Relationship between body mass index and physical fitness of children and adolescents in Xinjiang, China: a cross

The presented study estimated the relationship between BMI and physical fitness for children and adolescents in Xinjiang, China. We found that the physical fitness performance (grip strength, standing long jump, sit-and-reach, 50 m dash, and endurance running) in Xinjiang children and adolescents aged 7–18 years increased with age. Our results are consistent with findings among children and adolescents in China [26], Brazil [27], and adults in Germany [28]. The association between the BMI Z-score and Z-score of grip strength, standing long jump, and sit-and-reach showed an inverted U-curve, which was consistent with the results from Casonatto [29] and Gulías [30]. The association between BMI Z- score and Z-score of 50 m dash and endurance running showed a U-curve, which was in line with the conclusions of Li [31] and Huang [32]. Children and adolescents with normal BMI had the best performance in physical fitness. Whereas children and adolescents with a BMI above or below the normal range performed poorly. With regard to gender differences, the impact of BMI on fitness was more evident among boys than girls, consistent with the findings of Dong et al. [26] and Santos et al. [33].

Grip strength and standing long jump reflect the muscle strength of the upper and lower limbs, respectively. The association between BMI and grip strength of children and adolescents in the presented study was curvilinear, but almost linear in girls and young boys. The curvilinear association within boys aged 13–15 and 16–18 was more obvious, while in the 7–9 age group, the association was almost linear. These findings suggest that BMI has a stronger influence on grip strength among older Xinjiang children and adolescents. However, our results were inconsistent with the results of the study by Zaqout et al. [34], which suggested that the association between BMI and grip strength was linear, and grip strength performance was better in children and adolescents with higher BMI. One possible reason is that grip strength does not require support or movement of body weight. Boys with high BMI can be much stronger and more powerful, leading to better grip strength compared to their peers [35].

Our results also showed that BMI has a curvilinear association with standing jump, which is consistent with the studies on European children and adolescents [34]. We also found that BMI had a more obvious impact on standing long jump in boys than in girls, which is consistent with the research by Kwiecinski et al. [16] in Polish youth. Whereas the association between BMI and standing jump performance in girls aged 16–18 in our study tended to be linear. The different associations between boys and girls can be explained by the difference in muscular content in different genders.

Compared with other physical fitness, the performance of sit-and-reach was less affected by BMI, especially among girls. Unlike other physical fitness, sit-and-reach is not significantly affected by overweight since such activities do not have to overcome the resistance bought by high weight, and thus equally attractive to both underweight and overweight children and adolescents [36].

The 50 m dash and endurance running reflect speed ability and cardiorespiratory endurance level, respectively. Our results showed that the influence of BMI on 50 m dash performance in children and adolescents aged 7–18 in Xinjiang was more evident in boys than in girls. Girls aged 13–15 and 16–18 years showed a nearly positive linear relationship. Whereas girls aged 7–9 and 10–12 and boys aged 7–18 had a clear association in the U-curve. These results are consistent with the results for Lopes [37] and Rodrigues [38]. A possible explanation for this can be the fact that lower BMI means less muscle mass in adolescents, which can affect speed. Participants with higher BMI should overcome their resistance to weight, which results in a lower performance of the 50 m dash. BMI and endurance running performance in our study showed a clear association of the U - curve for boys and girls aged 7–18 years, which was consistent with previous studies [16, 17, 39,40,41]. However, the results should be interpreted with caution, since overweight or obese persons with a high BMI must overcome greater resistance to weight during the test [39, 42]. Artero et al. [35] found that, after adjusting fat mass, the association between 10 m × 4 round running test performance and weight status in girls became non-significant, which was signed before adjustment.

The results of this study found out an inverted U-shaped or U-shaped curve relationship between BMI and physical fitness in Xinjiang children and adolescents. Given the importance of physical fitness, children and adolescents can keep fit by maintaining a reasonable and normal BMI, thus reducing the incidence of disease caused by low physical fitness. Therefore, to improve physical fitness, targeted actions should be developed to address BMI-related effects in children and adolescents in Xinjiang. For example, the Physical Education and Health Curriculum Model of China, which was widely recognized in the field of physical education in China [43,44,45], should be carried out to help students maintain a normal BMI; Health courses also should be included in schools to make children and adolescents aware that they should keep a balanced diet and regular exercise. The government, communities, and families should also be united to take health promotion measures to keep the BMI of children and adolescents within the normal range [46].

There are some strengths in this study. The first strength is the large provincial representative sample, which has improved the objectivity and accuracy of the results, which has provided help to promote the healthy development of children and adolescents in Xinjiang, China. The second strength is that our study gives a picture of the patterns of how physical fitness changes with BMI in Xinjiang children and adolescents throughout the age (from 7 to 18 years old). However, there are also many limitations. The first limitation lies in the cross-sectional design which prevented the drawing of causal conclusions. Addressing the increasing prevalence of obesity and reduced fitness among children and adolescents, longitudinal studies are still needed to make causal inferences possible. The second limit is that, except for age and gender, we did not take into account other determinants of physical fitness (e.g. physical activity). The third limitation of the study is that we only measured five commonly used physical fitness and more accurate measures such as body composition were not included. The fourth limitation is that we didn’t control maturity status. This research also has some practical application value. First, it provides basis for the physical health intervention for children and adolescents in Xinjiang Uygur Autonomous Region, China. Secondly, it provides a reference for the government to formulate local public health policies and education policies in the future.