Study population
This study used raw data from KNHANES-VI and -VII (2014–2017). KNHANES is a nationwide cross-sectional survey that has been conducted since 1988 to ascertain the state of health and nutrition in the South Korean population, in accordance with Article 16 of the National Health Promotion Act, by the Korean Centers for Disease Control and Prevention (KCDC), as part of the Ministry of Health and Welfare. A representative sample of the households is selected by systematic stratified cluster sampling, and household members ≥1 years of age, excluding institutionalized individuals, are subjected to a health questionnaire, health examination, and nutritional survey. In the present study, out of 31,207 total respondents in the survey, we excluded those aged <19 years (6,386 individuals) as well as those with missing values for handgrip strength (3,667 individuals) and exercise data (1,504 individuals), and we included the remaining 19,650 respondents (8,676 men, 10,974 women) in our analysis (Fig. 1).
From visual inspection and interview with the participants, the surveyors excluded those in whom measuring handgrip strength was difficult due to functional limitations, history of hand or wrist surgery within the last 3 months, or hand or wrist pain within the last week. For the measurement of handgrip strength, participants adopted a standing position with their lower arms hanging naturally, away from the body, at the height of their thigh, and grasped the dynamometer without bending their elbow or wrist. Measurements were taken for a duration of up to 3 seconds. Three measurements were taken from each hand, alternatively, for a total of 6 measurements. The maximum value from the dominant hand was used as the final measurement of grip strength. The device used for measurement was a digital grip strength dynamometer (TKK 5401; Takei Scientific Instruments Co, Ltd, Tokyo, Japan). In a previous study using KNHANES data by Kim et al. [30], cut-off values for handgrip strength were defined to be <28.9 kg for men and <16.8 kg for women, and applying the same values in our study, we classified individuals with handgrip strength below the cut-off value as “low muscle strength” (LMS) and those with handgrip strength above the cut-off values as “normal muscle strength” (NMS).
Aerobic exercise performance rate was defined as the fraction of participants who performed, per week, at least 2 h and 30 min of moderate physical activity, 1 h and 15 min of vigorous physical activity, or an equivalent mixture of moderate and vigorous physical activity (where 1 min of vigorous activity is equivalent to 2 min of moderate activity). Using questionnaire items on aerobic exercise, we first identified participants who responded “Yes” to the question “Does your day include at least 10 min of vigorous/moderate physical activity, during which you are continuously very/slightly out of breath or your heart beats much/slightly faster?,” or to the question “Do you usually perform at least 10 min of vigorous/moderate sports, exercise, or leisure activities, during which you are continuously very/slightly out of breath or your heart beats much/slightly faster?”; then, based on the responses of these participants to the question “In a typical week, how many days/hours due to perform vigorous/moderate physical activity related to work/leisure?,” we calculated the number of people who performed at least 2 h and 30 min of moderate physical activity, 1 h and 15 min of vigorous physical activity, or an equivalent mixture of moderate and vigorous physical activity per week. The measurement results for vigorous/moderate physical activity were converted into minutes, and when the amount of exercise was calculated in terms of metabolic equivalent (MET-min/week, hereafter MET), participants were classified into aerobic exercisers (≥600 MET-min/week) and non-exercisers (<600 MET-min/week).
As sociodemographic characteristics of the participants, we included age, sex, educational level, marital status, and income level. Similarly, as lifestyle habit factors, we included weight, height, smoking status, and heavy alcohol drinking; as disease- and health-related factors, we included diagnosis of hypertension and diabetes, body mass index (BMI), and forced expiratory volume in 1 second (FEV1). Educational level was categorized into elementary school graduation or below, middle school graduation, high school graduation, and college graduation or above; income level was categorized into 4 quartiles: low, lower-middle, upper-middle, and high. BMI was calculated as the weight in kilograms divided by the square of the height in meters. Smoking status was categorized as never smoker, past smoker, or current smoker. Heavy alcohol drinking was calculated as the number of participants (n) who responded that they drank on an average ≥7 servings at a time for men or ≥5 servings at a time for women at least twice per week. Hypertension prevalence was calculated as the number of participants (n) with systolic blood pressure ≥140 mmHg, diastolic blood pressure ≥90 mmHg, or who reported as taking antihypertensive medication. Diabetes prevalence was calculated as the number of participants (n) with fasting blood glucose ≥126 mg/dL, diagnosed with diabetes by a doctor, reported as taking blood glucose lowering agents or insulin treatments. FEV1 was measured in patients aged ≥40 years using dry rolling-seal spirometry (Vmax series 2130; SensorMedics, USA).
As KNHANES is a sample survey rather than a census, all data analyses were performed using a weighted complex sample design. For participants’ sociodemographic data, measurements, and examination results, categorical variables were expressed as frequency and percentage (%), whereas continuous variables were presented as mean and standard deviation. Categorical and continuous variables were analyzed using chi-square test and t-test, respectively, and p<0.05 was considered significant. In order to analyze the effects of aerobic exercise on handgrip strength, we performed a logistic regression analysis and obtained the odds ratios (ORs) and confidence intervals. SPSS version 25.0 (IBM Corp., Armonk, NY, USA) was used for all statistical analyses in this study.
KNHANES-VI and -VII were conducted by the KCDC, and all participants provided prior written consent. All survey protocols were approved by the institutional review board of the KCDC (approval numbers 2013–12EXP–03–5C and 2015–01–02–6C). Data from KNHANES are available on the “Korea National Health and Nutrition Examination Survey” website (http://knhanes.cdc.go.kr), specifically the needed annual reports. Our study was approved by an institutional review board (JASENG 2019–04–006).