Exercise, hormones and skeletal adaptations during childhood and adolescence

The Effect of Sex, Maturity, and Training Status on Maximal Sprint Performance Kinetics

PURPOSE

The development of sprint running during youth has received renewed interest, but questions remain regarding the development of speed in youth, especially the influences of sex, training, and maturity status.

METHODS

One hundred and forty-seven team sport trained (69 girls; 14.3 [2.1] y) and 113 untrained (64 girls; 13.8 [2.7] y) youth completed two 30-m sprints separated by 2-minute active rest. Velocity was measured using a radar gun at >46 Hz, with power and force variables derived from a force-velocity-power profile.

RESULTS

Boys produced a significantly higher absolute peak power (741 [272] vs 645 [229] W; P < .01) and force (431 [124] vs 398 [125] N; P < .01) than girls, irrespective of maturity and training status. However, there was a greater sex difference in relative mean power and peak velocity in circa peak height velocity adolescents (46.9% and 19.8%, respectively) compared with prepeak height velocity (5.4% and 3.2%) or postpeak height velocity youth (11.6% and 5.6%).

CONCLUSIONS

Sprint development in youth is sexually dimorphic which needs considering when devising long-term training plans. Further research is needed to explore the independent, and combined, effects of sex, training, and maturity status on sprint performance kinetics in youth.

Evaluating and optimizing bone health in children with chronic health conditions

Paediatric health care providers (HCPs) play an important role in optimizing bone health. Early intervention is essential to maximize the accrual of peak bone mass in adolescence and young adulthood and to reduce osteoporosis and fracture risk later in life. Children and adolescents with chronic health conditions may have several risk factors for poor bone health, including underlying inflammatory conditions, reduced weight-bearing activity, delayed puberty, and inadequate intake of calcium and vitamin D. Some medications-particularly glucocorticoids-can compromise bone mass and place a child at risk for fragility fractures. This practice point describes a targeted approach to identifying bone health risk factors in children and youth with chronic health conditions, highlights office initiatives aimed at optimizing bone mass accrual, and links HCPs to useful web-based tools and medical references. Indications for referral to a bone health specialist and bone-specific pharmacotherapeutic interventions are also reviewed.

L'évaluation et l'optimisation de la santé osseuse chez les enfants ayant des affections chroniques

Les professionnels de la santé des enfants jouent un rôle important dans l'optimisation de la santé osseuse. Il est essentiel d'intervenir rapidement pour maximiser le pic de masse osseuse à l'adolescence et au début de l'âge adulte et d'ainsi réduire le risque d'ostéoporose et de fractures plus tard dans la vie. Les enfants et les adolescents ayant une affection chronique peuvent présenter plusieurs facteurs de risque de mauvaise santé osseuse, notamment des maladies inflammatoires sous-jacentes, des activités avec mise en charge limitées, un retard pubertaire et un apport insuffisant de calcium et de vitamine D. Certains médicaments, et particulièrement les glucocorticoïdes, peuvent compromettre la masse osseuse et exposer l'enfant à un risque de fractures de fragilisation. Le présent point de pratique décrit une approche ciblée pour déterminer les facteurs de risque liés à la santé osseuse chez les enfants et les adolescents ayant une affection chronique, expose les mesures à prendre en cabinet pour optimiser l'acquisition de la masse osseuse et propose des outils en ligne utiles et des références médicales à l'intention des professionnels de la santé des enfants. Les indications pour diriger les patients vers un spécialiste de la santé osseuse et pour procéder à des interventions pharmacologiques visant les os sont également abordées.

Influence of Maturity Status on Kinanthropometric and Physical Fitness Variables in Adolescent Female Volleyball Players

The aim of this research was to analyze differences in kinanthropometric characteristics and physical performance in relation to maturity status, as well as to determine if age, maturity offset or kinanthropometric variables could predict better performance in physical fitness tests. A total of 152 female volleyball players (14.16 ± 1.25 years old) underwent a kinanthropometric assessment, followed by a physical fitness assessment composed of different tests. The age at peak height velocity (APHV) was calculated, and the sample was divided according to biological maturation into three groups. Significant differences were observed in kinanthropometric variables (p < 0.001–0.026), with early maturers showing higher values. Age, body mass, Cormic index, relative arm span, ∑8 skinfolds, fat mass, corrected arm and thigh girths, muscle mass and biacromial and biiliocristal breadths were the variables that best predicted performance in the physical tests (p < 0.001–0.024). The more mature players showed higher values in most of the kinanthropometric variables, with the more remarkable differences being in body mass, height, arm span and sitting height, and those related to adiposity and absolute body composition, and with structural variables being the most influential on the physical tests. Age had a determinant influence on the differences found between groups in strength and power-related test performance.

Associations between match participation, maturation, physical fitness, and hormonal levels in elite male soccer player U15: a prospective study with observational cohort

OBJECTIVES

The aims of this study were to analyze the relationships between minutes of play (MP) and maturity status, fitness, and hormonal levels and to explain how those measures influence the time of play.

METHODS

Twenty-six youth soccer players U15 participated in this study over a full-season period. Anthropometric measures, maturity status, growth hormone (GH), insulin-like growth factor and physical levels such as maximal oxygen uptake (VO2max), fatigue index, countermovement jump (CMJ) performance were collected. At the end-season, players were assessed in 6 different tests over four days.

RESULTS

VO2max largely correlated with GH (r = 0.57) and CMJ (r = 0.51). Also, GH largely correlated with CMJ (r = 0.55). MP had moderate correlations with VO2max (r = 0.44) and CMJ (r = 0.42). Multiple linear regression with maturation, physical fitness and hormonal levels explained R2 of 0.62 of the MP (F (8, 17) = 3.47, p = 0.015). Although each independent variable alone was not able to determine the playing time, when using the interactions, the model significantly explained the MP.

CONCLUSIONS

The combination of maturity status, physical fitness, and hormonal levels seem to play a determinant role in explaining the match participation in youth soccer players.

Musculoskeletal Changes Across the Lifespan: Nutrition and the Life-Course Approach to Prevention

Aging is characterized by the progressive decline of muscle mass and function, the so-called sarcopenia. Also bone loss is widespread among older people. Sarcopenia and osteopenia/osteoporosis are associated with several adverse outcomes including falls, risk of fractures, functional decline, frailty, and mortality. Recently, the life-course approach to prevent or delay functional decline has become very popular. Regarding musculoskeletal health, there is suggestive evidence that acting during critical or sensitive periods of life in which each person build-up its biological reserves may influence the rate of functional decline in the later stages of life. A life-course approach to musculoskeletal health should take place during early life when plasticity allows more easily the attainment of the peak of the musculoskeletal system driven by environmental stimuli. The rate of the subsequent decline will depend on the peak previously reached. Nutrition and physical exercise are important environmental factors that can influence musculoskeletal development by favoring and maintaining peak bone and muscle mass and strength. Here we provide an overview of body composition changes occurring across the lifespan and strategies based on nutrition and physical exercise to support musculoskeletal health as well as minimizing losses during older life.

Relationship of Anterior Cruciate Ligament Volume and T2* Relaxation Time to Anterior Knee Laxity

Background:

High anterior knee laxity (AKL) has been prospectively identified as a risk factor for anterior cruciate ligament (ACL) injuries. Given that ACL morphometry and structural composition have the potential to influence ligamentous strength, understanding how these factors are associated with greater AKL is warranted.

Hypothesis:

Smaller ACL volumes combined with longer T2* relaxation times would collectively predict greater AKL.

Study Design:

Cross-sectional study; Level of evidence, 3.

Methods:

College-aged active male (n = 20) and female (n = 30) participants underwent magnetic resonance imaging (MRI) and AKL testing. T2-weighted MRI scans were used to assess ACL volumes, and T2* relaxation times were used to assess ACL structural composition. AKL was measured via a commercial knee arthrometer. Forward stepwise linear regression with sex and weight (first step; suppressor variables) as well as ACL volume and T2* relaxation time (second step; independent variables) was used to predict AKL (dependent variable).

Results:

After initially adjusting for sex and weight (R² = 0.19; P = .006), smaller ACL volumes combined with longer T2* relaxation times collectively predicted greater AKL (R² = 0.52; P < .001; R²_Δ = 0.32; P_Δ < .001). A smaller ACL volume was the primary predictor of greater AKL (R²_Δ = 0.28; P < .001), with a longer T2* relaxation time trending toward a significant contribution to greater AKL (R²_Δ = 0.04; P = .062). After adjusting for ACL volume and T2* relaxation time, sex (partial r = 0.05; P = .735) and weight (partial r = 0.05; P = .725) were no longer significant predictors.

Conclusion:

AKL was largely predicted by ACL volume and to a lesser extent by T2* relaxation time (and not a person’s sex and weight). These findings enhance our understanding of how AKL may be associated with a structurally weaker ACL. The current study presents initial evidence that AKL is a cost-effective and clinically accessible measure that shows us something about the structural composition of the ACL. As AKL has been consistently shown to be a risk factor for ACL injuries, work should be done to continue to investigate what AKL may tell a clinician about the structure and composition of the ACL.

Celiac Disease and Bone Health in Children and Adolescents: A Systematic Review and Meta-Analysis

CONTEXT

Celiac disease is characterized by deficits in bone mineral accrual and longitudinal growth.

OBJECTIVE

The purpose of this study was to determine the differences in bone health and stature among children and adolescents with celiac disease versus healthy controls.

DATA SOURCES

Articles published before February 27, 2018 were located using searches of the Physical Education Index (n = 186), PubMed (n = 180), Scopus (n = 3), SPORTDiscus (n = 3), and Web of Science (n = 4).

STUDY SELECTION

Bone mineral content (BMC) and areal bone mineral density (aBMD) were assessed via dual-energy X-ray absorptiometry, and height was measured using a stadiometer.

DATA EXTRACTION

Effect sizes (ES) were calculated as follows: the mean difference of the celiac disease group and healthy control group, divided by the pooled standard deviation. The inverse variance weight was used to calculate the overall mean ES. Random-effects models were used to aggregate a mean ES, 95% confidence intervals (CIs) and to identify potential moderators.

RESULTS

The results of 30 effects gathered from 12 studies published between 1996 and 2017 indicated BMC (ES = -0.54, 95% CI: -0.69 to -0.40; p < 0.0001) and aBMD (ES = 0.72, 95% CI: -0.96 to -0.47; p < 0.0001) were lower in youth with celiac disease.

LIMITATIONS

These results were limited to only cross-sectional and baseline data from longitudinal studies reporting BMC and BMD, however did not assess changes in bone health over time.

CONCLUSION

Children and adolescents with celiac disease have suboptimal bone health and shorter stature.

Sex-Specific Muscular Mediation of the Relationship Between Physical Activity and Cortical Bone in Young Adults

Muscle mass is a commonly cited mediator of the relationship between physical activity (PA) and bone, representing the mechanical forces generated during PA. However, neuromuscular properties (eg, peak force) also account for unique portions of variance in skeletal outcomes. We used serial multiple mediation to explore the intermediary role of muscle mass and force in the relationships between cortical bone and moderate-to-vigorous intensity PA (MVPA). In a cross-sectional sample of young adults (n = 147, 19.7 ± 0.7 years old, 52.4% female) cortical diaphyseal bone was assessed via peripheral quantitative computed tomography at the mid-tibia. Peak isokinetic torque in knee extension was assessed via Biodex dynamometer. Thigh fat-free soft tissue (FFST) mass, assessed via dual-energy X-ray absorptiometry, represented the muscular aspect of tibial mechanical forces. Habitual MVPA was assessed objectively over 7 days using Actigraph GT3X+ accelerometers. Participants exceeded MVPA guidelines (89.14 ± 27.29 min/day), with males performing 44.5% more vigorous-intensity activity relative to females (p < 0.05). Males had greater knee extension torque and thigh FFST mass compared to females (55.3%, and 34.2%, respectively, all p < 0.05). In combined-sex models, controlling for tibia length and age, MVPA was associated with strength strain index (pSSI) through two indirect pathways: (i) thigh FFST mass (b = 1.11 ± 0.37; 95% CI, 0.47 to 1.93), and (i) thigh FFST mass and knee extensor torque in sequence (b = 0.30 ± 0.16; 95% CI, 0.09 to 0.73). However, in sex-specific models MVPA was associated with pSSI indirectly through its relationship with knee extensor torque in males (b = 0.78 ± 0.48; 95% CI, 0.04 to 2.02) and thigh FFST mass in females (b = 1.12 ± 0.50; 95% CI, 0.37 to 2.46). Bootstrapped CIs confirmed these mediation pathways. The relationship between MVPA and cortical structure appears to be mediated by muscle in young adults, with potential sex-differences in the muscular pathway. If confirmed, these findings may highlight novel avenues for the promotion of bone strength in young adults. © 2019 American Society for Bone and Mineral Research.

The effect of constant-intensity endurance training and high-intensity interval training on aerobic and anaerobic parameters in youth

INTRODUCTION

High-Intensity Interval Training (HIIT) and Constant-Intensity Endurance Training (CIET) improves peak oxygen uptake (V̇O₂) similarly in adults; but in children this remains unclear, as does the influence of maturity.

METHODS

Thirty-seven boys formed three groups: HIIT (football; n = 14; 14.3 ± 3.1 years), CIET (distance runners; n = 12; 13.1 ± 2.5 years) and a control (CON) group (n = 11; 13.7 ± 3.2 years). Peak V̇O₂ and gas exchange threshold (GET) were determined from a ramp test and anaerobic performance using a 30 m sprint pre-and-post a three-month training cycle.

RESULTS

The HIIT groups peak V̇O₂ was significantly higher than the CON group pre (peak V̇O₂: 2.54 ± 0.63 l·min^-1 vs 2.03 ± 0.53 l·min^-1, d = 0.88; GET: 1.41 ± 0.26 l·min^-1 vs 1.13 ± 0.29 l·min^-1, d = 1.02) and post-training (peak V̇O₂: 2.63 ± 0.73 l·min^-1 vs 2.08 ± 0.64 l·min^-1, d = 0.80; GET: 1.32 ± 0.33 l·min^-1 vs 1.15 ± 0.38 l·min^-1, d = 0.48). All groups showed a similar magnitude of change during the training (p > 0.05).

CONCLUSION

HIIT was not superior to CIET for improving aerobic or anaerobic parameters in adolescents. Secondly, pre- and post-pubertal participants demonstrated similar trainability.

Impact of HIV-1 Infection and Antiretroviral Therapy on Bone Homeostasis and Mineral Density in Vertically Infected Patients

Daily assumption of antiretroviral drugs and HIV-related immune activation lead to important side effects, which are particularly evident in vertically infected patients. Bone homeostasis impairment and reduction of bone mineral density (BMD) is one of the most important side effects. Primary aim of this study is to assess the prevalence of bone homeostasis alterations in a group of vertically infected patients; secondary aim is to analyze the relationship between bone homeostasis alterations and anthropometric data, severity of HIV infection, and antiretroviral therapy. We studied 67 patients with vertically transmitted HIV-1 (aged 6-31 years), followed by the Pediatric Infectious Disease Unit of the University Hospital of Padua, Italy. We analyzed bone turnover markers (P1NP and CTx) and we performed lumbar spine and femoral dual energy X-ray absorption densitometry (DXA). Personal and anthropometric data and information on HIV-infection severity and antiretroviral therapy were collected for all patients. We found that BMD values recorded by DXA showed a significant correlation with age, race, BMI, physical activity, and antiretroviral therapy duration. P1NP was increased in 43% of patients, while CTX in 61% of them. P1NP alteration was related to age, race, BMI, physical activity, therapy duration, and ever use of protease inhibitors and nucleotide reverse transcriptase inhibitors. CTX alteration was found to be correlated only with age. In conclusion, our study confirms that a wide percentage of HIV vertically infected patients show reduced BMD and impaired bone homeostasis. Strict monitoring is needed in order to early identify and treat these conditions.

The Effects of Light and Moderate Intensity Exercise on the Femoral Bone and Cerebellum of d-Galactose-Exposed Rats

Aging causes the degeneration of organs of the locomotor system, including the cerebellum and bones. Exercise may reverse this deterioration. d-galactose has been frequently used in rodents to accelerate aging. The present study aimed at investigating the effects of exercise on cerebellar and serum growth factors, motor activity, and the number of bone cells of the femoral head of d-galactose-treated rats. Twenty-four male Wistar rats were divided randomly into four groups, that is, three treated groups injected with 300 mg/(mL·kg) body weight (bw) d-galactose solution daily for 4 weeks, and a control group injected with normal saline. Following the 4-week administration of d-galactose solution, two of the treated groups performed light- (45% VO₂max) and moderate- (55% VO₂max) intensity exercise, by running on a treadmill 4 × a week for 4 weeks. Locomotor activity was examined in rotarod and open field tests. The cerebellar and serum Insulin-like Growth Factor 1 (IGF-1) and Brain-Derived Neurotrophic Factor (BDNF) levels were measured using enzyme-linked immunosorbent assay (ELISA). The number of osteoblasts and osteoclasts of femoral head was estimated using unbiased stereological methods. It was found that the number of osteoclasts was higher in the d-galactose-treated group than the normal control and moderate-intensity exercise groups. No significant difference between groups was found in the rotarod and open field test performance, IGF-1 and BDNF levels, as well as number of osteoblasts. In conclusion, a 4-week administration of high-dosed-galactose caused the increase of the number of osteoclasts. A subsequent 4-week moderate-intensity exercise reversed this increase to the normal level.

Taekwondo Training Improves Mood and Sociability in Children from Multicultural Families in South Korea: A Randomized Controlled Pilot Study

Purpose: Children from multicultural families face physical, social, mental, and intellectual hurdles; however, relative interventions are lacking in South Korea (hereafter Korea) in this regard. The purpose of this study was to investigate the effects of regular Taekwondo (TKD) training on physical fitness, mood, sociability, and cognitive functions in these children. Methods: This study included 30 children from multicultural families in Korea who were randomly assigned to a TKD group (n = 15) and control group (n = 15). The children in TKD group underwent 16 weeks of TKD training once a week for 60 min. Each participant underwent a basic fitness test and sociability questionnaire before and after the intervention. Furthermore, we examined the changes in the mood and cognitive function by determining the profile of mood states (POMS), and Stroop color and word test, respectively. Results: Results of the Stork test of balance were significantly higher in the TKD group after intervention (p < 0.05). In terms of sub-variables, POMS, tension, and depression scores were significantly lower (p < 0.05) after the intervention, while the vigor score was significantly higher in the intervention group than those in the control group (p < 0.05). Furthermore, sociability and ‘being left out’ score, a sub-variable of sociability, was significantly lower (p < 0.05) after the intervention, while sociability score was significantly higher (p < 0.05). Conclusions: Our findings suggest that participation in regular TKD training can be effective for balanced improvements in variables of basic fitness and that it exerts a positive effect on the mood and development of sociability.

Exercise for Bone in Childhood-Hitting the Sweet Spot

PURPOSE

The goal of the current work is to challenge the enduring notion that prepuberty is the optimum timing for maximum bone response to exercise in childhood and to present the evidence that early puberty is a more potently receptive period.

METHOD

The relevant literature is reviewed and the causes of the misconception are addressed in detail.

RESULTS

Contrary to prevailing opinion, ample evidence exists to suggest that the peripubertal years represent the developmental period during which bone is likely to respond most robustly to exercise intervention.

CONCLUSION

Public health initiatives that target bone-specific exercise interventions during the pubertal years are likely to be the most effective strategy to harness the increased receptiveness of the growing skeleton to mechanical loading.

A shared biomechanical environment for bone and posture development in children

BACKGROUND CONTEXT

In each specific habitual standing posture, gravitational forces determine the mechanical setting provided to skeletal structures. Bone quality and resistance to physical stress is highly determined by habitual mechanical stimulation. However, the relationship between bone properties and sagittal posture has never been studied in children.

PURPOSE

This study aimed to investigate the association between bone physical properties and sagittal standing postural patterns in 7-year-old children. We also analyzed the relationship between fat or fat-free mass and postural patterns.

STUDY DESIGN

Cross-sectional evaluation.

PATIENT SAMPLE

This study was performed in a sample of 1,138 girls and 1,260 boys at 7 years of age participating in the Generation XXI study, a population-based cohort of children followed since birth (2005-2006) and recruited in Porto, Portugal.

OUTCOME MEASURES

Sagittal standing posture was measured through photographs of the sagittal right view of children in the standing position. Three angles were considered to quantify the magnitude of major curves of the spine and an overall balance measure (trunk, lumbar, and sway angles). Postural patterns were identified using latent profile analysis in Mplus.

METHODS

Weight and height were measured. Total body less head fat or fat-free mass and bone properties were estimated from whole-body dual-energy X-ray absorptiometry scans. The associations of fat or fat-free mass and bone physical properties with postural patterns were jointly estimated in latent profile analysis using multinomial logistic regressions.

RESULTS

The identified patterns were labeled as Sway, Flat, and "Neutral to Hyperlordotic" (in girls), and "Sway to Neutral," Flat, and Hyperlordotic (in boys). In both genders, children in the Flat pattern showed the lowest body mass index, and children with a rounded posture presented the highest: mean differences varying from -0.86 kg/m² to 0.60 kg/m² in girls and from -0.70 kg/m² to 0.62 kg/m² in boys (vs. Sway or "Sway to Neutral"). Fat and fat-free mass were inversely associated with a Flat pattern and positively associated with a rounded posture: odds ratio (OR) of 0.23 per standard deviation (SD) fat and 0.70 per SD fat-free mass for the Flat pattern, and 1.85 (fat) and 1.43 (fat-free) for the Hyperlordotic pattern in boys, with similar findings in girls. The same direction of relationships was observed between bone physical properties and postural patterns. A positive association between bone (especially bone mineral density) and a rounded posture was robust to adjustment for age, height, and body composition (girls: OR=1.79, p=.006 fat-adjusted, OR=2.00, p=.014 fat-free mass adjusted; boys: OR=2.02, p=.002 fat-adjusted, OR=2.42, p<.001 fat-free mass adjusted).

CONCLUSIONS

In this population-based pediatric setting, there was an inverse association between bone physical properties and a Flat posture. Bone and posture were more strongly positively linked in a rounded posture. Our results support that both bone properties and posture mature in a shared and interrelated mechanical environment, probably modulated by pattern-specific anthropometrics and body composition.

Predictors of Bone Mineral Density among Asian Indians in Northern Mississippi: A Pilot Study

BACKGROUND

Osteoporosis is a systemic skeletal disorder characterized by low bone mineral density (BMD) that leads to an increase in bone fragility, causing an individual to be at an increased risk for fractures. Asian-Indians are at an increased risk for developing osteoporosis. Considering the number of Asian-Indians in the US is rapidly growing, they likely could be an underappreciated population at risk for bone fractures. The aim of this study was to investigate bone health and determine the factors affecting BMD in Asian-Indians living in the US.

METHODS

Asian-Indians residing in Northern Mississippi (n = 87) were enrolled in this cross-sectional study from June 2013 to August 2014. Eligible participants completed a self-administered Osteoporosis Risk Factor Assessment questionnaire. BMD and body composition were measured using a dual energy x-ray absorptiometry scan (DXA).

RESULTS

Eight-seven Asian-Indians (male: 62.1%) participated, with the average age being 28.49 yr old (SD = 6.62). Overall, 31.0% and 48.3% had low femoral neck BMD and spinal BMD, respectively. Multiple regression analysis revealed that age, percent body fat, and body mass index (BMI) significantly predicted BMD at femur neck (P<0.05). Additionally, percent body fat, BMI, childhood milk consumption, and gender were statistically significant predictors of spinal BMD (P<0.05).

CONCLUSIONS

The findings from this study should be beneficial to healthcare providers that work with Asian-Indian population groups. Health promotion programs focusing on osteoporosis prevention are needed among Asian-Indians to prevent the risk of fractures.

Bone Mass and Strength in School-Age Children Exhibit Sexual Dimorphism Related to Differences in Lean Mass: The Generation R Study

Bone strength, a key determinant of fracture risk, has been shown to display clear sexual dimorphism after puberty. We sought to determine whether sex differences in bone mass and hip bone geometry as an index of strength exist in school-age prepubertal children and the degree to which the differences are independent of body size and lean mass. We studied 3514 children whose whole-body and hip scans were measured using the same densitometer (GE-Lunar iDXA) at a mean age of 6.2 years. Hip dual-energy X-ray absorptiometry (DXA) scans underwent hip structural analyses (HSA) with derivation of bone strength indices. Sex differences in these parameters were assessed by regression models adjusted for age, height, ethnicity, weight, and lean mass fraction (LMF). Whole-body bone mineral density (BMD) and bone mineral content (BMC) levels were 1.3% and 4.3% higher in girls after adjustment by LMF. Independent of LMF, boys had 1.5% shorter femurs, 1.9% and 2.2% narrower shaft and femoral neck with 1.6% to 3.4% thicker cortices than girls. Consequent with this geometry configuration, girls observed 6.6% higher stresses in the medial femoral neck than boys. When considering LMF, the sexual differences on the derived bone strength indices were attenuated, suggesting that differences in muscle loads may reflect an innate disadvantage in bone strength in girls, as consequence of their lower muscular acquisition. In summary, we show that bone sexual dimorphism is already present at 6 years of age, with boys having stronger bones than girls, the relation of which is influenced by body composition and likely attributable to differential adaptation to mechanical loading. Our results support the view that early life interventions (ie, increased physical activity) targeted during the pre- and peripubertal stages may be of high importance, particularly in girls, because before puberty onset, muscle mass is strongly associated with bone density and geometry in children. © 2015 American Society for Bone and Mineral Research.