Association of Physical Activity and Sedentary Behaviors with the Risk of Refractive Error in Chinese Urban/Rural Boys and Girls

Abstract

Background: Research shows physical activity (PA) is negatively associated with refractive error, especially outdoor activity. Our study aimed to examine the association of PA levels and sedentary time (SED) with refractive error in boys and girls living in urban and rural areas. Methods: A total of 8506 urban/rural boys and girls (13.5 ± 2.8 years old) in Shaanxi Province, China participated in this study. Questions about PA, SED, outdoor exercises, and digital screen time were asked in the study survey. Non-cycloplegic refractive error was measured by an autorefractor. The differences between sex/area groups have been analyzed by one-way ANOVA. The association of PA/SED with spherical equivalent (SE) and cylinder power was analyzed by general linear regression. The association between PA/SED and the risk of refractive error was determined using the binary logistic regression model. Results: Of the 8506 participants, the prevalence of refractive error was significantly higher in girls and urban students (p < 0.05). Less SED and digital screen time, and more outdoor activity were significantly associated with SE (p < 0.05), respectively. More PA and less SED were significantly associated with lower cylinder power (p < 0.05), respectively. More PA and less SED were significantly associated with lower risks of myopia and astigmatism, respectively (p < 0.05). Conclusions: PA and SED were associated with the risk of refractive error. Maintaining a healthy lifestyle can help to reduce the risk of refractive error in boys and girls.

1. Introduction

Refractive error is the leading cause of visual impairment, and it has also been one of the key tasks of the global plan to prevent blindness [1,2]. Refractive error includes hyperopia, myopia, and astigmatism [3]. According to the World Health Organization, at least 2.2 billion people in the world suffer from visual impairment, and at least 1 billion of them have visual impairment problems that could have been prevented or solved [4]. The prevalence of myopia is still rising sharply, and it has been estimated that by 2050, half of the world’s population will be affected [5].

The rates of myopia in China are the highest in the world. The prevalence of myopia in boys and girls aged 3 to 19 was estimated to reach about 84% by 2050 [6]. High levels of myopia can lead to cataracts, glaucoma, retinal detachment, and pathological myopia also increase the risk of blindness [7,8]. Myopia has a significantly negative impact on the mood states of boys and girls [9]. With the advancement and evolution of technology, a variety of digital screen electronic devices have become more prevalent and present health concerns, such as vision impairment and subsequent vision loss. Most researchers have implied that digital screen use negatively impacts refractive error. Long-term studying, watching TV and parents’ myopia were risk factors for myopia [10,11,12]. The prolonged use of electronic devices also seems to be related to an increase in the risk of developing myopia among children [13]. Daily screen use has been significantly associated with myopia [14,15].

Previous studies have shown that physical activity (PA) has a great positive impact on boys’ and girls’ refractive error. PA has been proven as a protective factor for myopia [16]. A large amount of literature has shown outdoor activity is associated with a reduced risk of myopia [17,18,19]. A study found more time spent outdoors is associated with lower rates of myopia [20]. Outdoor activity can reduce the prevalence of myopia in school-aged children [21]. Exposure to sunlight contributes to Vitamin D synthesis and calcium absorption, reducing the risk of myopia [17,18]. Currently, the exact association of varied intensity PA (light, moderate, or vigorous) and sedentary behaviors with refractive errors is still unclear. Studies have found that astigmatism is known to lead to amblyopia or myopia [22,23]. It has been shown that playing video games and computer use may increase the risk of astigmatism [24]. In addition, lack of outdoor activity may be an important cause of astigmatism [25]. However, few studies have investigated the association between PA/SED and astigmatism.

Research Gaps and Objectives

To date, research on the exact association between varied intensity PA (light, moderate, or vigorous) and refractive errors is still unknown. To our knowledge, limited research has discussed the association between sedentary behaviors and refractive error, and few studies on PA and astigmatism have been found. Thus, this study aimed to investigate the association of PA, sedentary time (SED), and digital screen time with refractive error in 8506 boys and girls aged 9–18 years living in urban and rural areas in Shaanxi Province. We hypothesized that PA and SED were associated with the refractive error among those Chinese boys and girls, and higher PA and less SED were related to a lower risk of refractive error, respectively. This study could provide theoretical support for the prevention of refractive error and encourage boys and girls in China to take part in more physical exercise to decrease the occurrence and development of refractive error in boys and girls.

2. Materials and Methods

2.1. Participants

The total population of boys and girls aged 9–18 in China was over 167 million. Participants in this study were selected from the cohort of the 2019 National Student Physical Fitness and Health Survey and the National Student Physical Fitness Tests organized by the Ministry of Education of the People’s Republic of China. The project was conducted in a sample of 32 provinces, cities, and autonomous regions within China, and investigated the physical fitness and PA of students aged 6–22. A total of 241,536 students were recruited in the whole country, and a total of 11,572 people were selected in Shaanxi Province. Students ranging from the fourth grade in primary school to the third year in high school were asked to fill out a health and lifestyle questionnaire. In this study, the number of students who completed the questionnaire was 8839. There were 8820 students in the age range of 9–18 years old. Excluding boys and girls with eye disease or physical disease, the final sample size in this analysis was 8506 boys and girls living in urban or rural areas in Shaanxi Province. Physical and common disease examination, as well as a lifestyle questionnaire, were included in the study. A baseline visual examination was conducted to examine the eye health condition of those students. Participants were divided into urban boy, urban girl, rural boy, and rural girl groups for further analyses. The protocol was explained to the participants and their parents by teachers at school. Written consent was provided by the parents of the participants in our study. The study was approved by the ethics committee of Shaanxi Normal University (202016001).

2.2. PA and SED Survey

PA status was investigated using a survey from the Evaluation Index System of Physical Activity and Fitness of Youth (EISPAFY), developed by Yueying Hu et al. from the Shanghai University of Sport. The survey has been validated in the Physical Activity and Fitness in China—The Youth Study (PAFCTYS) [26]. The questionnaire of the survey included questions about PA, SED, outdoor exercises, and digital screen time as below:

• In the last seven days, how many of these four activities (LPA, MPA, VPA, outdoor activity) did you do? What is the average number of minutes per day for each?
• In the last seven days, how many hours (outside of class time) did you spend on the following activities (watching television/using electronic devices) on average per day?
• In the last seven days, how many classes did you have on average per day in the classroom?

2.3. Eye Measurements

Refractive status was measured by spherical power (right or left), and cylinder power (right or left). Non-cycloplegic autorefraction of both eyes was measured using a refractometer (TOPCON RM-800 Auto Kato, Topcon Corporation, Tokyo, Japan). Each eye was measured three times and the average spherical and cylindrical power were analyzed. The spherical equivalent (SE) of the refractive error was calculated as SE = (spherical power) + (cylinder power)/2. All the eye measurements were conducted and recorded by a professional eye care doctor.

2.4. Statistical Analysis

All statistical analyses were performed by SPSS 25.0 (IBM, Chicago, IL, USA). One-way ANOVA (Tukey post hoc test) was used to compare the differences in PA and refraction data in different sex/area groups. General linear regression was used to examine the association of PA/SED with spherical power, and cylinder power, respectively, controlling for sex, urban/rural areas, age, and daily average number of classes in the classroom. The association between PA/SED and risks of myopia or astigmatism was analyzed by binary logistic analysis, respectively, controlling for sex, urban/rural areas, age, and daily average number of classes in the classroom. Myopia was defined as SE ≤ −0.50 diopters (D) [27]. Astigmatism was defined as cylinder power < 1.00 diopters (D) [25,28]. The statistical significance was set at p < 0.05 (two-tailed).

3. Results

3.1. Descriptive Characteristics of Participants

The baseline characteristics of each group are displayed in

Table 1. Descriptive characteristics of participants (n = 8506).

	Total (n = 8506)		Urban Boys (n = 2198)		Urban Girls (n = 2095)		Rural Boys (n = 2092)		Rural Girls (n = 2121)		p
	Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD
LPA (min/day)	37.48	40.63	39.12	45.91	35.74	35.69	37.62	37.98	37.34	41.85	0.058
MPA (min/day)	24.30	30.42	25.90	38.86	21.51	29.19	25.71	24.64	24.00	26.35	<0.001 **
VPA (min/day)	17.82	24.37	21.93	28.77	15.20	24.72	19.66	24.23	14.35	17.42	<0.001 **
Outdoor activity (min/day)	48.17	54.88	50.44	57.21	43.09	50.43	52.34	56.33	46.72	54.76	<0.001 **
SED (h/day)	4.23	2.78	4.61	2.86	5.04	2.87	3.41	2.55	3.86	2.54	<0.001 **
Digital screen time (h/day)	3.77	3.45	3.31	3.06	2.90	2.76	4.69	3.97	4.20	3.62	<0.001 **
Spherical power (right) (D)	−2.00	2.23	−2.10	2.29	−2.30	2.29	−1.68	2.13	−1.93	2.17	<0.001 **
Spherical power (left) (D)	−1.68	2.25	−1.75	2.30	−1.94	2.35	−1.42	2.17	−1.63	2.17	<0.001 **
Cylinder power (right) (D)	−0.57	0.61	−0.71	0.80	−0.60	0.68	−0.56	0.69	−0.66	0.71	<0.001 **
Cylinder power (left) (D)	−0.72	0.73	−0.87	0.83	−0.76	0.74	−0.66	0.71	−0.57	0.61	<0.001 **

Notes: The statistical significance between groups is marked with *. * Indicates p < 0.05, ** indicates p < 0.01, LPA = light physical activity; MPA = moderate physical activity; VPA = vigorous intensity physical activity; SED = sedentary time.

. Of these 8506 participants, 4290 (50.4%) were boys while 4216 (49.6%) were girls. Among them, there were 2198 (25.8%) urban boys, 2095 (24.6%) urban girls, 2092 (24.6%) rural boys and 2121 (24.9%) rural girls. The level of PA was higher in boys and lower in urban girls. The level of SED was the highest in urban girls while the lowest in rural boys. SED in boys or students living in urban areas was less than that in girls or rural students. Digital screen time was highest in rural boys and lowest in urban girls. There were significant differences regarding MPA, VPA, outdoor activity, spherical power (right), spherical power (left), cylinder power (right), and cylinder power (left) among sex and area groups, respectively (p < 0.05). Overall, eyesight was worst in urban girls and best in rural boys. There were no significant differences regarding LPA among groups (p > 0.05).

3.2. The Association of PA/SED with Spherical Power and Cylinder Power

In

Table 2. The association ^a between physical activity/SED and spherical power, and cylinder power. (n = 8506).

	Spherical Power (Right) (n = 7989 b)			Spherical Power (Left) (n = 7989 b)			Cylinder Power (Right)			Cylinder Power (Left)
	B	SE	p	B	SE	p	B	SE	p	B	SE	p
LPA (h/day)	0.038	0.033	0.239	0.027	0.034	0.415	0.030	0.011	0.005 **	0.027	0.012	0.019 *
MPA (h/day)	0.052	0.044	0.243	0.044	0.046	0.337	0.040	0.015	0.007 **	0.051	0.016	0.001 **
VPA (h/day)	0.050	0.056	0.368	0.034	0.057	0.556	0.043	0.018	0.020 *	0.045	0.019	0.021 *
Outdoor activity (h/day)	0.035	0.025	0.153	0.031	0.025	0.221	0.014	0.008	0.085	0.009	0.009	0.274
SED (h/day)	−0.038	0.008	<0.001 **	−0.039	0.008	<0.001 **	−0.013	0.003	<0.001 **	−0.017	0.003	<0.001 **
Digital screen time (h/day)	0.023	0.007	<0.001 **	0.025	0.007	<0.001 **	<0.001	0.002	0.830	−0.001	0.002	0.771

Notes: The statistical significance between groups is marked with *. * Indicates p < 0.05, ** indicates p < 0.01 ^a Sex, urban/rural areas, age, and the number of classes in the classroom were controlled in the regression models; ^b Participants with hyperopia (n = 517) were not included in the analyses; B = regression coefficient and intercept; SE = standard error; p = p-value. LPA = light physical activity; MPA = moderate physical activity; VPA = vigorous physical activity; SED = sedentary time.

, the data for the analysis of spherical power did not include those with hyperopia (n = 517) in order to better focus on myopia (because the prevalence of hyperopia decreases with increasing age in boys and girls), and the sample size was 7989. It has been shown that digital screen time was positively associated with spherical power (right: B = 0.023, SE = 0.007, p < 0.001 and left: B = 0.025, SE = 0.007, p < 0.001), while negatively associated with SED (right: B = −0.038, SE = 0.008, p < 0.001 and left: B = −0.039, SE = 0.008, p < 0.001). There was no significant association between LPA, MPA, VPA, outdoor activity, and spherical power in this population (p > 0.05). The LPA totals were positively associated with cylinder power (right: B = 0.030, SE = 0.011, p = 0.005 and left: B = 0.027, SE = 0.012, p = 0.019). The MPA totals were positively associated with cylinder power (right: B = 0.040, SE = 0.015, p = 0.007 and left: B = 0.051, SE = 0.016, p = 0.001). The VPA totals were positively associated with cylinder power (right: B = 0.043, SE = 0.018, p = 0.020 and left: B = 0.045, SE = 0.019, p = 0.021). SED was negatively associated with cylinder power (right: B = −0.013, SE = 0.003, p < 0.001 and left: B = −0.017, SE = 0.003, p < 0.001). There was no significant association between outdoor activity, digital screen use, and cylinder power in this population (p > 0.05).

3.3. The Association between PA, SED, Myopia, and Astigmatism

The analysis of the association between PA/SED and myopia/astigmatism is presented in

Table 3. The association ^a between physical activity, sedentary time, and myopia, astigmatism. (n = 8506).

	Myopia (Yes or No)			Astigmatism (Yes or No)
	O.R.	95% C.I.	p	O.R.	95% C.I.	p
LPA (h/day)	0.926	0.852–1.006	0.068	0.885	0.810–0.967	0.007 **
MPA (h/day)	0.845	0.765–0.932	<0.001 **	0.806	0.703–0.922	0.002 **
VPA (h/day)	0.821	0.719–0.938	0.004 **	0.750	0.644–0.873	<0.001 **
Outdoor activity/h	0.950	0.893–1.011	0.104	0.999	0.998–1.000	0.108
SED (h/day)	1.050	1.024–1.076	<0.001 **	1.050	1.030–1.070	<0.001 **
Digital screen time (h/day)	1.001	0.984–1.018	0.915	0.989	0.973–1.006	0.197

Notes: The statistical significance between groups is marked with *. * Indicates p < 0.05, **indicates p < 0.01 ^a Sex, urban/rural areas, age and the number of classes in the classroom were controlled in the regression models; O.R. = odds ratio; 95% C.I. = 95% confidence interval; p = p-value. LPA = light physical activity; MPA = moderate physical activity; VPA = vigorous physical activity; SED = sedentary time

. Myopia was defined as SE ≤ −0.50 diopters (D). Astigmatism was defined as cylinder power < 1.00 diopters (D). More MPA was significantly associated with a lower risk of myopia (O.R.: 0.845, 95% C.I.: 0.765–0.932). More VPA was significantly associated with a lower risk of myopia (O.R.: 0.821, 95% C.I.: 0.719–0.938). Less SED was significantly associated with a lower risk of myopia (O.R.: 1.050, 95% C.I.: 1.024–1.076). No significance was found between LPA, outdoor activity, digital screen time, and myopia (p > 0.05). More LPA was significantly associated with a lower risk of astigmatism (O.R.: 0.885, 95% C.I.: 0.810–0.967). More MPA was significantly associated with a lower risk of astigmatism (O.R.: 0.806, 95% C.I.: 0.703–0.922). More VPA was significantly associated with a lower risk of astigmatism (O.R.: 0.750, 95% C.I.: 0.644–0.873). Less SED was significantly associated with a lower risk of astigmatism (O.R.: 1.050, 95% C.I.: 1.030–1.070). No significance was found between outdoor activity, digital screen use and astigmatism (p > 0.05).

4. Discussion

This study examined the association between PA, SED, and the risk of refractive error among boys and girls aged 9–18 years living in urban or rural areas in Shaanxi Province. Our paper found higher PA and outdoor exercise levels were associated with a lower risk of myopia, respectively. Longer SED and digital screen time were related to a higher risk of myopia, respectively. Higher PA and outdoor activity levels were associated with a lower risk of astigmatism, respectively. Shorter SED was associated with a higher risk of astigmatism. This study provides theoretical guidance for boys and girls to protect their eyes and increase physical exercise.

In this study, the level of PA and outdoor activities was higher in boys and lower in urban girls. The level of SED was the highest in urban girls while the lowest was in rural boys. SED in boys or students living in urban areas were less than girls or those living in rural areas. Previous studies have shown sex and urban–rural differences in PA levels. The overall level of PA was higher among boys compared to girls, and it was higher in urban areas than in rural areas [29,30]. Research has shown rural boys spent more time on outdoor activity than urban boys, rural girls, or urban girls [31,32]. It was interesting that the level of using digital screen time in our study was the highest in rural boys and the lowest in urban girls. The reason may be that most Chinese parents in rural areas are busy with farm work or get a job in urban areas and spend less time on home education. The students in rural areas also have less homework and less schoolwork pressure. Nowadays, cell phones are widely used in rural areas in China and can be accessed by boys and girls through their parents or grandparents. Therefore, rural students, especially boys with less self-control, have shown increased digital device usage. Further follow-up study or intervention research on the digital device time in rural boys and girls may help to find the solution to this issue.

In our study, the risk of refractive error in boys was significantly lower than those in girls, and those in rural areas were lower than those in urban areas. This finding was consistent with those of previous studies [28,33]. A survey of vision screening in 2200 students found urban students had a higher prevalence of myopia than rural students; the risk of myopia was significantly greater in girls than in boys [34]. This reason may be that students in urban schools were under heavier academic pressure than those in rural schools. Additionally, girls spend less time on outdoor activities and physical exercise than boys.

Our study has found that PA was negatively associated with the risk of myopia in this population. Previous studies have proven that less near work and increased moderate-to-vigorous PA (MVPA) can reduce the risk of myopia [27,35]. A Chinese study among 1294 students in grades 1–3 in primary school has indicated a significant negative association between outdoor time and myopia using vision at 5 m non-cycloplegia refractive examination [36]. Another Chinese study has examined the effect of outdoor activity on 4890 teenagers aged 10–15 years and reported that a longer time spent on outdoor activities was significantly associated with a lower risk of myopia [37]. The results were also consistent with those found in research in the context of other countries. For example, one study in 12- and 13-year-olds at 15 schools in Northern Ireland found that refractive error was associated with PA [38], and longer time spent on outdoor activities was significantly associated with a lower risk of myopia. Another study found PA and screen time were related to myopia through a survey of Danish teenagers aged 16–17, and there was a doubled risk of having myopia if teenagers were physically active < 3 h/week or used screen devices > 6 h/day [39]. One randomized controlled trial found the addition of 40 min of outdoor activity resulted in a reduced incidence rate of myopia over the next 3 years [33]. The mechanism may be that outdoor light stimulates retinal activity to release more dopamine, which delays myopia progression and improves nearsightedness in teenagers [18,40,41], but the specific mechanism remains to be further studied.

In addition, our study found SED and digital screen time were associated with refractive error. Research has shown that using electronic devices for more than 2 h presents an independent risk factor for myopia, and heavy use of electronic products is one of the important risk factors for ametropia in boys and girls [42]. O’Donoghue et al. investigated the risk factors for myopia in children aged 12–13 years in Northern Ireland and found a positive correlation between SED and myopia [38]. In a study about the prevalence of myopia in Spain, a positive correlation was found between the time spent using electronic devices and SE [43]. Liu et al. explored the association between digital screen use and myopia progression in Chinese myopic children during the outbreak of COVID-19 and reported that, along with the increment in digital screen time, the risk of myopia increased [44]. One study has also found screen exposure in early life increases a higher risk of myopia [45]. A review article indicated watching television for more than 2 h/day and playing computer/video/mobile games increased the risk of developing myopia [46]. However, some studies found that the effect of watching TV was not statistically significant since it is an intermediate distance activity. Longer reading and writing time had a greater impact on refractive errors than using mobile phones and computers [47]. More intervention studies are needed to focus on the exact effect of SED or exercise on myopia.

Our study found that PA/SED was associated with astigmatism, while outdoor activity and digital screen use were not associated with astigmatism. This is the opposite of previous studies. A study [25] in younger children aged 5–6 years found time spent outdoors was a compensating factor for astigmatism. Different from our study, the questionnaire in that study was filled out by the parents, while participants in our study completed the PA/SED questions by themselves. More studies are needed to distinguish between the indoor and outdoor activities of boys and girls and explore whether astigmatism is related to outdoor activities.

4.1. Strengths and Limitations

The strengths of our study included: first, this study selected four cities from Northern Shaanxi, Central Shaanxi, and Southern Shaanxi, which were representative of each part of the province. Second, the sample size in our study was large and the response rate was high. Third, the questionnaire was filled out under the guidance of the teachers, resulting in high reliability and validity.

The current study also had some limitations. First, this study was a cross-sectional survey in China, and we were unable to determine the causal association between PA, SED, and refractive error. More intervention studies in the context of different countries are in need. Secondly, there was some subjective bias in the PA survey. Only SED and digital screen time were measured and cumulative near work hours were not calculated. Future studies should consider using a professional myopia questionnaire that includes near work, intermediate distance activities, and outdoor activities [48]. Third, the ciliary muscle paralysis state was not performed when optometry was performed. The non-cycloplegic refraction may provide overestimates of myopia. However, this study was a part of a national project in China with a large sample size. There were also parental disagreements about performing cycloplegic refraction, which they believe may have other side effects. It is hard to maintain a high completion rate in such a large national study when using cycloplegia. It has been [49] reported in the COMET study that noncycloplegic refractions are 0.23 diopters more myopic than cycloplegic refractions. Some previous studies on the association between outdoor activity and myopia used noncycloplegic refraction [50,51]. Fourth, the risk of hyperopia was not analyzed in this study because the prevalence of hyperopia decreases with increasing age among boys and girls. Further investigation is needed to indicate the association between PA and hyperopia.

In the future, long-term cohort studies are necessary to better understand the longitudinal association between PA/SED and refractive error. The measures of PA, outdoor activities as well as SED, and cumulative near work hours can also be improved, for example, by using PA accelerometers and the cumulative near work hours formula. More randomized controlled trials should be conducted to identify the exact effect of PA or exercise on refractive error and reduce the risk of refractive error.

4.2. Practical Applications

Our study provides theoretical support for interventions in refractive error and theories of sufficient quality for public policy. Prevention and alleviation of vision problems in boys and girls require the participation of schools, parents, and the state to encourage boys and girls to accumulate more outdoor activity, reduce sedentary behaviors, take a regular visual examination, and set up system file information, which can effectively reduce the problem of boys’ and girls’ refractive error risks.

5. Conclusions

The risk of refractive error is higher in girls than boys and higher in students living in urban areas than those in rural areas. Higher levels of PA were significantly associated with less risk of refractive error in this population. Lower levels of SED were also significantly related to a lower risk of refractive error. PA had a significant effect on the development of astigmatism. Maintaining a healthy lifestyle can help to reduce the risk of refractive error in boys and girls living in urban or rural areas in China.