites measured, including hip, radius, wrist, and spine.
Notably, the study with the Health ABC study cohort including 3041 community-dwelling older adults [79
] showed that the association between hip region BMD and PP was sex and race specific, with the strongest association in black women, followed by white women and men. In this study, ANCOVA results adjusted for age, study site, height, weight, medication (thiazides, corticosteroids, estrogen for women), smoking pack-years, and physical activity showed that the mean value of femoral neck and trochanter BMD was significantly different among groups classified according to the quartiles of performance measures including timed chair STS, 6-meter walking speed, and OLS. The graded associations between femoral neck and trochanter BMD and groups classified by all PP measures were significant among black women, showing a higher BMD with better PP (P
< .05, except standing balance and femoral neck BMD; P
= .163 for trend). In white women, there was a moderate trend observed between femoral neck BMD and STS only. In white and black men, there was graded association found in trochanter BMD among the groups based on STS quartiles, but the difference among the groups was not significant. Therefore, the conclusions drawn included that the association between BMD and PP measures was strongest in black women, followed by white women and men. Since the analyses already included physical activity, other factors like nutritional, hormonal, or environmental could be considered to further explain the differences under the genetic basis among races.