Physical functioning represents an integrated marker of aging, influenced by a broad array of physiological and clinical characteristics interacting with behavior and the social environment [1]. In old age, decreased muscle strength predisposes people to functional limitations and disability [2]. Older people had less appendicular skeletal muscle than younger, the decline in muscle mass of -0.4 kg/decade in women and 0.8 kg/decade in men starting at the age of 20 [3]. According to Rantanen et al. [4], the intra-individual strength changes over time were significant in all ages. Muscle strength was found to influence life-span. Those who have higher grip strength during midlife remain stronger than others in old age [4] and people who have greater muscle strength during midlife then have a lower risk of becoming disabled because of their greater ability to maintain that strength regardless of chronic conditions that may develop [5]. Grip strength is known as a good predictor of unhealthy outcomes such as mortality and may indicate subclinical disease, which later develops into clinical disease [5] and functional limitation [6]. Grip strength is also known to be correlated with bone mineral density (BMD) in middle-aged men [7]. Age-related bone loss is reported to be associated with decreased BMD [8] and with weakened muscle strength [9]. Grip strength tests are convenient, safe, and reliable, and do not require large or expensive equipment [10]. Consequently, grip strength has been used as an indicator of overall muscle strength in many population studies [11].

Smoking is one of the major components determining BMD [12]. Smoking is associated with lower BMD of neck and spine in both male and female elderly and is known to increase hip fracture rates compared with nonsmokers [10]. In Ward and Klesges [11] study, absolute effects at most bone sites were greatest for current smokers compared with those who had never smoked.

Elderly with lower socioeconomic status (SES) according to educational attainment, income, and occupation have significantly higher morbidity and mortality than individuals with higher SES [13,14]. A study has shown that SES is related to mobility disability in later life [15]. Levels of education and/or income have also been significantly associated with decreased physical function [14,16]. Studies have found that higher SES is associated with higher BMD at total hip [17], femoral neck [18], and spine [19].

This study was conducted to elucidate whether smoking and social economic status are associated with grip strength in community-dwelling elderly men in Korea.

Table 1 shows overall characteristics of study subjects according to level of grip strength. The subjects with high grip strength were younger, more educated, more likely to be smokers, and more likely to participate in regular exercise than those with low grip strength (Table 1). The grip strength of nonsmokers adjusted for age, smoking status, level of education, and obesity was higher than that of past and current smokers (Figure 2).

After adjustment for age, education, income, regular excise, and body mass index, the risk of decreased grip strength gradually increased among current smokers (aOR, 4.5; 95% CI, 1.10 to 9.17), those with low educational attainment (aOR, 2.88; 95% CI, 1.08 to 7.66) and those with low monthly income (aOR, 2.86; 95% CI, 1.08 to 7.54), compared with non-smokers, those with high educational attainment, and those with high monthly income, respectively (Table 2).

Current smokers were found to have higher risk of decreased grip strength compared with non-smokers. Education and income also affected the risk of the decline of grip strength among the elderly men in this study.

Hands have many physiological and anatomical changes associated with aging. Impaired hand abilities are observed among the elderly with metabolic and skeletal diseases such as osteoarthritis and rheumatoid arthritis [22]. Starting in middle age, men lose approximately a fifth of BMD over their lifetimes [23]. Smokers were reported to have less bone mass at all measured sites, and smoking cessation had a beneficial effect on bone mass as past smokers had an intermediate bone phenotype compared with either lifelong non-smokers or current smokers [24]. Smoking is significantly associated with BMD in middle-aged men [25,26] since the nicotine from cigarettes is involved in the metabolism of calcium and vitamin D and affects bone health [27]. Smokers may have different taste perception, leading them to choose unhealthy food [28], and may take less exercise [29] compared with non-smokers. Kanis et al. [30] reported that current smoking was associated with a 25% increase in fracture risk compared with the risk in subjects who had never smoked. One study of elderly men and women reported grip strength can predict BMD of the proximal radius [31] and another study of athletes also reported that grip strength was a predictor of radial BMD [32].

Low income and low educational attainment can affect the risk of overall health and functional limitations among the elderly [33] and are known to be associated with lower levels of self-efficacy in specific behavioral domains [34]. High income and educational attainment are more likely to have a higher level of perceived control and thus situation-specific self-efficacy [35]. This means that higher socioeconomic status may increase the likelihood of individuals having the confidence necessary necessary to attempt socially prescribed behavioral changes and having the resources necessary to ease the adoption process (e.g., time, money) [33]. Higher education may allow someone to access more knowledge on health, practice more healthy behaviors, have more employment opportunities, and earn higher income, which all affect health [36]. Income affects health in various ways, such as availability of material resources and health services [15].

Given the public health implications of smoking on bone health, it is important that this information be incorporated into smoking prevention and cessation efforts targeting the elderly [37]. Elderly people commonly have difficulties in terms of hand function and hand strength can affect simple everyday actions. Grip strength is a simple test and an important index of the body's muscle situation and can predict disability in the nearer future. Declining physical fitness as a natural aging process may not be completely prevented, however.

One of the strengths of this study is that the long-term effect of smoking on physical function and occurrence of chronic diseases can be assessed in a relatively short time through a test of grip strength, which can be regarded as a surrogate indicator predicting these health conditions. This study may not appropriate to demonstrate the effect of smoking on health outcomes directly since we observed them at the same time in this cross-sectional study.

A limitation of this study is that there may be residual confounding owing to categorical variables such as smoking (non-, ex-, and current) and socioeconomic status. It is hard to exclude the possibilities of unknown confounders between groups with high and low grip strength. In addition, because participants in this study at least maintained their health, it was not representative of all elderly people, but the findings can be applied to healthy elderly people residing in the community.

Though we cannot change some factors such as age and education among the elderly, we can improve health behaviors such as smoking, and reverse or delay the process of decline of muscle strength.