Comparison of performance-based measures among native Japanese, Japanese-Americans in Hawaii and Caucasian women in the United States, ages 65 years and over: a cross-sectional study

Hip fractures result in increased health care utilization, suffering, and functional disability, and are one of the most serious fractures among elderly people [1]. Japanese (both in Japan and Hawaii) have a lower incidence of hip and other nonspine fractures than Caucasians in North America or northern Europe [2–4]. This lower fracture rate among Japanese is not explained by greater bone mass [5–9].

The lower fracture rates might have resulted from a lower prevalence of risk factors for fractures among Japanese. Hip fractures usually occur after a fall, but there are multiple risk factors for hip fracture, including bone mass and fall-related factors [10, 11]. We reported earlier that the incidence and prevalence of falls among Japanese (both in Japan and Hawaii) is approximately half that of Caucasians in North America and Britain [12, 13], but the reason for these differences are not clear.

Risk factors for falls, such as dementia, visual impairment, stroke, previous falls, medications and environmental hazards have been reported in previous studies [14–20]. Most of these risk factors indicate difficulty with neurologic and musculoskeletal functions that contribute to physical stability [16]. Previous studies showed that performance-based measures of physical function are strongly associated with risk of falling among the elderly, including studies of two of the populations reported here [16, 18, 21]. We recently reported that Japanese women performed better than Caucasian women on some neuromuscular tests, but Caucasians performed better on others, and that the differences in performance did not explain the observed differences in rates of falls [22]. Nevertheless, some of the observed differences in performance may be associated with lower risk of hip fracture independent of falling, if they influence protective responses in the event of a fall. The purpose of the current study was to compare performance-based measures (chair stand time, usual and rapid walking speed, and grip strength) among native Japanese, Japanese-Americans in Hawaii and Caucasian women in United States aged 65 years and over, to explore whether there are any obvious differences in physical performance, because there are few studies comparing neuromuscular function between races [22]. Hypotheses generated from the current study may be useful in planning future studies to explore whether differences in performance-based characteristics might be responsible for differences in fracture risk between populations.

Grip strength was greatest in Japan, followed by Caucasians, Japanese-Americans in Hawaii. Grip strength was about 10% stronger among native Japanese than Caucasians, whereas it was about 7% weaker among Japanese-Americans than Caucasians. Additional adjustment for body size (height, weight and BMI) increased adjusted means of chair stand time and grip strength among either group of Japanese, whose body size was smaller compared with Caucasians, but had little or no effect on the statistical significance, and the general relationships described above remained relatively unchanged.

Since there have been few comparative studies of neuromuscular function between races [22], we compared performance-based measures among native Japanese, Japanese-American in Hawaii and Caucasian women in United States aged 65 and over after careful standardization. Both native Japanese and Japanese-American women in Hawaii had better performance than Caucasians on the chair stand and walking speed tests. Decreased performance on the chair stand test is associated with an increased risk of falls [18] and hip fracture [10]. Slower gait speed was also an independent fall-related predictor of hip fracture [11]. Thus, better neuromuscular function in old age may partly explain why the rates of falls and hip fractures among Japanese are lower than those among North American Caucasians. Although grip strength is also reported to be associated with risk of falls and hip fractures [11, 17], the difference did not consistently favor Japanese over Caucasians.

Life-long squatting behaviors, a component of traditional Japanese lifestyle, may have had a long-term effect on preventing falls and hip fractures [28–30]. Since Japanese elderly spent most of their lives using tatami mats and futon bedding, and Eastern-style toilets (all of which are within inches of ground level and require squatting), they may have developed a better sense of balance and lower extremity strength by sitting, squatting, and rising to stand from floor level. An international cross-cultural study among residents of Japanese and American nursing homes demonstrated maintenance of quadriceps strength and fewer falls among Japanese [29]. A recent case-control study of risk factors for hip fractures in Japanese elderly revealed a lower risk among those sleeping in futon bedding than in Western-style beds [30]. These daily activities of the traditional Japanese lifestyle may in part contribute to better performance among native Japanese, which could help in preventing falls and hip fractures.

Grip strength was greatest in Japan, which may reflect aspects of physical activity in the rural farming district that this sample was drawn from, but information on physical activity was not available to explore this hypothesis. Many older people in rural Japan grow rice and vegetables using both manual labor and machinery. In contrast, the Japanese-Americans and Caucasians were drawn from urban/suburban settings.

Performance on all tests (chair stand time, walking speed and grip strength) decreased with age within each of the three populations, which is consistent with previous studies [27, 31, 32]. Age-related poor performance may have two distinct roles in the occurrence of hip fractures: it may not only increase the risk of falling but also influence an individual's speed, coordination, and protective responses during a fall [11, 33]. Tests of neuromuscular performance may be useful in primary care and clinical practice because they are simple, recreate situations in which falls are likely to occur, and provide a dynamic, integrated assessment of physical capacity [34].

Some performance-based measures may depend on body size [27, 32, 35]. In the current study, Caucasian women had the greatest body size (height, weight and BMI), followed by Japanese-Americans in Hawaii, and then native Japanese women. Additional adjustment for body size increased adjusted means of chair stand time and grip strength among Japanese, which indicates that greater body size is associated with poorer chair stand performance, but with stronger grip strength.

Both neuromuscular impairments and femoral neck bone mass are independent predictors of the risk of hip fracture in elderly women, and their combined assessment improves the prediction of hip fractures [10, 11]. Thus, intervention programs to prevent hip fractures may be most effective if they target both fall-related factors and maintenance of bone mass. Effective programs for improving neuromuscular function, such as exercise and strength training may reduce the risk of falling [18, 36] and improve the effectiveness of protective responses during a fall [33], which may reduce the risk of hip fracture. This may be especially relevant among elderly women whose bone density is already considerably below the fracture threshold [33, 37].

Geometric characteristics of the femur as well as falls may be important for hip fracture risk [38, 39]. Japanese have shorter hip axis lengths than Caucasians, which may be related to the smaller body size of Japanese, and may partly explain the lower hip fracture risk among Japanese compared to Caucasians in the US [38, 39]. However, the incidence of hip fracture among Japanese was similar to that among Caucasians in Hawaii, suggesting that factors other than hip axis length are important [40]. Measurements of hip axis length in native Japanese were not available for analysis in the current study.

This study had several limitations. The participants were community-dwelling women over the age of 65, so these findings may not be generalizable to men, younger women, or nursing home residents. Backgrounds of participants were different; native Japanese were drawn from a rural area, but Japanese-American and Caucasian women were from urban/suburban settings. Thus, the differences of performance-based measures among populations may be in part influenced by current or past differences in physical activity (including occupation), or other factors. Finally, we did not have data from more detailed neurologic or musculoskeletal examinations, or other risk factors for falls and fractures, such as dementia, visual impairment, stroke, previous falls, medications and environmental hazards, calcium intake, physical activity, previous fractures and family history of fractures, any of which may be associated with hip fracture risk [14–20, 41].

In summary, performance on chair stand and walking tests of both native Japanese and Japanese-American women in Hawaii were better than Caucasians. These differences indicate better neuromuscular function in older Japanese that may confer advantages in balance and lower extremity strength. However, whereas the native Japanese had the greatest grip strength, on average, the Japanese in Hawaii had lower grip strength than Caucasians. The biologic significance of these observations are uncertain, but may partly explain the lower incidence of falls and hip fractures among Japanese compared to Caucasians. Other factors, such as differences in health, disease, medication use, risk taking, or social supports, may also contribute to the observed differences in the rates of falls and hip fractures [22]. Further study is needed to control potential confounders to determine if neuromuscular risk factors for falls are independent risk factors for hip fracture.