Study design and participants
This cross-sectional study is based on de-identified data from the Taiwan’s Scientific Physical Fitness Testing Program (TSPFTP). The TSPFTP was conducted by the Sports Administration, Ministry of Education in Taiwan, to obtain annual data on health-related physical fitness tests of Taiwanese adults aged 23 to 64 years. The design of this survey used convenience sampling. Participants were recruited from 18 physical fitness test stations in Taiwan. This survey included face-to-face interviews followed by a standardized structural questionnaire, anthropometric measurements, and physical fitness tests conducted by trained examiners and medical specialists (usually nurses or doctors). This study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Institutional Review Board of Fu Jen Catholic University in Taiwan (FJU-IRB C110113).
The data collection took place from September to November 2017 and contained three different approaches. First, trained examiners and medical specialists preliminarily checked the participants’ resting heart rate and their systolic and diastolic blood pressure, as well as assessing their potential safety risks by the Physical Activity Readiness Questionnaire (PAR-Q). All participants were required to pass the preliminary safety assessment and then they were allowed to proceed to the next test step. Second, participants were requested to fill out (or verbally answer the questions) the demographic questionnaire, as well as to complete the anthropometric measurements. After completing the second step, the participants were instructed to warm up (dynamic and static muscle stretching) for approximately 10 minutes. Then, a series of physical fitness measurements were performed, and interval breaks were permitted for 2–4 minutes between measurements. For this study, we finally included a total of 16,939 participants (7,761 men and 9,178 women) in the analysis who passed the potential safety risk assessment, completed the questionnaire and underwent the physical fitness measures.
A standardized, structured questionnaire was used to collect the data on demographic characteristics (i.e., age and gender), socioeconomic status (i.e., education, monthly income, marital status, and relationship status), and the residence zip code through face-to-face interviews. Education was divided into elementary school or lower, junior or senior school, and college or higher. Currently employed status was divided into yes, no, and other. Monthly income was divided into ≦ 20,000 NTD (new Taiwan dollar), 20,001–40,000 NTD, and ≧ 40,001 NTD. Marital status was divided into married, never married, and divorced/separated/widowed. Relationship status was divided into living with someone and not living with someone.
Anthropometric measurements included body height, weight, and BMI, taken after the participants were asked to remove their shoes and heavy clothes and stand in a normal posture. Body weight and height were recorded in meters to the nearest 0.1 kg and 0.1 cm with an electronic height-weight scale. In addition, BMI was calculated based on body weight and height (weight [kg]/height [m]2), and according to the Health Promotion Administration in Taiwan , BMI categories such as normal weight, overweight, and obesity were defined as a BMI of 18.5 ≦ BMI < 24 kg/m2, 24 ≦ BMI < 27 kg/m2, and BMI ≧ 27 kg/m2, respectively.
Health-related Physical Fitness Measurements
The following tests of physical fitness were conducted: cardiorespiratory fitness was measured via the 3MPKS test (ml/kg/min) [11, 19], muscular fitness was measured via hand grip strength (kg) , and flexibility was measured via the sit-and-reach test (cm) . Hand grip strength was measured with an electronic hand grip dynamometer by taking the average of the two dominant handgrip attempts. The sit-and-reach test needed to be performed twice with a sit-and-reach box with a measuring scale, where 30 cm was at the level of the feet, and the average distance from the two attempts was used for analysis.
Participants were asked to avoid any other vigorous- or moderate-intensity physical activity before performing these tests. A 10-min warm-up was introduced by the examiner, and the participant did this before the physical fitness assessment. All participants performed the tests in the following order with a sufficient break period (3–5 min) between tests: hand grip strength, sit-and-reach test, and 3MPKS test.
All statistical analyses were performed with SAS version 9.4 software (SAS Institute, Cary, NC, USA). Differences in demographic characteristics, anthropometric variables and health-related physical fitness measurements between BMI categories were analyzed using one-way analysis of variance (ANOVA) or chi-square tests. When a significant F value was found (p < 0.05), Tukey’s post-hoc test was performed to determine the differences between the pairs of means. Multiple linear regression analysis with health-related physical fitness measurements as the dependent variable was used to examine the associations between health-related physical fitness measurements and BMI after adjustment for potential confounders such as age, education, occupation, monthly income, marital status, and relationship status. To examine the dose–response relationship of health-related physical fitness performance with BMI and obesity status, four different categories (quartiles) were applied for each health-related physical fitness measurement according to gender. The low quartile was comprised of participants who had the best performances on each physical fitness measurement, and it was assigned as the reference group for further analysis. Unconditional logistic regression analyses were conducted to evaluate the linear association among cardiorespiratory fitness, muscle fitness, or flexibility, and obesity risks. All regression models were adjusted for age, education, occupation, monthly income, marital status, relationship status and other health-related physical fitness measurements. Then, the adjusted odds ratios (ORs) with 95% confidence intervals (CIs) were calculated. All data are expressed as the means ± standard deviation (SD) or frequency (percentage). The significance level adopted to reject the null hypothesis was p < 0.05.