This is a cross-sectional study design of the Korean population from the KNHANES, an annual study that has been conducted since 2007. The study targets only Koreans and is based on a complex stratified multistage probability cluster survey of a cross-sectional and nationally representative sample of individuals from the non-institutionalized civilian population by the Division of Chronic Disease Surveillance, Korea Centres for Disease Control and Prevention.
Data from the KNHANES VI -VII (2014–2017), including data relating to handgrip strength was included. Participants were excluded from the study if they were < 19 years of age (n = 6,386), provided insufficient information of questionnaires relating to multimorbidity (n = 2,975), and had no measurement of handgrip strength for the right or left hand (n = 2,067). Following exclusion of these participants, a total of 19,779 participants (male: 8,730, female: 11,049) remained that were eligible for the study (Fig. 1).
Definitions of multimorbidity
The KNHANES VI-VII questionnaires included 25 chronic diseases (hypertension, dyslipidaemia, stroke, myocardial infarction, angina, osteoarthritis, rheumatic arthritis, tuberculosis, asthma, allergic rhinitis, depression, kidney failure, atopic dermatitis, diabetes mellitus, thyroid disease, stomach cancer, liver cancer, colon cancer, breast cancer, cervical cancer, lung cancer, thyroid cancer, liver cirrhosis, hepatitis B, and hepatitis C). The presence of chronic disease was confirmed by a clinician and confirmed by participants using self-reports. Participants also reported if they still had the disease or if the disease had been treated. Participants were defined as multimorbidity if they had two of more confirmed chronic diseases.
Measurement of handgrip strength
Handgrip strength was measured three times in each hand, using a digital grip strength dynamometer (TKK 5401; Takei Scientific Instruments Co., Ltd., Tokyo, Japan). Trained medical technicians instructed participants to hold the dynamometer with the distal interphalangeal finger joints of the hand at 90° to the handle and to squeeze the handle as firmly as they could whilst being seated. After participants had slowly stood up, handgrip strength was measured during expiration. Study participants conducted three attempts per hand, with a one-minute rest period between each attempt to reduce the effects of fatigue. Handgrip strength of the participants’ dominant hand was defined using questionnaires, and an average of the three measurements was used [13]. Relative handgrip strength was defined as grip strength per unit of BMI.
Covariates
The self-reported questionnaire provided information relating to participants education, smoking status, alcohol consumption, and physical activity. As education levels are lower in the elderly, education was divided into two categories of ≤ 9 and > 9 years of school. Smoking status was classified as never smoking, former smokers and currently smoking. Current smokers self-reported that they smoked > 100 cigarettes in their lifetime and were currently still smoking. Past smokers self-reported that they had a history of smoking. Participants who consumed alcohol were defined as those who drink at least one glass of alcohol every month over the last year. Physically activity participants were defined as those who had performed exercise for least 150 minutes per week at a moderate intensity (fast walking, carrying light objects, cleaning, parenting, etc.); 75 minutes per week at a high intensity (lifting or carrying heavy items, digging, labor at construction sites, carrying objects on stairs) (or a combination of medium and high-intensity physical activity (where one minute of high-intensity activity = two minutes of medium intensity activity). BMI was calculated as weight divided by height squared (kg/m2).
Statistical analyses
Statistical guidance was provided by the Korean Centre for Disease Control and Prevention, raw data from the VI-VII (2014–2017) were combined and complex sample analysis was utilised using the weighting data from KNHANES. Sex and age were used to divide participants when investigating associated between handgrip strength and multimorbidity. The groups utilised were 19–49 and 50–80 years old, for both male and female participants.
Differences in demographic and anthropometric characteristics were compared using the Student’s t-test or Chi-square test, as appropriate. Complex sample multivariate logistic regression was performed to analyse the relationship between relative handgrip strength and multimorbidity, and odds ratio (OR) and 95% confidence intervals (CI) were calculated. Model 1 adjusted for age and Model 2 adjusted for age, education (≤ 9, > 9 years of school), alcohol consumption (yes vs. no), smoking status (never, former, current), and physical activity (yes vs. no). Relative handgrip strength was divided into quartiles. ORs were calculated using complex samples logistic regression analysis to examine associations between the quartile of relative handgrip strength and the presence of multimorbidity based on the highest quartile. Logistic regression analysis was repeated for all age groups using a stratified approach with a BMI < 25 kg/m2 and a BMI ≥ 25 kg/m2. Statistical significance was defined as P < 0.05. Continuous and categorical variables were expressed as mean ± standard deviation (± SD) and number (n) (%), respectively. All statistical analysis were performed using SPSS software (IBM Corp. Released 2017. IBM SPSS Statistics
for Windows, Version 25.0. Armonk, NY: IBM Corp). All graphical representations were prepared using the R software suite, version 3.3.3 (The Comprehensive R Archive Network: http://cran.r-project.org).