Association of body mass index with muscle strength and cardiorespiratory fitness: A cross-sectional study based on Chinese adolescents
Abstract
Background
Muscle strength and cardiorespiratory fitness are important components of physical fitness and are important for the physical and mental health development of university students. However, obesity is also an important factor affecting physical fitness, and there are few studies on how body mass index (BMI), which reflects obesity, is associated with muscle strength and cardiorespiratory fitness among Chinese university students. Therefore, this study analyzed the association between BMI and muscle strength and cardiorespiratory fitness among Chinese university students in order to provide a reference and basis for promoting the development of physical fitness among Chinese university students.
Methods
A stratified whole-group sampling method was used to test physical fitness items in 27 973 (15 527 boys, 55.51%) university students in 800 first- to fourth-year university classes in Anhui, Fujian, Xinjiang, Shanghai, and Jiangxi, China. The physical fitness items included height, weight, standing long jump, 1000 m running (boys), and 800 m running (girls) items. Curvilinear regression analysis was used to analyze the correlations that existed between BMI and standing long jump and VO2max.
Results
The BMI of Chinese 19–22 years old university students was (21.14 ± 2.92) kg/m2. The mean standing long jump score was (197.31 ± 34.07) cm. In general, the BMI reached the highest point of 207.92 cm when the BMI was 26.25 kg/m2, and then showed a decreasing trend with the increase of BMI. The overall relationship between BMI and vertical jump showed an inverted “U”-shaped curve. In terms of VO2max, the overall trend of VO2max increased gradually with the increase of BMI, and when BMI reached 40 kg/m2, VO2max was 4.34 L-kg−1-min−1.
Conclusion
Chinese university students showed an inverted “U” curve relationship between BMI and standing long jump in general, while VO2max showed a gradual increase with increasing BMI. Compared with the cardiorespiratory fitness of Chinese university students, the effect of BMI changes on muscle strength was greater.
1 INTRODUCTION
The problem of adolescent obesity is a public health problem of common concern in the world. Body mass index (BMI) is widely used as an important indicator to evaluate adolescent obesity (Hughes et al., 2022). BMI is closely associated with physical health and is an important indicator of physical health status. Studies have shown that a 1-unit increase in BMI leads to a 5% increase in the risk of death in overweight participants and a 9% increase in the risk of death in obese participants (Sun et al., 2019). Studies have also confirmed that an increase in BMI leads to an increase in the incidence of hypertension, diabetes, and chronic cardiovascular disease, posing a serious health risk (Vithayathil et al., 2021). For university students at the peak of their development, BMI is of great importance for their health. Chen et al. (2022) confirmed that the relationship between BMI and physical fitness indicators (grip strength, standing long jump, sit and reach, 50 m dash, endurance run) in adolescents showed a u-shaped or inverted u-shaped curve (R2 range−–0.001–0.182) in most age groups. Adolescents with normal BMI scored higher on fitness tests, more significantly in boys (R2 range −0.001–0.182) than in girls (R2 range 0.001–0.031). It has also been confirmed that with the outbreak of COVID-19, home confinement and isolation have led to a trend of elevated BMI in adolescents, posing a serious threat to physical and mental health (Woolford et al., 2021). In the past 20 years, the BMI of Chinese university students has hardly changed significantly in the lower percentiles, and the differences are mainly concentrated in the upper percentiles, where the curves show a significant increase. For example, the P95 of boys and girls Chinese university students increased from 23.12 and 22.78 in 1985 to 26.72 and 24.86 in 2005, with an increase of 15.57% and 9.13%, respectively (Yingxiu, 2010).
Muscle is the “engine” of movement, and strong muscles and the strength they bring are naturally the most essential guarantee of survival and health for adolescents. Studies have shown (Wiik et al., 2020) that healthy women have 23% of their body weight in muscle mass, while men have more than 40%. Studies have also shown (Fyfe et al., 2022) that muscle strength is directly related to many diseases. Studies have shown (Schauer et al., 2021) that patients with heart failure and cancer are characterized by rapid muscle atrophy, and the degree of atrophy has been shown to be one of the important predictors of their survival. It has also been shown (McLeod et al., 2016) that a decrease in muscle strength will accelerate the decline in physical function and make them age more rapidly. This suggests that there is a strong association between muscle strength and health. However, with the change in lifestyle, the increasing time of light physical activity has led to a decrease in resistance exercise that facilitates muscle strength improvement in life, bringing about a decrease in muscle strength levels. Dong et al. (2020) showed that Chinese adolescents showed a decreasing trend in muscle strength, and necessary measures should be taken to intervene and improve it in response to the influencing factors.
Cardiorespiratory fitness, as a core element of physical health, is of great importance for adolescent health. Several studies have confirmed that cardiorespiratory fitness is a key predictor of various types of adverse health problems in adolescents and has an important role and significance for health (Alberga et al., 2016). In addition, other studies have confirmed that the effects of cardiorespiratory fitness on health in adolescence continue into adulthood, with trajectory effects on health (Berenson et al., 1998). The continued decline in adolescent cardiorespiratory fitness has become a common public health concern worldwide. From the mid-1970s to the early 21st century, the level of child and adolescent cardiorespiratory fitness decreased by about 15% in some developed countries in Europe and the United States, and by nearly 30% points in emerging developed countries such as South Korea and Singapore (Dong et al., 2019). The results of the 2014 China student physical fitness and health survey showed that the level of cardiorespiratory fitness among children and adolescents in China was still at a low level, although it showed a steady trend toward improvement (CNSSCH Association, 2016). The problem of declining cardiorespiratory fitness in adolescents should be given adequate attention. Previous studies also showed (Rankinen et al., 2007)that physical activity, lifestyle, dietary behavior, and nutritional status are all important factors affecting adolescent cardiorespiratory fitness, among which the BMI index reflecting nutritional status has an important impact on cardiorespiratory fitness, and a reasonable BMI range should be maintained to better promote the healthy development of adolescent cardiorespiratory fitness.
There is a strong association between BMI and muscle strength and cardiorespiratory fitness in adolescents. Changes in BMI will have an impact on muscle strength and cardiorespiratory fitness in adolescents. Studies have shown that there is an association between BMI and muscle strength. Due to the continuous increase in BMI, it leads to a decreasing trend of muscle strength in the lower limbs of the body. It has also been shown (de Souza et al., 2022) that adolescents whose bodies are in the overweight stage have higher muscle strength levels than lean, normal and obese adolescents. In terms of cardiorespiratory fitness, studies have found that poor nutritional status is an important factor in the decrease of cardiorespiratory fitness levels in adolescents (Galan-Lopez et al., 2022). A mixed study in Chile and Colombia (Garcia-Hermoso et al., 2019) found an inverted “J” shaped relationship between adolescent cardiorespiratory fitness and BMI, with wasting, overweight and obesity as risk factors for high cardiorespiratory fitness in adolescents. Bovet et al. (2007) showed that the cardiorespiratory fitness of students in the wasted group was lower than that of the normal group, but higher than that of the overweight and obese groups. Ming et al. (2017) found that the cardiorespiratory fitness levels of Han Chinese adolescents in the wasted, overweight and obese groups were lower than the normal group, suggesting that we must not only consider the effect of overweight and obesity on cardiorespiratory fitness in children and adolescents, but also be alert to the effect of wasting on cardiorespiratory fitness, but also the negative effect of wasting on cardiorespiratory fitness. However, how the effects of BMI with cardiorespiratory fitness and muscle strength in Chinese university students need to be further studied and analyzed.
In China, due to the unbalanced economic development in different regions, the nutritional status of adolescents presents a situation of malnutrition and overnutrition. The health status of university students will have an important impact on their future career achievement because they are under the pressure of both employment and study. In order to better promote the improvement of cardiorespiratory fitness and muscle strength in university students in the future, it is necessary to understand the correlation between BMI and cardiorespiratory fitness and muscle strength. Previous studies have found few studies on the relationship between BMI and muscle strength and cardiorespiratory fitness in Chinese university students. Therefore, in this study, 27 973 (15 527 boys students, 55.51%) university students in China were tested for physical fitness items and BMI to analyze the association between BMI and muscle strength and cardiorespiratory fitness in order to better promote the healthy development of university students' healthy development of physical fitness.
2 METHODS
2.1 Participants
The participants of our study were selected in the following steps: In the first step, based on the geographical distribution of Chinese provinces, Anhui, Fujian, Xinjiang, Shanghai, and Jiangxi in China were selected as the provinces to be investigated and tested in this study. In the second step, two higher education schools were randomly selected as test schools in each province. In the third step, 20 classes were randomly selected in each school from the first to the fourth year of university. All eligible students in the class served as the survey subjects of this study. The inclusion criteria for participants in this study were: (1) university students aged 19–22 years old who were enrolled in school; (2) able to participate in the physical fitness program; (3) voluntarily accepted the survey of this study and gave their informed consent.
In this study, a total of 29 145 university students in 800 teaching classes were tested on physical fitness items. After proposing copies of invalid data after the survey, a total of 27 973 valid data (15 527 boys students, 55.51%) were retrieved, and the valid recovery rate of the questionnaire was 95.98%. The effective return rates of questionnaires in Anhui, Fujian, Xinjiang, Shanghai, and Jiangxi were 96.37% (5624/5836), 96.85% (5495/5674), 95.81% (5647/5894), 95.56% (5683/5947), and 95.34% (5524/5794), respectively. The specific sampling process of subjects in this study is shown in Figure 1.
This study investigation was approved by the human ethics committee of the center for health promotion in sports, Chizhou University (202211457). Written informed consent was obtained from the subjects themselves prior to the survey. The survey questionnaire was completed anonymously by number to strictly protect the subjects' personal privacy.
2.2 Body mass index
We calculated BMI based on the tested height and weight with the formula of weight (kg)/height (m)2. The testing standards for height and weight were based on the instruments and methods required by the China student physical fitness survey (CNSSCH Association, 2016). The height test required the subjects to take off their shoes, with their heels and heads leaning against the uprights of the height meter and the staff's line of sight parallel to the level of the scale to read the test data. Height was measured using a ZL23-SZG-210 height meter, and the test results were accurate to 0.1 cm. The weight test required students to wear light clothing and take off their shoes, and the test subject stood on the scale to read the weight data after the numbers stabilized. The weight test was conducted using a DT-200 scale, and the results were accurate to 0.1 kg. In order to ensure the accuracy of the test results, both height and weight tests were conducted in the morning and the staff was asked to calibrate the height and weight scales before each day's test.
2.3 Standing long jump
The standing long jump test was conducted according to the method required by the Chinese student physical fitness standard (CNSSCH Association, 2016). The test subject is required to jump in a standing manner. The subject stands behind the starting line, both arms hang down naturally, jumps forward as far as possible, and tests the shortest vertical distance between the heel and the starting line. The test result was accurate to 0.1 cm.
2.4 Cardiorespiratory fitness
The test of cardiorespiratory fitness was based on the requirements of Chinese students' physical fitness standards (CNSSCH Association, 2016), and the test was conducted using 1000 m run for boys and 800 m run for girls, and the test results were accurate to 1 s. The maximum oxygen uptake was calculated based on the test time (s) and through the derivation formula of Haiyun (2019), the specific derivation formula was: VO2max (L/min) = 1.640–0.004 × sex (boys 1, girls 2) × time (s) + 0.037 × weight (kg). Subjects were asked to prepare for the 1000 and 800 m running tests before the test, and the staff used a stopwatch to time the test.
2.5 Quality control
The items tested in this study included height, weight, standing long jump, and endurance running (1000 m for boys and 800 m for girls). The testing staff was all trained physical education teachers, and each testing item was tested by a fixed staff. The testing apparatus is required to be calibrated every day after the test.
2.6 Statistical analysis
The physical fitness of different genders of university students was expressed by means and standard deviations (M ± SD). The analysis of the relationship between BMI of different genders and standing long jump and VO2max was performed by curve regression analysis, using standing long jump and VO2max as dependent variables and BMI and the square of BMI as independent variables, respectively, and a curve regression model was established, yielding standing long jump/VO2max = a BMI2 + b BMI + c, where a, b, and c are constants.
Data were double-entered using EpiData 3.0, and the completed data were verified again by another group of students, and the data were imported into SPSS 25.0 software (version 25.0; IBM Inc., Armonk, NY) for processing and analysis. GraphPad Prism 8.0.2 (GraphPad Software, Inc., CA) was used for image production. A two-sided test level of P < 0.05 was used.
3 RESULTS
Table 1 shows that our study tested height, weight, standing long jump, and 1000 (boys)/800 m (girls) running in 27 973 university students (boys 15 527, 55.51%) aged 19–22 years in China. The average age of the subjects was (20.25 ± 1.11) years. After passing the height and weight tests, BMI was calculated according to weight (kg)/height (m)2. The BMI of the university students in this study was (21.14 ± 2.92) kg/m2. The mean score of standing long jump was (197.31 ± 34.07) cm. VO2max was (2.27 ± 0.89) L kg−1-min−1.
| Projects | Boys(n = 15 527) | Girls(n = 12 446) | Total(n = 27 973) |
|---|---|---|---|
| Age (year) | 20.33 ± 1.12 | 20.15 ± 1.10 | 20.25 ± 1.11 |
| Height (cm) | 175.16 ± 5.38 | 163.68 ± 5.24 | 170.05 ± 7.80 |
| Weight (kg) | 67.58 ± 9.84 | 53.78 ± 7.42 | 61.44 ± 11.19 |
| BMI (kg/m2) | 22.01 ± 2.93 | 20.06 ± 2.52 | 21.14 ± 2.92 |
| Standing long jump (cm) | 221.93 ± 21.61 | 166.58 ± 18.05 | 197.31 ± 34.07 |
| 1000/800 m run (s) | 292.61 ± 57.47 | 278.32 ± 41.38 | 286.25 ± 51.43 |
| VO2max (L kg−1 min−1) | 2.97 ± 0.43 | 1.40 ± 0.42 | 2.27 ± 0.89 |
Table 2 shows that the curvilinear regression analysis was performed with BMI as the independent variable and standing long jump as the dependent variable, after stratifying by gender. Separate regression equations were obtained for boys: standing long jump = −0.130BMI2 + 4.990BMI + 176.429 (R2 = 0.021); regression equation for girls students: standing long jump = −0.088BMI2 + 3.649BMI + 129.506 (R2 = 0.003); overall regression equation: standing long jump = −0.310BMI2 + 16.250BMI-5.032 (R2 = 0.060).
| Independent variables and constants | B-value | Standard error | β-value | t-value | p-value |
|---|---|---|---|---|---|
| Boys(n = 15 527) | |||||
| BMI | 4.990 | 0.665 | 0.677 | 7.508 | <0.001 |
| BMI 2 | −0.130 | 0.015 | −0.807 | −8.945 | <0.001 |
| (Constants) | 176.429 | 7.494 | 23.542 | <0.001 | |
| Girls (n = 12 446) | |||||
| BMI | 3.649 | 0.657 | 0.510 | 5.554 | <0.001 |
| BMI 2 | −0.088 | 0.015 | −0.526 | −5.727 | <0.001 |
| (Constants) | 129.506 | 6.932 | 18.682 | <0.001 | |
| Total (n = 27 973) | |||||
| BMI | 16.250 | 0.733 | 1.394 | 22.160 | <0.001 |
| BMI 2 | −0.310 | 0.017 | −1.179 | −18.746 | <0.001 |
| (Constants) | −5.032 | 8.035 | −0.626 | 0.531 | |
- Abbreviation: BMI, body mass index.
Table 3 shows that curvilinear regression analysis was performed after stratifying by gender using BMI as the independent variable and VO2max as the dependent variable. The regression equations were obtained for boys students: VO2max = −0.002BMI2 + 0.215BMI−0.654 (R2 = 0.602); girls students: VO2max = −0.001BMI2 + 0.137BMI−0.949 (R2 = 0.323); overall regression equation: VO2max = −0.005BMI2 + 0.410BMI−4.059 (R2 = 0.368).
| Independent variables and constants | B-value | Standard error | β-Value | t-value | p-value |
|---|---|---|---|---|---|
| Boys (n = 15 527) | |||||
| BMI | 0.215 | 0.008 | 1.461 | 25.395 | <0.001 |
| BMI 2 | −0.002 | 0.000 | −0.691 | −12.005 | <0.001 |
| (Constants) | −0.654 | 0.095 | −6.867 | <0.001 | |
| Girls (n = 12 446) | |||||
| BMI | 0.137 | 0.013 | 0.818 | 10.816 | <0.001 |
| BMI 2 | −0.001 | 0.000 | −0.252 | −3.325 | 0.001 |
| (Constants) | −0.949 | 0.134 | −7.078 | <0.001 | |
| Total (n = 27 973) | |||||
| BMI | 0.410 | 0.016 | 1.350 | 26.166 | <0.001 |
| BMI 2 | −0.005 | 0.000 | −0.750 | −14.539 | <0.001 |
| (Constants) | −4.059 | 0.172 | −23.627 | <0.001 | |
- Abbreviation: BMI, body mass index.
Figure 2 shows the trend change of the curvilinear regression relationship between BMI and standing long jump and VO2max in Chinese university students. In general, when BMI was 26.25 kg/m2, the standing long jump performance reached the highest point, that is 207.92 cm, and then showed a decreasing trend with the increase of BMI, and the overall relationship between BMI and standing long jump showed an inverted “U” curve. In terms of VO2max, in general, with the increase of BMI, VO2max showed a gradually increasing trend, and when BMI reached 40 kg/m2, VO2max was 4.34 L kg−1-min−1.
4 DISCUSSION
In this study, 27 973 (15 527 boys students, 55.51%) university students in China were assessed for physical nutritional status, muscle strength, and cardiorespiratory fitness items. Curvilinear regression analysis was used to analyze the association that exists between BMI and muscle strength and cardiorespiratory fitness. The results of our study showed that BMI and standing long jump in Chinese university students showed an inverted “U” curve relationship in general, while VO2max showed a gradual increase with the increase of BMI. The effect of BMI change on muscle strength was greater than that of cardiorespiratory fitness in Chinese university students. Our findings are consistent with those of adolescent studies in China (Dong et al., 2019), Brazil (Lopes et al., 2017) and Germany (Tittlbach et al., 2017).
The standing long jump reflects the muscle strength of the lower limbs in adolescents. The results of this study showed that both adolescent boys and girls showed a curvilinear relationship between BMI and standing long jump, which was more pronounced in boys than in girls. The reason for this result may be that as development reaches its peak, adolescents' muscle fibers begin to thicken, thus increasing lower limb muscle strength. The increase in BMI may be dominated by the increase in adolescent body fat, which further increases body weight and requires greater body weight resistance to be overcome during the standing long jump test, so the increase in BMI is not effective in improving the lower extremity muscle levels of adolescents. Of course, a lower BMI can also affect muscle strength. A lower BMI means that a deeper body is in a leaner state, with lower body muscle mass, smaller muscle cross-sectional area and thinner muscle fibers, which inevitably affects muscle strength levels. Boys and girls in this study showed different trends in BMI and muscle strength, which may be related to the different muscle content of boys and girls due to gender differences. The results of a study on European adolescents showed that BMI showed a curvilinear relationship with lower limb muscle strength (standing long jump test) (Zaqout et al., 2016), which is consistent with the results of the present study. Research also has to show that obese adolescents have relatively low muscle strength compared to normal people (Zembura et al., 2023).
Endurance running reflects the level of cardiorespiratory fitness of the test participants. In this study, the VO2max test was conducted using 1000 m run for boys and 800 m run for girls to reflect the level of cardiorespiratory fitness of the participants. This test method is recognized by the Chinese student physical fitness standard test method and is conducted annually throughout China on children and adolescents aged 7–22 years. The results of this study showed that VO2max showed a gradual increase with increasing BMI. Chen et al. confirmed that the trend of BMI and endurance running performance of adolescents showed a “U”-shaped curve in the age group of 7–18 years for both boys and girls, and it was more obvious, indicating that endurance performance was best when BMI was in the normal range, and high or low BMI would affect the endurance performance of adolescents. High or low BMI affects the endurance performance of adolescents. However, the reasons for the differences between the results of this study and other studies need to be interpreted with caution, because there are some differences in the groups, geographical and ethnic composition of the different studies tested. And these factors are also important factors affecting cardiorespiratory fitness. The study shows that overweight and obese adolescents tested for cardiorespiratory fitness improved after adjusting for the amount of fat that even reversal occurred (Artero et al., 2010). Similarly, after adjusting for body fat, there was no significant difference in cardiorespiratory fitness between obese and normal weight adolescents. Therefore, the use of non-weight-bearing tests, such as the power bicycle test, can better reflect the cardiorespiratory fitness status of adolescents with different body weights.
There are some strengths and limitations of this study. In terms of strengths, first, the sample size used in this study is large, involving 27 973university students from five regions in China, which is representative. Second, this study used the 1000 m running for boys students and 800 m running for girls students as the evaluation standard of cardiorespiratory fitness as specified in the Chinese national student physical fitness standard, which is a simple test method, easy for students to grasp, and can objectively reflect university students' cardiorespiratory fitness level of university students. However, there are some limitations in this study. First, this study only used the standing long jump to reflect the lower limb muscle strength of university students, but did not use items such as grip strength to test the upper limb muscle strength of the subjects, which has some limitations in evaluating the muscle strength. Second, this study only used BMI to evaluate the body composition of the participants and ignored waist circumference to evaluate centripetal obesity. At the same time, this study did not use more sophisticated instruments such as body composition analyzers for body composition evaluation, and the results are inevitably biased to some extent. In addition, this study indirectly projected the VO2max of university students by using 1000 m running for boys and 800 m running for girls, which is somewhat different from the real level. A more accurate test method should be used for VO2max assessment in the future.
5 CONCLUSIONS
In this study, the association between BMI and muscle strength and cardiorespiratory fitness in Chinese university students was analyzed. In general, the relationship between BMI and standing long jump in Chinese university students showed an inverted “U” curve, while VO2max showed a gradual increase with the increase of BMI. Compared with the cardiorespiratory fitness of Chinese university students, the effect of BMI changes on muscle strength was greater. This study further suggests that both muscle strength and cardiorespiratory fitness in Chinese university students are influenced by BMI. Both lower and higher BMI resulted in a decrease in muscle strength, whereas the effect on cardiorespiratory fitness was inconsistent. This also suggests that future improvements in muscle strength should pay more attention to changes in body composition, especially for lean and obese individuals. However, the improvement of cardiorespiratory fitness in university students should pay more attention to the improvement of cardiorespiratory fitness level in the wasting group.
AUTHOR CONTRIBUTIONS
Conceptualization: Lixin Liu and Junmin Yang. Data curation: Lixin Liu. Formal analysis: Junmin Yang. Funding acquisition: Ruisheng Jiao. Investigation: Yan Wang. Methodology: Yan Wang. Project administration: Ruisheng Jiao. Resources: Ruisheng Jiao. Software: Ruisheng Jiao. Supervision: Lixin Liu. Validation: Lixin Liu. Visualization: Lixin Liu and Ruisheng Jiao. Writing—original draft: Lixin Liu and Ruisheng Jiao. Writing—review & editing: Lixin Liu and Ruisheng Jiao. All authors have read and agreed to the published version of the manuscript.
ACKNOWLEDGMENTS
We thank the students and parents who participated in this study, as well as the staff who participated in the data testing of this study.
FUNDING INFORMATION
This research was supported by 2022 Youth Project of Anhui Provincial Philosophy and Social Science Planning (AHSKQ2022D132).
CONFLICT OF INTEREST STATEMENT
The authors declare no conflict of interest.
Open Research
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.
