Participants
The participants came from the Chinese Longitudinal Healthy Longevity Study (CLHLS). The CLHLS is a nationwide investigations that aimed to investigate factors affecting the health of Chinese older people (≥65 years)[21]. This investigation started in 1998, continued in 2000, 2002, 2005, 2008, 2011, 2014 and 2018. It randomly selected half of the counties, cities, and autonomous regions in 22 of 31 provinces, covering 85% (985 million) of the total population[22]. This representative survey has been recognized by the international and domestic academic circles and a large segment of studies have been conducted[1, 23].
The participants inclusion criteria in this study were as follows: (1) died between 2005 and 2014; (2) had two consecutive follow-up (three years apart) within the last six years of life; (3) did not become disabled at the earlier interview; (4) no missing data.
Exercise behaviour
At each investigation, the exercise behaviour of participants was measured with the question “Do you do exercises regularly at present? such as walking, playing balls, running, and qigong.”[24] And the regular exercise behaviour did not refer to all types of bodily movement but to purposeful fitness activities. Therefore, housework, garden work, and physical labor were not considered exercise behaviour. The participants were coached and were able to distinguish the difference between physical activity and exercise before they answered “Yes” or “No”. For each participants, their last 6 years to 4 years’ data were considered as baseline exercise status. The last 3 years to 1 year’s were measured as outcomes of exercise status. Two investigations’ exercise behaviour were combined and stratified into four groups:long-term non-exercising (NN), non-exercising to exercise (NY), exercise to non-exercising (YN), and long-term exercising (YY).
Disability
Disabilities in activities of daily living (ADL) were measured through the Katz Index of Activities of Daily Living scale (Cronbach’s α = 0.87)[25] according to self-reported responses to questions that involved six daily activities of eating, dressing, transferring, using the toilet, bathing, and continence. ADL had been proved as a good scale for measuring participants’ functional performance[26, 27]. Participants answered on a three-point scale, with “1” for “able to perform independently”, “2” for “receives assistance only for one part of process” and “3” for “receives more assistance”. The total score ranged from 6 to 18. The classification of disability adopted in other studies was followed in this study[23, 28]. “Good” was defined as needing assistance for none or only one of the six items. “Moderately disabled” was defined as needing assistance for two of the six items. When the person requires assistance for three or more, he is considered as “severely disabled.”
Physical functioning
Three objective PF performance tests were used to measure participants’ PF and tested in the participants’ homes by trained interviewers from the local center for disease prevention[23]. Since self-reported measures of disability are subjective, they are recommended as a complementary measures and have been used in Chinese older population[29]. For each investigation (three years apart), the participants were asked to ⅰ) stand up from a chair (1=can without using arms, 2=can using arms, and 3=cannot); ⅱ) pick up a book from floor (1=can while standing, 2=can while sitting, and 3=cannot). “1,” “2,” and “3” three codes represented normal, moderate impairment, and severe impairment, respectively; and ⅲ) turn around 360° (1=yes and 2= no)[23, 30]. As mentioned previously, the baseline data were collected from the last 6 years to 4 years, and the outcome data were collected from the last 3 years to 1 year.
Covariables
Relevant control variables were adjusted in bivariate and unordered multiclassification logistic regression analyses. Age at death was reported by family members. Body weight was measured by objective examination. The number of chronic diseases and arthritis-suffering situations (yes/no) were diagnosed by hospital. Chronic diseases included hypertension, diabetes, heart disease, stroke, cerebrovascular disease, bronchitis, emphysema, asthma, pneumonia, and pulmonary tuberculosis. Education level (0, 1–6, 7–9, 10–12, or >12 years), place of residence (city, town, or rural), living companions (with household member(s), alone, or in an institution), financial support (enough/not enough), current marital status (married and living with spouse/else), medical service status (enough/not), main occupation characteristics before 60 years (physical work/else), and the current intake status of smoke and alcohol were collected through face-to-face interview. The smoke and alcohol intake status were defined based on the baseline investigation phase. In addition, the time duration between the first follow-up and death were calculated. The objective PF were determined from the last 6 years to 4 years investigation.
Statistical analysis
Differences in demographic characteristics were examined using the Kruskal-Wallis H test or the chi-square test, as appropriate. Data were presented as mean and standard deviation (SD) for continuous variables and as number and percentage for categorical variables.
Bivariate and unordered multi-classification logistic regression analyses were used to compare disability level and three PF performance among four-set groups controlling for various confounding factors. Crude and adjusted odds ratio (ORs) with 95% confidence intervals (CIs) were calculated. Data analyses were conducted using the SPSS 20.0 statistical software (SPSS Inc). Statistical significance was considered at P<0.05.