Table of Contents
Datasets and study participants
This study used data from the KNHANES, a nationally representative survey of non-institutionalized Korean citizens conducted annually by the Korean Center for Disease Control and Prevention. The KNHANES collects data on socioeconomic status, health-related behaviors, anthropometric indices, and biochemical and clinical profiles of noncommunicable diseases. Each survey year includes a new sample of randomly enrolled participants. The detailed KNHANES procedures have been described previously32.
We collected data from KNHANES from 2014 to 2019, where of 61,754 total individuals, 47,309 (76.6%) responded to the survey. Since pulmonary function testing (PFT) was only performed in adults aged ≥ 40 years, participants aged < 40 years were excluded (n = 31085). Individuals without airflow limitation were also excluded (n = 13,595). Among the remaining 2629, 1763 men and 722 women were identified. Because our study purpose was to investigate the association of a combination of RT and aerobic MVPA with respiratory symptoms and sleep duration compared to aerobic MVPA alone, 1056 individuals (794 men and 262 women) who were engaged in aerobic MVPA were selected. However, of the 261 women, only 54 underwent additional RT. Therefore, our analysis was restricted to men with airflow limitation (n = 794).
Spirometry
PFT was performed using dry rolling seal spirometers (Model 2130; SensorMedics, Yorba Linda, CA, USA) from 2014 to 2015 and Vyntus Spiro (CareFusion, San Diego, CA, USA) from 2016 to 2019. Calibration and quality control followed the standardization criteria of the American Thoracic Society and European Respiratory Society33. FEV1 (Liters), forced vital capacity (FVC, Liters), and the ratio of FEV1/FVC (%) were obtained from the pre-bronchodilator test. However, post-bronchodilator testing was not performed in the KNHANES.
Airflow limitation was defined as when pre-bronchodilator FEV1/FVC ratio is < 0.7. The GOLD recommends use of post-bronchodilator spirometry for COPD diagnosis2. Nonetheless, this operational definition of airflow limitation has been utilized in several major epidemiological studies representative for COPD, despite the possibility of overdiagnosis and no consideration of respiratory symptoms34,35. Severity of airflow limitation was classified as mild when the FEV1 was ≥ 80% pred and moderate-to-severe when the FEV1 was < 80% pred2.
Aerobic MVPA and RT
The main exposure in this study was RT in addition to aerobic MVPA. Therefore, individuals who reported engaging in regular aerobic MVPA were further categorized based on whether they underwent RT. Attainment of aerobic MVPA was collected using self-reported questionnaires to assess regular aerobic exercise. A definition of aerobic MVPA was made when the following conditions were met: (1) vigorous-intensity PA for > 20 min per day on ≥ 3 days per week, (2) moderate-intensity PA for > 30 min per day on ≥ 5 days per week, or (3) an equivalent combination of moderate- and vigorous-intensity PA17. RT was assessed according to the number of days that exercises such as push-ups, sit-ups, use of dumbbell or weights, and chin-ups were performed per week and marked “yes” if the participant performed RT more than 2 days per week17.
Sleep problems
Sleep duration was recorded using a self-reported questionnaire, assessed on weekdays and weekends separately, and the average sleep duration calculated. Adequate sleep was defined as having 6–8 h of sleep, and sleep problems were defined as when the individual showed sleep deprivation (sleep ≤ 5 h) or oversleep (sleep ≥ 9 h)36,37,38,39.
Cough and sputum
Data on self-reported respiratory symptoms, specifically chronic cough and sputum production, were acquired using the following question: “Have you experienced sputum production or coughing persistently for a duration of more than three months within the past year?”. The answers options were either yes or no.
Other variables
Hand grip strength (HGS) was measured using a digital hand dynamometer (Digital grip strength dynamometer, T.K.K 5401, Takei Scientific Instruments Co., Ltd., Tokyo, Japan). HGS was measured in the standing position with the forearm away from the body at the level of the thigh. A resting interval of at least 30 s was allowed between the measurements. HGS was defined as the mean value of the measured grip strength of the dominant hand. Sarcopenia was defined as a HGS < 28 kg in men40.
Other variables included BMI (kg/m2), smoking status (never, former, and current), high-risk drinking (seven [alcohol 60 g] or more drinks for men on one occasion), residence (rural or urban), education (middle school or lower, high school, and college or higher), and household income (lowest, lower middle, higher middle, and highest). The smoking status was categorized based on the National Health Interview Survey of the United States. Current smokers were defined as individuals who smoked more than 100 cigarettes in their lifetime and who currently smoked. Former smokers were defined as individuals who had smoked more than 100 cigarettes in their lifetime but had stopped smoking for more than 1 year.
Statistical analysis
The KNHANES is designed to represent non-institutionalized South Korean citizens. To ensure representativeness, a stratified multistage probability sampling method was employed in the KNHANES design. Therefore, all statistical analyses conducted in this study utilized a complex sample analysis method, considering the sampling weights, stratification, and clustering of the KNHANES data.
Continuous variables were presented as weighted means and standard error (SE) and compared using complex sample linear regression analysis. Categorical variables were presented as weighted percentages with SE and compared using the chi-squared test.
Complex sample logistic regression analysis was used to estimate the weighted OR and 95% CI for sleep problems, chronic cough, and chronic sputum production. For multivariable analysis, Model 1 was adjusted for age; Model 2 was additionally adjusted for BMI, smoking status, and high-risk drinking status; Model 3 was additionally adjusted for residence, education, and household income level; and Model 4 was additionally adjusted for FEV1%pred.
Subgroup analyses were additionally conducted, stratified by the degree of airflow limitation (FEV1 ≥ 80 or < 80% pred).
All statistical analyses were performed using SPSS version 24 software Windows, Armonk, NY, USA). For all analyses, a P value < 0.05 was considered statistically significant.
Ethical approval
The Institutional Review Board of Samsung Medical Center (no. 2023–09-028) approved the study and waived the requirement for informed consent because of the retrospective nature of this study and the KNHANES data were de-identified. The study was conducted in accordance with the principles of the Declaration of Helsinki. All procedures were performed in accordance with relevant guidelines and regulations.
