Mediating Role of Echo-Intensity in Sex Differences of Physical Fitness among Children

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Mediating Role of Echo-Intensity in Sex Differences of Physical Fitness among Children

https://doi.org/10.21203/rs.3.rs-4257280/v1

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Numerous studies have documented disparities in Physical Fitness (PF) between boys and girls. This cross-sectional study investigates if echo-intensity (EI) explains the relationship between sex and PF among children. The study cohort consisted of 497 children aged 4.0–10.9, including 288 boys and 209 girls. Anthropometric measures, including height, weight, and waist circumference, were assessed. PF was evaluated using age-appropriate tests: the ALPHA Fitness battery for school children and the PREFIT battery for preschool children. Ultrasound (US) was employed to evaluate EI in the rectus femoris muscle. Our analysis revealed boys exhibited significantly higher PFcan components and echo-intensity levels compared to girls (all PF p < 0.001, EI p = 0.001). Mediation analysis revealed that the total effect of sex on overall fitness (z-score) was 0.51 (95% CI, 0.36 to 0.66), with an indirect effect mediated by EI of 0.07 (95% CI, 0.02 to 0.13), and a direct effect on overall fitness (z-score) of 0.44 (95% CI, 0.30, 0.58; 15.68% mediation). EI in the rectus femoris muscle serves as a factor elucidating how a child's sex influences their PF. These findings emphasize the importance of considering sex in exercise planning and interpreting the results of fitness assessments. These insights hold potential implications for public health strategies targeting children and adolescents.

Physical fitness

echo-intensity

sex-differences and children

Sex differences can be attributed to specific aspects of human biology [1]. Significant sexual dimorphism in body composition concerning lean mass, fat mass, and bone mass becomes evident very early in life and undergoes the most pronounced changes during puberty [2]. Numerous studies have investigated disparities in Physical Fitness (PF) between boys and girls [3]. For instance, Latorre et al. reported that boys exhibited superior performance in CRF endurance, reaction time, strength, and running speed across various preschooler groups [4]. Consequently, reference values for PF have been established for each age group, differentiated by sex [5].

In contrast, limited research has explored gender-based disparities in muscle structure during early childhood. García-Alonso et al. are among the few who have established age-specific normal ranges for echo-intensity (EI) and developed reference values for different age groups and sexes [6]. Both PF and EI are potent indicators of children's health [7]. This association has also been observed in conditions such as type 2 diabetes and other cardiovascular risk factors among older adults [8]. Similarly, EI is a practical method for assessing age-related risks associated with reduced muscle quality [9]. Okura et al. found an inverse relationship between physical activity levels and rectus femoris EI in older adult patients [10].

The relationship between PF and EI has been explored previously, revealing an inverse association, even after adjusting for potential confounding factors such as sex in prepuberal children [11]. Therefore, it is essential to account for sex when examining these two variables.

In research investigating the relationship between sex and PF, potential confounding factors are typically managed using statistical analyses such as ANOVA [4], linear regression [5], or logistic regression [12], selected based on the study's objectives and the nature of the outcome under investigation. Mediation, a statistical approach, is employed to elucidate the mechanisms underlying the connection between two variables and to assess to what extent another variable may mediate, modify, or distort this relationship [13]. A mediation effect occurs when a third variable, known as the mediator, elucidates how an independent variable influences a dependent variable.

This study aims to address the hypothesis of how sex influences PF and whether EI serves as a potential mediator in this relationship. In other words, it seeks to determine whether EI can elucidate how an individual's sex impacts their PF. Therefore, the objectives of the present study are twofold: (i) to examine the relationship between PF and EI in schoolchildren, considering sex as a factor, and (ii) to examine whether the association between sex and PF is mediated by EI.

2.1 Study design and population

In this cross-sectional study, we investigated PF and muscle ultrasound (US) parameters among Spanish schoolchildren, utilizing data from the "Observatorio de Actividad Física en escolares" (Physical Activity Observatory in Schoolchildren) available at https://observatorioactividadfisica.es. We analyzed data from 497 children, aged 4.0 to 10.9 years, comprising 288 boys and 209 girls, with a mean age (95% confidence interval [CI]) of 7.39 years, to assess the mediation effect of EI on sex differences in PF.

To ensure a representative sample, participant recruitment locations were purposefully selected across various sites in the Metropolitan Region of Pamplona, Navarra, Spain. These locations encompassed a diverse range of settings, including a private school (Santa María la Real-Maristas), three state schools (San Juan de la Cadena, Lago de Mendillorri, and Garcia Galdeano), two sports centers (a private sports center, S.C.D.R Anaitasuna, and a football club, Gazte Berriak C.F), as well as a primary care facility (C.S Iturrama).

Ethical considerations were paramount in our study. We obtained informed consent from parents or guardians, and children provided informed assent before participating. The study protocol adhered to the principles outlined in the Helsinki Declaration and received approval from the Ethics Committee of the Universidad Pública de Navarra (CENEDUCA1/2019).

2.2 Measurements

We conducted a thorough assessment of anthropometric measures, adhering to the CDC-NHANES Survey protocol [14]. Height measurements were taken in the Frankfurt position using a SECA 213® stadiometer, with a precision of 1 mm. Weight was measured with participants wearing light clothing and bare feet, utilizing a Tanita DC-430MAS® scale with a precision of 100 g. Waist circumference was measured in centimeters, employing a Seca 201® non-elastic flexible tape with a precision of 1 mm. Body mass index (BMI) was calculated using the standard formula (kg/m²).

PF was evaluated through age-appropriate tests: the ALPHA Fitness battery [15] for school children and the PREFIT battery [16] for preschool children. Cardiovascular fitness was assessed using the Course Navette test, which measured the number of laps completed in a single trial. Muscle fitness was evaluated for both upper and lower body strength. Upper body strength was determined using a handgrip strength (HGS) test, where children underwent the test twice with a Takei 5001® analog dynamometer, providing isometric grip strength measurements in kilograms of force (kgf) with a precision of 100 g. Lower body strength was assessed through the standing long jump (SLJ) test, with children performing the test three times, and the best performance was recorded. Speed and agility were measured using the 4 × 10m shuttle run test. Children completed this test twice, and their best time was recorded, with shorter times indicating better performance. To establish an overall PF score, we combined the following parameters: CRF (Course Navette - laps), HGS, SLJ, and speed-agility (4 × 10 test). In the case of the speed-agility test, where a longer time denoted poorer performance, we inverted the time by multiplying it by -1 for consistency in the scoring process.

The evaluation of EI within the rectus femoris muscle was conducted using real-time two-dimensional B-mode US, specifically utilizing the Esaote MyLabTM50 system from Genova, Italy. Participants were instructed to lie on their backs with extended knees and maintain a state of rest during the image capture process. To ensure accurate probe placement and image capture location, a point was marked at 50% of the distance between the anterior superior iliac spine and the midpoint of the patella. The image acquisition was performed by a skilled operator (YGA) employing a multi-frequency linear transducer with a range of 4–15 MHz.

To minimize potential bias, enhance spatial resolution, and maintain consistency in muscle structure measurements, all measurements of muscle structure were acquired using fixed settings that remained constant for all participants. The image gain was set at 55 decibels (dB), the dynamic range was maintained at 72, and the image depth ranged from 45 to 50 mm. Any optional post-processing features within the software were deactivated, and the time gain compensation settings were kept neutral. US gel (Ultrasound GEL® Ref: 33273, Gima s.p.A Laboratories, Inc., Gessate, MI, Italy) was applied to ensure proper acoustic contact, and gentle pressure was exerted to visualize the muscle border partially. Subsequently, images were captured, followed by panoramic images.

EI was determined based on the average gray level within the region of interest (ROI) in 8-bit resolution images, where gray levels ranged from 0 to 255 (with 0 representing black and 255 indicating white). To perform this analysis, ImageJ software (ImageJ, National Institutes of Health, USA, version 1.45s) was employed. Higher values on the grayscale spectrum indicated greater intramuscular fat and interstitial fibrous tissue accumulation [9]. Mean and standard deviation (SD) calculations were carried out for each histogram. Data processing was executed using internal software integrated into the Esaote MyLabTM50, employing a methodology similar to established procedures (9, 7).

2.3 Statistical analysis

We employed Statistical Package for SPSS release 3.5.3 (IBM Corp., Portsmouth, Hampshire, UK) to investigate the potential mediation effect of EI in the relationship between sex (boys vs. girls) and our primary outcomes, which included various aspects of PF.

Mediation analysis is a statistical approach that serves to elucidate the underlying mechanisms that connect two variables and assess the extent to which this connection may be influenced, mediated, or confounded by a third variable. A mediation effect occurs when a third factor, known as the mediator, plays a role in explaining the impact of an independent variable on a dependent one. This analytical approach employs nested models to estimate the proportion of the total association, while accounting for various covariates, that can be attributed to an indirect pathway facilitated by the mediator. Confidence intervals, calculated through a nonparametric bootstrapping method with 5000 iterations, were used to estimate the significance of these effects.

Throughout our statistical analyses, a significance level of 0.05 for two-tailed P-values was considered meaningful.

Table 1 displays the characteristics of the study sample, segmented by sex. The cohort consisted of 497 children aged 4.0 to 10.9, comprising 288 boys and 209 girls. Notably, anthropometric parameters (except for BMI) and Our analysis revealed boys exhibited significantly higher PF components and echo-intensity levels compared to girls. Future studies could explore intervention strategies that consider EI's mediating role to enhance physical fitness equally across sexes.

Table 1

Descriptive characteristics of full sample, boys and girls participating in the study.
Variables	Full sample (n = 497)	Boys (n = 288)	Girls (n = 209)	P value
Anthropometric parameters
Age (years)	7.39 (7.22; 7.56)	7.64 (7.41; 7.88)	7.03 (6.80; 7.27)	< 0.001
Height (cm)	124.82 (123.65; 126.00)	126.61 (125.02; 128.21)	122.37 (120.69; 124.05)	< 0.001
Weight (kg)	27.21 (26.48; 27.93)	28.16 (27.19; 29.14)	25.89 (24.83; 26.96)	0.002
Body mass index (kg)	17.08 (16.87; 17.28)	17.18 (16.92; 17.45)	16.93 (16.61; 17.25)	0.225
Waist circumference (cm)	58.27 (57.65;58.90)	58.88 (58.05; 59.70)	57.45 (56.50; 58.39)	0.026
Physical fitness components
CRF (laps)	40.23 (38.36; 42.11)	46.59 (43.90; 49.29)	31.72 (29.74; 33.70)	< 0.001
Absolute HGS (kgf)	11.44 (11.11; 11.76)	12.14 (11.69; 12.59)	10.47 (10.02; 10.92)	< 0.001
SLJ (cm)	119.26 (116.74; 121.78)	125.79 (122.24; 129.34)	110.25 (107.15; 113.34)	< 0.001
Speed agility (s)	13.52 (13.37; 13.68)	13.19 (12.98; 13.40)	13.97 (13.75; 14.20)	< 0.001
Overall fitness (z-score)	0.01 (-0.07; 0.09)	0.23 (0.12; 0.34)	-0.28 (-0.37; -0.19)	< 0.001
Echo-intensity	44.82 (43.61; 46.03)	43.13 (41.45; 44.81)	47.15 (45.48; 48.82)	0.001

The prevalence of EI estimated through US, and PF components measured by HGS, SLJ, speed-agility, cardiorespiratory fitness (CRF), and overall fitness, were significantly higher in boys than in girls. Specifically, HGS was significantly greater in boys than in girls (mean [SD], 12.14 [3.86] vs. 10.47 [3.32] kg; difference, 1.67 kg; P < 0.001) (Fig. 1A). Similarly, SLJ was longer in boys than in girl (mean [SD], 125.79 [30.58] vs. 110.24 [22.61] cm; difference, 15.55 cm; P < 0.001) (Fig. 1B). Likewise, speed agility (mean [SD], 13.19 [1.75] vs. 13.97 [1.62] sec; difference, 0.78 sec; P < 0.001) and CRF (mean [SD], 46.59 [22.79] vs. 31.71 [14.47] laps; difference, 32.12 laps; P < 0.001) were significantly higher in boys than in girls (Fig. 1, C and D). Overall PF (z-score) was also significantly greater in the boys than in girls (mean [SD], 0.23 [0.90] vs. -0.28 [0.64]; difference, 1.67; P < 0.001) (Fig. 2).

A mediation analysis was conducted to estimate the effect of the sex on PF. The results of the mediation analysis indicated that a significant portion of the total effect of boys on HGS (1.70 kg; 95% CI, 1.05, 2.36 kg) could be attributed to an indirect effect mediated by EI (0.33 kg; 95% CI, 0.13, 0.56 kg; 19.10% mediation). Even after accounting for the EI association, a statistically significant direct effect of sex on HGS remained at 1.37 kg (95% CI, 0.74, 1.99 kg) (Fig. 1A). Similarly, the analysis revealed that a substantial portion of the total effect of sex on SLJ (15.57 cm; 95% CI, 10.61, 20.53 cm) was mediated by EI (2.74 cm; 95% CI, 0.99, 4.80 cm; 17.43% mediation), leaving a statistically significant direct effect of sex on SLJ at 12.82 cm (95% CI, 8.14, 17.50 cm) (Fig. 1B).

Furthermore, mediation analysis indicated that the total effect of sex on speed-agility (-0.76 sec; 95% CI, -1.07 to -0.45 sec) was partially mediated by EI (-0.13 sec; 95% CI, -0.23 to -0.04 sec), with a remaining direct effect of -0.63 sec (95% CI, -0.93, -0.33 sec; 15.11% mediation) (Fig. 1C). Similarly, the total effect of sex on CRF (15.13 laps; 95% CI, 11.55 to 18.71 laps) was mediated in part by EI (1.87 laps; 95% CI, 0.61 to 3.31 laps), leaving a statistically significant direct effect of sex on CRF at 13.26 laps (95% CI, 9.86, 16.66 laps; 12.30% mediation) (Fig. 1D).

In summary, the mediation analysis indicated that a significant portion of the total effect of sex on overall fitness (z-score) of 0.51 (95% CI, 0.36 to 0.66) was mediated by EI (0.07; 95% CI, 0.02 to 0.13), with a remaining direct effect of overall fitness (z-score) at 0.44 (95% CI, 0.30, 0.58; 15.68% mediation) (Fig. 2).

This study is the first to employ mediation analysis to explore the role of EI in elucidating the relationship between sex and PF in schoolchildren. Intriguingly, our investigation revealed that various components of PF, including HGS, SLJ, speed-agility, CRF, and the overall fitness (z-score), were mediated by EI. These findings not only reaffirm the significant disparities in PF and EI between boys and girls but also underscore the relationship between lower levels of EI and enhanced PF. Therefore, our results propose that EI serves as a comprehensive mediator in the intricate connection between sex and PF in children.

These findings may be attributed to multiple factors. The associations observed between somatotype and PF assessments predominantly stem from a shared genetic foundation among children and adolescents, irrespective of sex [17]. Furthermore, the study by Zaqout et al. highlights that modifiable targets such as BMI, physical activity, dietary habits, and psychosocial factors play pivotal roles in augmenting PF [18]. Sexual dimorphism is another crucial aspect to consider. Historically, 'sexual dimorphism' has characterized systematic anatomical differences between individuals of different sexes (14, 15). These differences in fitness tests are already evident during preschool years and tend to magnify with age. Recent insights have elucidated sexual dimorphism in substrate metabolism during post-exercise recovery and the role of lipid kinetics in supporting resting metabolism during this phase [20]. linearity on running speed and aerobic capacity seem to be crucial in prepubescent boys and girls[21]. The impact of adiposity on explosive strength, musculoskeletal proportions on running speed, and relative linearity on both running speed and aerobic capacity emerge as critical factors in this context.

In exploring the mediation effect of echo-intensity on sex differences in physical fitness, it is crucial to consider the role of hormonal differences. For instance, testosterone and estrogen significantly influence muscle composition and function, potentially impacting echo-intensity and, by extension, physical fitness levels [22]. Testosterone, more prevalent in boys, is known to facilitate muscle growth and strength, possibly leading to higher echo-intensity values and better physical performance compared to girls. Conversely, estrogen, which is more dominant in girls, plays a role in fat distribution, which could influence echo-intensity measurements and physical fitness outcomes differently. Acknowledging the study's observational nature, these speculations about hormonal influences invite further research into the intricate biological pathways affecting physical fitness through muscle quality, as measured by echo-intensity.

While our analyses yielded robust results, it is important to acknowledge several limitations. The study is constrained by its relatively modest sample size, and the observational nature of the study design limits causal inferences.

Conclusion

EI in the rectus femoris emerges as a crucial factor that elucidates how a child's sex influences their PF. Importantly, our findings reveal a significant difference in physical fitness levels between boys and girls. This disparity can be partially attributed to echo-intensity, underscoring its potential role as a mediator. These insights hold potential implications for public health strategies targeting children and adolescents.

Physical fitness

Ultrasound

echo-intensity

HGS

Handgrip strength

SLJ

Standing Long Jump

CRF

Cardiorespiratory fitness

Arbitrary Units

Ethics approval and consent to participate

We obtained informed consent from parents or guardians, and children provided informed assent before participating. The study protocol adhered to the principles outlined in the Helsinki Declaration and received approval from the Ethics Committee of the Universidad Pública de Navarra (CENEDUCA1/2019).

Consent for publication

Not applicable

Availability of data and material

The datasets used and/or analyzed during the current study can be made available from the corresponding author on reasonable request.

Competing interests

All authors have nothing to declare.

Funding

Funding for the study was provided by the Gobierno de Navarra–Departamento de Salud, co-financed by the European Regional Development Fund through the ERDF Operational Program 2014–2020 (Navarra).

Authors' contributions

GLG, YGA, RRV, AMA, MI designed research; GLG, YGA, RRV, AMA conducted research; GLG, YGA, RRV, AMA analyzed data; GLG, YGA, RRV, MI AMA wrote the first draft of manuscript; GLG, AMA, RRV and MI had primary responsibility for final content. All authors read and approved the final manuscript.

Acknowledgments

None

Regitz-Zagrosek V (2012) Sex and gender differences in health. EMBO Rep 13:596–603
Wells JCK (2007) Sexual dimorphism of body composition. Best Pract Res Clin Endocrinol Metab 21:415–430
Tyler R, Mackintosh KA, Spacey HL, Stratton G (2020) A cross-sectional study on the deprivation and sex differences in health-related fitness measures in school children. J Sports Sci 38:70–78
Latorre Román PÁ, Del Moreno R, Lucena Zurita M, Salas Sánchez J, García-Pinillos F (2017) Mora López D. Physical fitness in preschool children: association with sex, age and weight status. Child Care Health Dev 43:267–273
Tomkinson GR, Carver KD, Atkinson F, Daniell ND, Lewis LK, Fitzgerald JS et al (2018) European normative values for physical fitness in children and adolescents aged 9–17 years: results from 2 779 165 Eurofit performances representing 30 countries. Br J Sports Med 52:1445–14563
García-Alonso Y, Alonso-Martínez AM, García-Hermoso A, Legarra-Gorgoñon G, Izquierdo M, Ramírez-Vélez R (2023) Centile reference curves of the ultrasound-based characteristics of the rectus femoris muscle composition in children at 4–11 years old. Front Pediatr 11:1168253
del Fonseca FJ, Alonso JV, Álvarez MV, Orr S, Cantarero FJL (2017) Physical fitness as an indicator of health status and its relationship to academic performance during the prepubertal period. Health Promot Perspect 7:197–204
Peterson MD, McGrath R, Zhang P, Markides KS, Al Snih S, Wong R (2016) Muscle Weakness Is Associated With Diabetes in Older Mexicans: The Mexican Health and Aging Study. J Am Med Dir Assoc 17:933–938
Stock MS, Thompson BJ (2021) Echo intensity as an indicator of skeletal muscle quality: applications, methodology, and future directions. Eur J Appl Physiol 121:369–380
Okura K, Iwakura M, Kawagoshi A, Sugawara K, Takahashi H, Shioya T (2022) Objective physical activity level is associated with rectus femoris muscle echo-intensity in patients with chronic obstructive pulmonary disease. Clin Respir J 16:572–580
García-Alonso Y, García-Hermoso A, Alonso-Martínez AM, Legarra-Gorgoñon G, Izquierdo M, Ramírez-Vélez R (2022) Associations between physical fitness components with muscle ultrasound parameters in prepuberal children. Int J Obes 46:960–968
Lu KD, Billimek J, Bar-Yoseph R, Radom-Aizik S, Cooper DM, Anton-Culver H (2016) Sex Differences in the Relationship between Fitness and Obesity on Risk for Asthma in Adolescents. J Pediatr 176:36–42
MacKinnon D (2012) Introduction to Statistical Mediation Analysis. Routledge
Questionnaires NHANES, Datasets, and, Documentation R (2023) accessed September 5, n.d. https://wwwn.cdc.gov/nchs/nhanes/Default.aspx
Ruiz JR, Castro-Piñero J, España-Romero V, Artero EG, Ortega FB, Cuenca MM et al (2011) Field-based fitness assessment in young people: the ALPHA health-related fitness test battery for children and adolescents. Br J Sports Med 45:518–524
Ortega FB, Cadenas-Sánchez C, Sánchez-Delgado G, Mora-González J, Martínez-Téllez B, Artero EG et al (2015) Systematic review and proposal of a field-based physical fitness-test battery in preschool children: the PREFIT battery. Sports Med Auckl NZ 45:533–555
Silventoinen K, Maia J, Jelenkovic A, Pereira S, Gouveia É, Antunes A et al (2021) Genetics of somatotype and physical fitness in children and adolescents. Am J Hum Biol Off J Hum Biol Counc 33:e23470
Zaqout M, Vyncke K, Moreno LA, De Miguel-Etayo P, Lauria F, Molnar D et al (2016) Determinant factors of physical fitness in European children. Int J Public Health 61:573–582
Ortega-Avila JG, García-Muñoz H, Segura Ordoñez A, Salazar Contreras BC (2022) Sexual dimorphism of leptin and adiposity in children between 0 and 10 years: a systematic review and meta-analysis. Biol Sex Differ 13:47
Henderson GC (2014) Sexual Dimorphism in the Effects of Exercise on Metabolism of Lipids to Support Resting Metabolism. Front Endocrinol ;5
Marta CC, Marinho DA, Barbosa TM, Carneiro AL, Izquierdo M, Marques MC (2013) Effects of body fat and dominant somatotype on explosive strength and aerobic capacity trainability in prepubescent children. J Strength Cond Res 27:3233–3244
Handelsman DJ, Hirschberg AL, Bermon S (2018) Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance. Endocr Rev 39:803–829
Baron RM, Kenny DA (1986) The moderator-mediator variable distinction in social psychological research: conceptual, strategic, and statistical considerations. J Pers Soc Psychol 51:1173–1182

No competing interests reported.

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Mediating Role of Echo-Intensity in Sex Differences of Physical Fitness among Children

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1. Introduction

2. Methods

2.1 Study design and population

2.2 Measurements

2.3 Statistical analysis

3. Results

4. Discussion

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