The results showed that during the period 1985–2000, the physical fitness of Chinese rural children and adolescents improved in most aspects except for flexibility, and most of them reached the largest increases; during the period 2000–2010, except for flexibility, all other fitness decreased significantly; during the period 2010–2019, the speed of boys rebounded and flexibility, explosive power and muscular strength continued to decline. The largest rates of decline in fitness were reached in general except for muscular strength in boys aged 13–18 years. Speed, flexibility and muscular strength of girls rebounded and explosive power continued to decline. The cardiorespiratory fitness of boys and girls had significant downward trends in general during the entire period but improved significantly for boys and girls in junior middle school, and the decline slowed down in primary school and junior high school. The largest increases in BMI in boys and girls occurred in the period between 2010 and 2019. In general, some components of fitness of Chinese children and adolescents in rural has shown positive trends in recent years, especially for girls.

The speed for both sexes improved after 2010, which was similar to other Chinese studies^2,11,12,35 and a Japanese study (2013–2019)³⁶, which both demonstrated positive trends in recent years. Some studies found that Chinese urban children and adolescents also show positive trends in speed after 2010 as well, but positive trends were more pronounced in rural areas^11,12. Speed in children and adolescents aged 8–15 years in Mozambique, Africa, declined continuously from 1992 to 2012³⁷. The trends in Slovenia are almost consistent with our study¹⁵, with speed quality first decreasing and then rebounding. However, studies from some developed countries^{6,38,39,40,41,42} showed different results that speed remained stable in children after 2000, having previously improved or worsened or stabilized. There are some exceptions, such as the decline in speed of Portuguese girls from 2003/2008 to 2008/2013³⁸ and in the Netherlands from 2006 to 2015/2017⁴³. Our study also found that during the period 2010–2019, the speed of adolescents aged 16–18 years had no significant changes, and some studies also provided some evidence^35,44. A systematic review by Fühner et al.⁴⁵ showed that speed in children and adolescents had been increasing since 2002 and declined to a minimum in the 1980s, while rural Chinese boys bottomed out in 1985 and girls in 2010. Other systematic reviews, which were not quantified, showed inconsistent trends in speed across countries^1,44. Moreover, we found significant differences in speed items between countries, with straight-line dashes (e.g., 30/50/60-m dash) being related to the ability to move quickly; speed-agility (e.g., 10 × 5 m shuttle run) also included the ability to change body position/orientation quickly and accurately in response to stimuli⁴⁶. Three types of speed were selected in Italy, and the results of dashes remained almost constant over the last 30 years, but the shuttle run declined⁴⁰. Therefore, we estimate that the differences in secular trends in speed may be related to the test items, although it is known that speed is highly genetically determined¹. In short, the current worldwide change trends of speed vary across the world.

The explosive power of boys and girls began to improve in 1985 but worsened after 2000, and the rate of decline increased. The trend was similar to other Chinese studies^2,11,12,35 in Chinese urban areas^11,12. However, we found that the explosive power of rural boys improved from 2015 to 2018 in Shanghai, China¹³, and improved in Japan in the last decade³⁶. Some developed countries, such as Slovenia (until 2014)²¹, Lithuania³⁹, Italy⁴¹, Poland⁴⁷, and Brazil⁴⁸, reported negative trends at the beginning of the twenty-first century or throughout the observed period, with many of these studies showing no significant changes^15,39,41,47 or improvement for girls in recent years²¹. The explosive power of Croatian and German boys had no significant changes and improved for girls slightly since the beginning of the twenty-first century^14,49. Slovenia had stable or positive trends (especially for girls) from 2014 to 2019 after the previous downward trend¹⁵. Positive trends were observed among boys and girls in the United States in the twentieth century and in Greece in the early twenty-first century^50,51. Tomkinson et al. converged on the standing long jump performance of 10,940,801 children and adolescents from 29 countries during the period 1960–2017 and found that the rate of improvement was steady from the 1960s to the 1980s, slowed in the 1990s, and then declined thereafter²⁰, which is generally consistent with the results of this paper. In addition to the above results using the standing long jump as an explosive power test, we also found that the performance of continuous leaping (Finland)⁵², horizontal jump (Portugal)³⁸, high jump (Netherlands)⁴³, and vertical jump (rural Poland)⁶ worsened or remained stable or in recent years stabilized⁵² and, in general, did not show positive trends. Several reviews also pointed to the negative trends of explosive power in most countries^1,44, but we found that explosive power remained stable in children and adolescents (especially girls) in some countries around the 2110s. Meanwhile, we found downward trends for girls and boys, and the decline for boys accelerated, suggesting the need to focus on explosive power in boys in China and globally.

We observed that flexibility worsened during the period 1985–2000 and then improved during the period 2000–2010 for both sexes, but the trends were different for boys (worsened) and girls (improved) during the period 2010–2019. Overall, the flexibility worsened in boys and improved in girls. The trends were similar to nationwide research and Chinese urban areas¹¹, but there were different trends in some regions of China (first stable then worsened or improved)^13,35. The flexibility of boys worsened and improved in girls in Hong Kong from 1998 to 2015⁵³ which was similar to our findings A 30-year negative trend was observed in Africa³⁷, but a positive trend was observed in Japan in the previous decade³⁶. For some developed countries, such as Croatia¹⁴, Portugal³⁸, Lithuania³⁹, rural Germany⁴², Dutch⁴³, Poland (Krakow)⁴⁷, and Brazil⁴⁸, a large number of studies have shown that flexibility has worsened. Other studies (e.g., rural Poland⁶, Slovenia¹⁵, Italy⁴¹, Germany⁴⁹, Canada⁵⁴, and Brazil⁵⁵) found negative or steady trends in boys and steady or positive trends in girls in recent years. Meanwhile, an upward trend was found in Greece for both sexes⁵⁰. In conclusion, most of the studies found that flexibility worsened, and there were positive trends for girls in recent years which was consistent with this study. In addition, we also observed a significant decline in higher-age boys, the exact reasons for which need to be explored in depth.

The trends of muscular strength showed differences among boys and girls with boys having mostly negative trends and girls having mostly positive trends during the period 1985–2019. Muscular strength in boys worsened and in girls improved in recent years. The trends were similar to nationwide research and Chinese urban areas^2,11,12. However, Xinjiang, China, found positive trends in recent years only in boys aged 13–18 and worsened in both boys and girls in Hong Kong (sit-ups for both sexes)^35,53, which differs from this study and suggests differences between different regions of China. The performance of sit-ups improved in Japan but declined after 2019³⁶. In some developed countries, increasing or stable sit-up performance was observed in Croatia¹⁴, Slovenia¹⁵, Greece⁵⁰ Portugal³⁸ and Germany^42,49,56, and negative trends were found only in Brazil^48,55 and the United Kingdom⁵⁷. On the other hand, for the bent-arm hangs test, we found that most studies showed downward trends, such as in Slovenia¹⁵, Dutch⁴³, Lithuania³⁹, and the UK⁵⁷. We also found a decline in backward overhead medicine ball throws in Poland (Krakow)⁴⁷. Kaster et al.¹⁹ estimated secular trends of sit-up performance for 9,939,289 children and adolescents aged 9–17 years from 31 countries/regions from 1964 to 2017 and found that most countries showed positive trends. Although there was a negative international trend after 2010, the lack of data for a large number of developing countries made the interpretation of the results incomplete¹⁹. These different secular trends might be related to the specifics of the different tests used for muscle strength, such as bent-arm hang/flexed-arm hang, pull-up, sit-up, handgrip, etc., which emphasize arm and shoulder belt strength, abdomen strength, etc. Meanwhile, the selection of tests varies between countries and is not always consistent between boys and girls.

Cardiorespiratory fitness significantly worsened in Chinese rural children and adolescents. The negative trends were similar to nationwide researchs and Chinese urban areas^2,11,35,53, but improvements were also observed for children aged 7–12 years from 2005 to 2014^11,12. Cardiorespiratory fitness improved in Japan but worsened after 2019³⁶. Previous studies have shown negative trends in cardiorespiratory fitness in children and adolescents in many countries around the world^{2,11,13,14,37,39,48,54,55} and some studies have found that cardiorespiratory fitness has been stable (boys or girls or both sexes) in recent years^{35,41,42,49,56}. However, there are also studies reporting positive trends, such as Greece⁵¹, rural Poland (girls)⁶, and Slovenia^15,58. Fühner et al.⁴⁵ and Tomkinson et al.¹⁹ identified stabilization and possible improvement after 2010 or 2000.

During the period 2000–2019, the BMI of boys and girls in all age groups increased and the increase accelerated, especially in those aged 16–18 years. It has been shown that both higher and lower BMI can have a detrimental effect on physical fitness^22,59. Overweight and obesity individuals tend to perform less physical activity, have longer screen time, consume more calories⁶⁰ and have a substantial decrease in physical activity. The study from CNSSCH showed that the prevalence and increased rate of overweight and obesity among rural Chinese boys were higher than those among girls²⁴, which may be one of the reasons for the more obvious improvement in girls’ physical fitness in recent years. However, a slow or stagnant or negative increase in the prevalence of overweight and obesity was observed in recent years^61,62, which partly explains the improvements in some physical fitness in children and adolescents during the period 2010–2019. In addition, some studies have noted that nutritional status is an important factor affecting physical fitness regardless of the levels of physical activity⁶³, and Dong et al.⁶⁴ also found that children and adolescents with high level of physical activity and high socioeconomic status were associated with better physical fitness, and children and adolescents with obesity and longer TV viewing time were associated with worse physical fitness. Most of these factors were independently and significantly associated with physical fitness. Physical fitness is influenced by several factors and there are also some interactions among these factors, but nutritional status might be the key factor of physical fitness in children and adolescents. In addition, children and adolescents in lower age groups were less resistant to food temptations than those in higher age groups⁶⁰, and their physical inactivity was more likely to lead to overweight or obesity⁶⁵.

It was worth noting that the secular trends in some fitness (cardiorespiratory fitness, speed, muscular strength) were found to be more “positive” for both sexes during junior middle school than for other educational phases. Other studies did not demonstrate such distinctly different trends across age categories as this paper (possibly due to the age range limitations of the participants or the lack of focus on age differences), and some worldwide studies found more positive trends in sit-ups (with a smaller rate of decline) in recent years in children than in adolescents¹⁹, while cardiorespiratory fitness showed almost no difference¹⁸. Since participants of our study were divided based on the Chinese educational stage and observed significantly different trends for this age category, we believe that it might be related to some Chinese-specific factors. One study found that the increased rate of overweight and obesity among Chinese adolescents aged 14–17 years was smaller than that of children aged 7–13 years in recent years⁶¹, while a study from CNSSCH (Henan, China) showed a rapid increase in the prevalence of overweight and obesity in students aged 10–12 and 16–18 years from 2010 to 2019⁶⁶. From our study, we found that the increase in BMI was faster in adolescents at higher ages from 2010 to 2019, but these findings cannot fully explain the positive trend of adolescents aged 13–15 years in recent years. Therefore, we estimate that another occurrence of this phenomenon is related to the junior high school entrance examination for physical education. Piloting from the twenty-first century, 31 administrative districts in China included the exam of physical education in the total score (physical education and culture scores) of the junior high school examination, signifying the full implementation that the exam of physical education was included among the junior high school entrance examination; initially, the exam of physical education accounted for 5% of the total score of the junior high school entrance examination, and then, in response to the decline in the physical fitness of students nationwide and the need for sports power strategy, the score of the physical education examination was constantly increased (other cultural scores remained almost unchanged) and even doubled in some districts⁶⁷. In addition, the test programs have become more diverse, but the 1000-m running for boys and 800-m running for girls belong to the mandatory test programs, which may explain the significant improvements in cardiorespiratory fitness in adolescents aged 13–15 years. Although we are unclear how this initiative has impacted them (e.g., voluntary or pressured participation in physical activity), it is clear from the CNSSCH that from 2005 to 2014, the rates of the good and excellent health status of physical fitness rose significantly for 13–15 years than for 16–18 years^68,69. On the other hand, they proves some evidence. In contrast, the National College Entrance Examination (NEMT or Gaokao) and junior middle school entrance examination do not include physical education subjects or do not count toward the total score, and students are not highly motivated to exercise. Meanwhile, students at junior high school are under more academic pressure and spend more time being sedentary,⁷⁰, which may worsen their health status.

In the early twenty-first century, the nation and society have paid great attention to the decrease in physical fitness of children and adolescents, and a series of initiatives have been taken. For example, the Healthy Physical Education Curriculum Model of China⁷¹, developed by Professor Ji, was introduced to improving students’ physical fitness by helping them enjoy physical education and engage in at least one sport as a hobby. The curriculum must focus on the three key elements of “sports loading, physical fitness training and motor skills”. Students should have approximately 10 min of physical readiness training in each session, and the exercise intensity should be at least 75% in each session, with an average heart rate of 140–160 beats per minute per session. Currently, the model has been promoted nationwide and a large number of Chinese physical education teachers have been trained to use the model^35,72. The government implemented the Opinions of the Central Committee of the Communist Party of China on Strengthening Youth Sports to Enhance the Physical Fitness of Young People in 2007⁴⁵ while emphasizing the importance of school physical education and ensuring that students exercise for one hour every day at school. For the first time, Children and adolescents’ sports health promotion has been elevated to the level of national strategy. In the years that followed, the government issued more than 88 policies including the promotion of sports, reductions in academic burden, and promotion of physical fitness^2,73. To improve the health of poor rural students, the state launched the “Nutrition improvement program for rural compulsory education students” in 2011, with the financial department providing nutritional meal subsidies for rural compulsory education students (approximately 7–15 years) in poor areas of central and western areas. In the beginning, each student was provided with a subsidy of 3 yuan per study day, which was increased to 4 yuan in 2014 and 5 yuan in 2021, and the subsidy amounted to 34.8 billion yuan in 2021^74,75. Through this, the average height and weight of students increased, and the gap between urban and rural areas narrowed; micronutrient deficiencies such as anemia decreased; the intake of foods rich in high-quality protein and micronutrients such as fish, poultry, meat, eggs and milk increased, and rural students’ nutrition levels improved^74,75. A study from CNSSCH found an increase in the percentage of children and adolescents meeting one hour of in-school physical activity⁷⁶. These findings may be related to improvements in physical fitness in recent years. In addition, the Central Committee of the Communist Party of China and the State Council issued the “Outline of the Healthy China 2030 Plan” to improve the physical fitness of the whole population as one of the strategic goals in 2016⁷⁷. In 2019, the Health China Action Promotion Committee issued the “Health China Action (2019–2030)”⁷⁸, which clearly states that by 2022 and 2030, the proportion of students meeting the national physical fitness standards (National student physical health standard, revised in 2014) will reach at least 50% and 60%, respectively. The Physical Education Law of the People’s Republic of China, which is amended by the Standing Committee of the 13th National People’s Congress on June 24, 2022, stipulates that physical education subjects will be included in NEMT from January 2023⁷⁹. We predict that in the future the physical fitness of Chinese children and adolescents will show more positive trends.

Our study spans more than three decades and a long time interval, providing not only a report of secular trends in physical fitness but also an exploration of changes in different year phases. Since there are differences between urban and rural areas, such as economic and political, separate analyses of the physical fitness of rural children and adolescents are beneficial for the development of future rural promotion strategies. In contrast to including only a few age groups, this study includes 12 age groups for each sex from childhood to adulthood and is divided again according to educational phases, which facilitates the characteristics of physical fitness in boys and girls at different stages of growth and development, and our study did find different secular trends in cardiorespiratory fitness. This study has several limitations. First, the muscular strength and cardiorespiratory fitness tests differ across age categories for boys and girls, which does not facilitate comparisons between them. Then, the assessment of rural area being done only in 1985 and urban areas are also possibly included in later study waves. This may increase the differences in economics, policies, etc. between the rural areas selected, and secular trends may not be applicable to all rural areas. Finally, it has been demonstrated that physical activity, nutritional status, and dietary habits can have an effect on physical fitness, and this study did not include these variables or conduct a correlation study.