This study was conducted as a substudy of the Korean Longitudinal Study on Health and Aging (KLoSHA). The KLoSHA was designed as a population-based prospective cohort study of health, aging, and common geriatric diseases in the elderly Korean population, conducted from September 2005 to August 2006, and involved residents of Seongnam City, a satellite of metropolitan Seoul, South Korea. The institutional review board of the authors' hospital approved this cohort-based study, and written informed consent was obtained from every participant. Candidates were randomly drawn from a roster of patients 65 years or older using a computer-generated list of resident registration numbers. We invited 1,118 individuals to participate in this study by letter and telephone, of which 696 agreed to participate (a response rate of 62.3%). A specially trained nurse recorded demographic data and conducted an interview, asking patients questions regarding a current or past medical history of illness or surgery. Several questionnaires such as general health status, upper and lower extremity functions, cognitive function, depression etc., were used at the point of enrollment. We excluded participants who had a history of trauma in the hand or wrist or prior surgery on the upper extremity. We also excluded those with symptoms of shoulder or elbow pain, and patients with rheumatoid arthritis. Finally, 266 participants (143 males, 123 females) were included in the analysis. The mean age of subjects was 71.7 ± 4.4 years (range, 65-87 years) in men, 70.7 ± 4.5 years (range, 65-86 years) in women.

We evaluated grip strengths of the dominant hand in a standardized manner.[20] In a neutral position of the arm, forearm, and wrist, 2 consecutive attempts with 1-minute intervalswere each measured in kilograms with a Jamar 5030J1 hydraulic dynamometer (Sammons Preston, Bolingbrook, IL, USA). For statistical analysis, we used average values.

Factors assessed for potential association with grip strength were; 1) demographics such as age and gender, 2) body constructs such as height, body mass index (BMI), and BMD, 3) upper extremity functional status using disabilities of the arm, shoulder and hand (DASH) scores,[21,22] and 4) mental health status using a depression scale (geriatric depression scale Korean version, GDS-K) and the short form-36 (SF36) mental health score.[23-25]

Height and body weight were measured by standard methods. BMI was calculated as weight in kilograms divided by the square of height in meters. The BMD (g/cm²) of the lumbar spine (L2-L4) and proximal femur was measured by dual-energy X-ray absorptiometry (DXA; model DPX, GE-Lunar Corp., Madison, WI, USA). All scans performed on the same machine by the same operator were analyzed with the same software. We used femur neck BMD for statistical analysis; Marin et al.[26] reported that handgrip strength and lean mass were the variables that were best explained by femoral neck BMD.

Upper extremity functional status was assessed by using the Korean version of the DASH scores.[21] DASH is a self-administered, upper extremity-specific questionnaire and contains 30 items: 21 on physical function, 5 on symptoms, and 4 on social role functions. The DASH has been widely used for evaluations of upper extremity functions,[22] and is also well known for its reliability and validity.[22] The DASH provides overall scores ranged from 0 and 100 (a best possible score of 0 and a worst possible score of 100).

Mental health status was assessed using a depression scale (GDS-K) and the SF36 mental health score. The GDS-K, which is a standardized self-questionnaire scores ranging from 0 to 15 (a score of 5 or more indicates depression) were used to specifically assess the influence of depression.[23] The SF36 is commonly used to represent broad aspects of patient-reported general health status, especially regarding musculoskeletal disorders.[24,25] The thirty-six items in the questionnaire are grouped on eight health subscales, and these 8 scales can be combined into two summary measures that provide overall estimates of physical health (physical component summary, PCS) and mental health (mental component summary, MCS). We used only MCS score to assess psychological status in this study.

Descriptive statistics were calculated to determine the demographic and clinical characteristics of participants. Comparison of variables between male and female was tested by using student t-test. Univariate analyses were performed initially between grip strength and potential factors, and then variables that were significant at P < 0.1 in the univariate analyses as well as those factors deemed clinically important were included in the multivariate analyses. Multiple linear regression analyses with stepwise variable selection were used to assess the relative contributions of factors that influence the grip strength. Multiple linear regression models assess the abilities of explanatory variables to account for variations in response variables while accounting for any confounding between explanatory variables, and produce a statistic called adjusted R², which reflects the percentage of overall variability in the dependant variable that can be explained by the explanatory variables included in a multiple linear regression model. Statistical analyses were conducted using SPSS for windows version 18.0. (SPSS Inc., Chicago, IL, USA) and statistical significance was accepted for P values of < 0.05.