Association of absolute and relative hand grip strength with all-cause mortality among middle-aged and old-aged people

Data were extracted from the Korean Longitudinal Study of Ageing (KLoSA) from 2006 to 2018 (follow-up). The KLoSA is designed to collect nationally representative longitudinal data on Koreans aged 45 years or older living in households, selected by multistage stratified probability sampling. A multistage stratified sampling design was used to select household units according to geographical area, including both urban and rural areas. In case of refusal to participate, another subject was selected from an additional, similar sample from the same district [16, 17]. The KLoSA was designed to help develop policies to address health and social issues that are emerging trends related to population aging. As this study used secondary data, additional individual informed consent was not required. The KLoSA survey data is publicly available, so ethical approval was not applicable for this study. This study did not collect inform consent from the participants, because their information was fully anonymized prior to analysis. All individuals were anonymized and de-identified prior to the analysis. To estimate the association between HGS and mortality among people aged 45 years or older, we included 9,102 participants at baseline in 2006 who had no missing information and followed up until 2018 to determine whether a respondent was in a state in which the measurement could be performed. They were followed up for 13 years for all-cause mortality, and as the KLoSA survey was measured every two years, participants were also assessed every two years.

The primary independent variable was HGS, which was performed in a sitting position with the elbow fixed at 90° on both sides and measured using a hand grip dynamometer (model number, NO6103; Manufacturer: Tanita Corp., Tokyo, Japan) [12]. The average of grip strength was calculated on both hands. If the subjects were unable to perform the grip test, the value from the other hand was used for the analysis [18]. Absolute HGS was measured in kilograms(kg). Relative HGS was defined as HGS (kg) divided by BMI (kg/m²) and was measured in kg/BMI. Subgroup analysis was conducted stratified into age, gender, and number of chronic diseases. For age, participants were divided into those 65 years of age or older, as age and muscle weakness are significantly related [19, 20]. In Korea, the age of 65 is generally considered the beginning of the old age, as workers typically retire at this age [21]. Additionally, the analyses included variables such as the Center for Epidemiologic Studies Depression scale (CES-D) score, Mini-Mental State Examination (MMSE) score, age, sex, education, smoking status, alcohol consumption, marital status, self-rated health, and number of chronic diseases. The CES-D scale is believed to be a good indicator of depression because of its high sensitivity, specificity, and internal consistency, which inquires whether the ten items were experienced or carried out during the past week [22]. MMSE, which is a global measurement that can quickly and easily measure cognitive functions and has been widely used in clinical evaluation, was used to calculate cognitive decline [23]. The number of chronic diseases was measured based on self-reported responses indicating whether the individual had ever been diagnosed with each disease by a physician. Chronic diseases included hypertension, diabetes, cancer, chronic obstructive pulmonary disease, liver disease, cardiovascular disease, cerebrovascular disease, mental illness, and arthritis.

All-cause mortality during the period from 2006 to the end of follow-up (2018) was the main outcome of the study. Death over a maximum follow-up period of 12 years was determined using death certificates.

Covariates were as follows: age (45–54, 55–64, 65–74 and ≥ 75 years), sex (male and female), education (elementary, middle, high school, and ≥ college), smoking status (non-smoker, former smoker, and smoker), alcohol consumption (never, former drinker, and drinker), marital status (married and single [including separated, divorced]), self-rated health (very good, good, moderate, bad, and very bad), and comorbidities such as hypertension, diabetes, cancer, chronic obstructive pulmonary disease, liver disease, heart disease, cerebrovascular diseases, mental illness, and arthritis or rheumatoid arthritis (0, 1, 2, and ≥ 3). Participants’ smoking status was categorized based on a self-reported question that asked whether they currently smoke, have smoked in the past, or have never smoked. Similarly, alcohol consumption was also determined based on self-reported questions that asked whether they usually consume alcohol.

Chi-square tests were performed to investigate the general characteristics of the study population. The Cox proportional hazards model was used to analyze the association between HGS and all-cause mortality, determined using the adjusted hazard ratio (HR) and 95% confidence interval (CI). Survival time was defined as the number of days from the date of HGS measurement to the date of death or December 31, 2018, whichever occurred first. The Akaike Information Criteria (AIC) were used for model selection, in which that a lower value indicates a better model. Differences were considered statistically significant at P < 0.05. Kaplan-Meier was used to evaluate the time taken to all-cause mortality to HGS that affected the time interval. Cumulative incidence of all-cause mortality was calculated by the product limit (Kaplan-Meier) method of survival probability. All data analyses were performed using the SAS software (version 9.4; SAS Institute Inc., Cary, NC, USA).

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the Declaration of Helsinki (1964) and its later amendments or comparable ethical standards. The KLoSA study was approved by the National Statistical Office (Approval number: 33,602), Institutional Review Board of Korea National Institute for Ethics Policy (P01–201,909–22 − 002). The survey was conducted after acquiring verbal consent of the participants by the trained study interviewer. Since the KLoSA database has been released to the public for scientific use, additional ethical approval was not required for this study.