The effect of mechanical loading on the size and shape of bone in pre-, peri-, and postpubertal girls: a study in tennis players

R-Spondin 1 promotes vibration-induced bone formation in mouse models of osteoporosis

Bone tissue adapts to its functional environment by optimizing its morphology for mechanical demand. Among the mechanosensitive cells that recognize and respond to forces in the skeleton are osteocytes, osteoblasts, and mesenchymal progenitor cells (MPCs). Therefore, the ability to use mechanical signals to improve bone health through exercise and devices that deliver mechanical signals is an attractive approach to age-related bone loss; however, the extracellular or circulating mediators of such signals are largely unknown. Using SDS-PAGE separation of proteins secreted by MPCs in response to low-magnitude mechanical signals and in-gel trypsin digestion followed by HPLC and mass spectroscopy, we identified secreted proteins up-regulated by vibratory stimulation. We exploited a cell senescence-associated secretory phenotype screen and reasoned that a subset of vibration-induced proteins with diminished secretion by senescent MPCs will have the capacity to promote bone formation in vivo. We identified one such vibration-induced bone-enhancing (vibe) gene as R-spondin 1, a Wnt pathway modulator, and demonstrated that it has the capacity to promote bone formation in three mouse models of age-related bone loss. By virtue of their secretory status, some vibe proteins may be candidates for pre-clinical development as anabolic agents for the treatment of osteoporosis.

KEY MESSAGE

Mesenchymal stem cells respond to low magnitude mechanical signals (vibration). R-Spondin 1 is upregulated by mechanical signals and secreted. R-Spondin 1 promotes bone formation in three mouse models of osteoporosis.

The impact on children's bone health of a school-based physical education program and participation in leisure time sports: the Childhood Health, Activity and Motor Performance School (the CHAMPS) study, Denmark

OBJECTIVE

To evaluate the effect of a school based physical education (PE) program and the amount of leisure time sport (LTS) on children's bone health and to examine if LTS influences the impact of school type on children's bone health.

METHODS

Children attending "sports" schools (6 × 45 min PE lessons per week) were compared to children at "traditional" schools (2 × 45 min of PE lessons per week) in Svendborg, Denmark. Whole-body DXA scans were performed at baseline (2008) and at a two-year follow-up (2010). Bone mineral content (BMC), bone mineral density (BMD), and bone area (BA) were measured. Multilevel regression analyses examined the impact of school type and LTS participation on bone.

RESULTS

742/800 (93%) invited children accepted to participate. 682/742 (92%) participated at two-year follow-up. Mean (SD) age was 9.5 years (0.9) at baseline. A positive association between LTS and BMC, BMD (p<0.001) and for BA (p<0.05) (total body less head (TBLH) and lower limb (LL)) was found. All effects regarding school type were insignificant.

CONCLUSION

A positive impact of attending LTS on bone traits was found. There was no effect on BMC, BMD and BA (TBLH, and LL) for children attending sports schools compared to traditional schools.

Osteocyte-derived insulin-like growth factor I is essential for determining bone mechanosensitivity

This study sought to determine whether deficient Igf1 expression in osteocytes would affect loading-induced osteogenic response. Tibias of osteocyte Igf1 conditional knockout (KO) mice (generated by cross-breeding Igf1 floxed mice with Dmp1-Cre transgenic mice) and wild-type (WT) littermates were subjected to four-point bending for 2 wk. Microcomputed tomography confirmed that the size of tibias of conditional mutants was smaller. Loading with an equivalent loading strain increased periosteal woven bone and endosteal lamellar bone formation in WT mice but not in conditional KO mice. Consistent with the lack of an osteogenic response, the loading failed to upregulate expression of early mechanoresponsive genes (Igf1, Cox-2, c-fos) or osteogenic genes (Cbfa-1, and osteocalcin) in conditional KO bones. The lack of osteogenic response was not due to reduced osteocyte density or insufficient loading strain. Deficient osteocyte Igf1 expression reduced the loading-induced upregulation of expression of canonical Wnt signaling genes (Wnt10b, Lrp5, Dkk1, sFrp2). The loading also reduced (by 40%) Sost expression in WT mice, but the loading not only did not reduce but upregulated (~1.5-fold) Sost expression in conditional KO mice. Conditional disruption of Igf1 in osteocytes also abolished the loading-induced increase in the bone β-catenin protein level. These findings suggest an impaired response in the loading-induced upregulation of the Wnt signaling in conditional KO mice. In summary, conditional disruption of Igf1 in osteocytes abolished the loading-induced activation of the Wnt signaling and the corresponding osteogenic response. In conclusion, osteocyte-derived IGF-I plays a key determining role in bone mechanosensitivity.

Site and sex effects on tibia structure in distance runners and untrained people

PURPOSE

The purpose was to study the relationship between mechanical environment and bone structure by comparing the tibia in people with different physical activities.

MATERIALS AND METHODS

Indicators of bone mass (bone mineral content), bone material "quality" (cortical volumetric mineral density (vBMD)), and diaphyseal design (endocortical and periosteal perimeters (EcPm and PoPm, respectively), cortical thickness (CtTh), circularity, and bending and torsion cross-sectional moments of inertia (CSMIs)) were determined in serial peripheral quantitative computed tomography scans taken at 5% steps of the tibia in 40 voluntary men and women age 25-40 yr who were either physically inactive or experienced distance runners (n = 10-12 per group).

RESULTS

Bone mass and design indicators were higher in runner than in nonrunner men, with a variable effect size along the tibia. In the distal tibia, runners had enhanced bone mineral content and CtTh (resistance to compression), but EcPm, PoPm, circularity, and CSMI were unaffected. In the midshaft, CSMIs (resistance to bending/torsion) were enhanced in runners, whereas bone mass was unaffected. In the proximal third, effects were observed for CtTh, EcPm, and PoPm. In female runners, these benefits were restricted to CSMIs only. Cortical vBMD, naturally lower in men than in women, was reduced in runners of either sex.

DISCUSSION

Results are coherent with previous findings in physically inactive people and with Frost's mechanostat theory. The observed group differences in cortical vBMD could reflect an increase in intracortical porosity (enhanced remodeling for damage repair), eventually compensated biomechanically by CSMI improvements. The sex specificity of exercise effects may suggest the interference by the endocrine environment. Results confirm that the mechanical environment is a strong determinant of regional tibia structure and suggest that the endocrine environment may reduce the effects of physical interventions on bone health in fertile women.

Bone mass following physical activity in young years: a mean 39-year prospective controlled study in men

This is a study on exercise-associated bone mineral density (BMD) which in men is maintained three decades after cessation of sports. In this prospective controlled cohort study active athletes had a BMD Z-score of 1.0 and after 39 years 0.5 to 1.2 depending on the measured region), using the same single-photon absorptiometry device, dual X-ray absorptiometry (DXA), and peripheral computed tomography (pQCT).

INTRODUCTION

The aims of this study were to prospectively evaluate BMD changes in male athletes from activity into long-term retirement and to simultaneously evaluate other bone traits.

METHODS

Bone mineral density (grams per square centimeter) was measured in 46 male athletes with a mean age of 22 years (range, 15-40) by using the same single-photon absorptiometry device, both at active career and a mean of 39 years (range, 38-40) later when they had long-term retired. At follow-up, BMD was also evaluated by DXA and pQCT. Twenty-four non-athletic males of similar age served as controls. Between-group differences are presented as means with 95% confidence intervals.

RESULTS

The active athletes (baseline) had a BMD Z-score of 1.0 (0.7, 1.4) in the femoral condyles. The retired athletes (follow-up) had a BMD Z-score of 0.5 to 1.2 depending on the measuring technique and the measured region. The tibial cortical area Z-score at follow-up was 0.8 (0.5, 1.2) and the tibial strength index Z-score 0.7 (0.4, 1.0). There were no changes in BMD Z-scores from activity to retirement, neither when estimated by the same device in different regions [∆ Z-score -0.3 (-0.8, 0.2)] nor in the same region with different devices [∆ Z-score 0.0 (-0.4, 0.4)]. The benefits remained after adjustments for anthropometrics and lifestyle. No correlation was seen with years since retirement.

CONCLUSIONS

Exercise-associated high BMD in young years seems, in men, to be maintained three decades after cessation of high-level physical activity.

The prevalence of disordered eating and possible health consequences in adolescent female tennis players from Rio de Janeiro, Brazil

The aim of this study was to estimate the prevalence of disordered eating and possible health consequences in adolescent female tennis players. This cross-sectional controlled study investigated the pubertal development (Tanner stages); body composition (dual energy X-ray absorptiometry-DXA); dietary intake (food record); presence of disordered eating (EAT-26, BITE and BSQ); menstrual status (questionnaire) and bone mineral density (DXA). The Female Athlete Triad (FAT) was divided into two severity stages. The study included 45 adolescents (24 athletes and 21 controls) at some pubertal developmental stage. The athletes exhibited better body composition profiles. We found that 91.7%, 33.3% and 25% of athletes and 71.4%, 9.5% and 33.3% of controls met criteria for disordered eating and/or low energy availability, menstrual irregularities and low bone mass, respectively. A greater percentage of athletes than controls presented with 1 and 2 FAT components (stage I), and 4.2% presented with the full syndrome. In conclusion, tennis players appear to present with more severe disorders than controls and should be monitored to avoid damage to their performance and health.

Femoral neck cross-sectional geometry and exercise loading

The aim of this study was to examine the association between different types of exercise loading and femoral neck cross-sectional geometry. Our data comprised proximal femur magnetic resonance (MR) images obtained from 91 female athletes and their 20 age-matched controls. The athletes were categorized according to typical training activity - high impact (high and triple jumping), odd impact (racket and soccer playing), high magnitude (power lifting), repetitive low impact (endurance running) and repetitive non-impact (swimming). Segmented MR images at two locations, narrowest cross-section of the femoral neck (narrowFN) and the cross-section at insertion of articular capsule (distalFN), were investigated to detect between group differences in shape, curvature and buckling ratio derived using image and signal analysis tools. The narrowFN results indicated that the high-impact group had weaker antero-superior (33% larger buckling ratio than controls) but stronger inferior weight-bearing region (32% smaller than controls), while the odd-impact group had stronger superior, posterior and anterior region (21% smaller buckling ratio than controls). The distalFN results indicated that the high-impact group had stronger inferior region (37% smaller buckling ratio), but the odd-impact group had stronger superior region (22% smaller buckling ratio) than the controls. Overall, the results point towards odd-impact exercise loading, with inherently varying directions of impact, associated with more robust cross-sectional geometry along the femoral neck. In conclusion, our one-dimensional polar treatment for geometrical traits and intuitive presentation of differences in trends between exercise groups and controls provides a basis for analysis with high angular accuracy.

Sex specific association of physical activity on proximal femur BMD in 9 to 10 year-old children

The results of physical activity (PA) intervention studies suggest that adaptation to mechanical loading at the femoral neck (FN) is weaker in girls than in boys. Less is known about gender differences associated with non-targeted PA levels at the FN or other clinically relevant regions of the proximal femur. Understanding sex-specific relationships between proximal femur sensitivity and mechanical loading during non-targeted PA is critical to planning appropriate public health interventions. We examined sex-specific associations between non-target PA and bone mineral density (BMD) of three sub-regions of the proximal femur in pre- and early-pubertal boys and girls. BMD at the FN, trochanter (TR) and intertrochanter (IT) regions, and lean mass of the whole body were assessed using dual-energy x-ray absorptiometry in 161 girls (age: 9.7±0.3 yrs) and 164 boys (age: 9.7±0.3 yrs). PA was measured using accelerometry. Multiple linear regression analyses (adjusted for body height, total lean mass and pubertal status) revealed that vigorous PA explained 3–5% of the variability in BMD at all three sub-regions in boys. In girls, vigorous PA explained 4% of the variability in IT BMD and 6% in TR BMD. PA did not contribute to the variance in FN BMD in girls. An additional 10 minutes per day of vigorous PA would be expected to result in a ∼1% higher FN, TR, and IT BMD in boys (p<0.05) and a ∼2% higher IT and TR BMD in girls. In conclusion, vigorous PA can be expected to contribute positively to bone health outcomes for boys and girls. However, the association of vigorous PA to sub-regions of the proximal femur varies by sex, such that girlś associations are heterogeneous and the lowest at the FN, but stronger at the TR and the IT, when compared to boys.

Body size and pubertal development explain ethnic differences in structural geometry at the femur in Asian, Hispanic, and white early adolescent girls living in the U.S

Variation in structural geometry is present in adulthood, but when this variation arises and what influences this variation prior to adulthood remains poorly understood. Ethnicity is commonly the focus of research of skeletal integrity and appears to explain some of the variation in quantification of bone tissue. However, why ethnicity explains variation in skeletal integrity is unclear.

METHODS

Here we examine predictors of bone cross sectional area (CSA) and section modulus (Z), measured using dual-energy X-ray absorptiometry (DXA) and the Advanced Hip Analysis (AHA) program at the narrow neck of the femur in adolescent (9-14 years) girls (n=479) living in the United States who were classified as Asian, Hispanic, or white if the subject was 75% of a given group based on parental reported ethnicity. Protocols for measuring height and weight follow standardized procedures. Total body lean mass (LM) and total body fat mass (FM) were quantified in kilograms using DXA. Total dietary and total dairy calcium intakes from the previous month were estimated by the use of an electronic semi-quantitative food frequency questionnaire (eFFQ). Physical activity was estimated for the previous year by a validated self-administered modifiable activity questionnaire for adolescents with energy expenditure calculated from the metabolic equivalent (MET) values from the Compendium of Physical Activities. Multiple regression models were developed to predict CSA and Z.

RESULTS

Age, time from menarche, total body lean mass (LM), total body fat mass (FM), height, total calcium, and total dairy calcium all shared a significant (p<0.05), positive relationship with CSA. Age, time from menarche, LM, FM, and height shared significant (p<0.05), positive relationships with Z. For both CSA and Z, LM was the most important covariate. Physical activity was not a significant predictor of geometry at the femoral neck (p≥0.339), even after removing LM as a covariate. After adjusting for covariates, ethnicity was not a significant predictor in regression models for CSA and Z.

CONCLUSION

Variability in bone geometry at the narrow neck of the femur is best explained by body size and pubertal maturation. After controlling for these covariates there were no differences in bone geometry between ethnic groups.

A cross-sectional study of the relationship between cortical bone and high-impact activity in young adult males and females

CONTEXT

The factors that govern skeletal responses to physical activity remain poorly understood.

OBJECTIVE

The aim of this study was to investigate whether gender or fat mass influences relationships between cortical bone and physical activity, after partitioning accelerometer outputs into low (0.5-2.1 g), medium (2.1-4.2 g), or high (>4.2 g) impacts, where g represents gravitational force.

DESIGN/SETTING

We conducted a cross-sectional analysis in participants from the Avon Longitudinal Study of Parents and Children.

PARTICIPANTS

We studied 675 adolescents (272 boys; mean age, 17.7 yr).

OUTCOME MEASURES

We measured cortical bone parameters from peripheral quantitative computed tomography scans of the mid-tibia, adjusted for height, fat mass, and lean mass.

RESULTS

High-impact activity was positively associated with periosteal circumference (PC) in males but not females [coefficients (95% confidence intervals), 0.054 (0.007, 0.100) and 0.07 (-0.028, 0.041), respectively; showing sd change per doubling in activity]. There was also weak evidence that medium impacts were positively related to PC in males but not females (P=0.03 for gender interaction). On stratifying by fat mass, the positive relationship between high-impact activity and PC was greatest in those with the highest fat mass [high impact vs. PC in males, 0.01 (-0.064, 0.085), 0.045 (-0.040, 0.131), 0.098 (0.012, 0.185), for lower, middle, and upper fat tertiles, respectively; high impact vs. PC in females, -0.041 (-0.101, 0.020), -0.028 (-0.077, 0.022), 0.082 (0.015, 0.148), P=0.01 for fat mass interaction]. Similar findings were observed for strength parameters, cross-sectional moment of inertia, and strength-strain index.

CONCLUSIONS

In late adolescence, associations between high-impact activity and PC are attenuated by female gender and low body fat, suggesting that the skeletal response to high-impact activity is particularly reduced in young women with low fat mass.

Human biology at the interface of paediatrics: measuring bone mineral accretion during childhood

BACKGROUND

Professor Tanner established a paradigm for the study of growth and development that demands precise growth measurements, description of normal variability through development to adulthood, consideration of the effects of tempo and the study of factors that influence growth outcomes. The relatively new field of paediatric bone health assessment fits this paradigm and reflects the collaboration of human biologists and paediatricians in understanding the growth of the human skeleton.

REVIEW

This review describes the reasons for clinical assessment of bone density in children, the technological developments in bone health assessment in children, the development of reference curves and the effects of growth, body composition, pubertal timing, genetics and lifestyle on bone health outcomes.

Physical activity as a strategy to reduce the risk of osteoporosis and fragility fractures

Childhood and adolescence are critical periods for the skeleton. Mechanical load has then been shown to be one of the best stimuli to enhance not only bone mass, but also structural skeletal adaptations, as both contributing to bone strength. Exercise prescription also includes a window of opportunity to improve bone strength in the late pre- and early peri-pubertal period. There is some evidence supporting the notion that skeletal gains obtained by mechanical load during growth are maintained at advanced age despite a reduction of physical activity in adulthood. The fact that former male athletes have a lower fracture risk than expected in their later years does not oppose the view that physical activity during growth and adolescence is important and it should be supported as one feasible strategy to reduce the future incidence of fragility fractures.

[Impact of physical activity and exercise on bone health in the life course : a review]

Physical activity and exercise are important determinants for metabolic and cardiovascular health. They also play an important role for bone health in childhood, adolescence, and adulthood. This review summarizes results from observational and intervention studies which evaluated the association between physical activity/exercise and bone health in different life course stages. In childhood and adolescence, physical activity and exercise induce improved bone accrual. In adulthood, mainly in postmenopausal women, long-term exercise programs reduce age-related bone loss. Especially weight-bearing activities seem to have an important osteogenic effect. Children and adolescent show a higher bone accrual until 5 years after cessation of an exercise program compared to their peers, who do not participate in an exercise program. In contrast, adults who quit exercising have a higher decrease in bone stiffness compared to adults who never exercised. This effect was particularly seen in postmenopausal women. Continuous physical activity and exercise over the life course and the implementation of exercise programs in schools and community-based intervention programs can help prevent or even reduce osteoporosis and osteoporosis-related fractures. Due to the lack of prospective longitudinal studies, the supposed long-term sustainable protective effect of physical activity and exercise in childhood and adolescent on bone health in later adulthood is not well established.

Increased physical activity is associated with enhanced development of peak bone mass in men: a five-year longitudinal study

Data supporting physical activity guidelines to optimize bone development in men is sparse. Peak bone mass is believed to be important for the risk of osteoporosis later in life. The objective of this study was to determine if an increased amount of physical activity over a 5-year period was associated with increased bone mineral content (BMC), areal (aBMD) and volumetric (vBMD) bone mineral density, and a favorable development of cortical bone size in young adult men. The original 1068 young men, initially enrolled in the Gothenburg Osteoporosis and Obesity Determinants (GOOD) study, were invited to participate in the longitudinal study, and a total of 833 men (78%), 24.1 ± 0.6 years of age, were included in the 5-year follow-up. A standardized self-administered questionnaire was used to collect information about patterns of physical activity at both the baseline and 5-year follow-up visits. BMC and aBMD were measured using dual energy X-ray absorptiometry, whereas vBMD and bone geometry were measured by peripheral quantitative computed tomography. Increased physical activity between the baseline and follow-up visits was associated with a favorable development in BMC of the total body, and aBMD of the lumbar spine and total hip (p < 0.001), as well as with development of a larger cortex (cortical cross sectional area), and a denser trabecular bone of the tibia (p < 0.001). In conclusion, increased physical activity was related to an advantageous development of aBMD, trabecular vBMD and cortical bone size, indicating that exercise is important in optimizing peak bone mass in young men.

Midhumerus adaptation in fast-pitch softballers and the effect of throwing mechanics

PURPOSE

Throwing is a vigorous activity that generates large internal loads. There is limited evidence of the effect of these loads on bone adaptation. The aim of this study was to investigate the 1) magnitude of bone adaptation within the midshaft humerus of female fast-pitch softball players and 2) influence of throwing mechanics (windmill vs overhand throwing) on the magnitude of adaptation.

METHODS

Midshaft humeral bone mass, structure, and estimated strength were assessed via peripheral quantitative computed tomography in fast-pitch softball players (throwers; n = 15) and matched controls (controls; n = 15). The effect of throwing was examined by comparing dominant-to-nondominant differences in throwers to controls, whereas the influence of mechanics was determined by comparing dominant-to-nondominant differences in throwers who primarily play as pitcher (windmill thrower), catcher (overhand thrower), or fielder (overhand thrower).

RESULTS

Throwers had greater dominant-to-nondominant difference in midshaft humeral bone mass, structure, and estimated strength relative to controls (all P < 0.05). The largest effect was for estimated torsional strength with throwers having a mean dominant-to-nondominant difference of 22.5% (range = 6.7%-43.9%) compared with 4.4% (range = -8.3% to 17.5%) in controls (P < 0.001). Throwing mechanics seemed to influence the magnitude of skeletal adaptation, with overhand throwers having more than double dominant-to-nondominant difference in midshaft humeral bone mass, structure, and estimated strength than windmill throwers (all P < 0.05).

CONCLUSIONS

Throwing induces substantial skeletal adaptation at the midshaft humerus of the dominant upper extremity. Throwing mechanics seems to influence the magnitude of adaptation, as catchers and fielders (overhand throwers) had twice as much adaptation as pitchers (windmill throwers). The latter finding may have implications for skeletal injury risk at the midshaft humerus in throwing athletes.

Estrogen, exercise, and the skeleton

Patterns of variation in bone size and shape provide crucial data for reconstructing hominin paleobiology, including ecogeographic adaptation, life history, and functional morphology. Measures of bone strength, including robusticity (diaphyseal thickness relative to length) and cross-sectional geometric properties such as moments of area, are particularly useful for inferring behavior because bone tissue adapts to its mechanical environment. Particularly during skeletal growth, exercise-induced strains can stimulate periosteal modeling so that, to some extent, bone thickness reflects individual behavior. Thus, patterns of skeletal robusticity have been used to identify gender-based activity differences, temporal shifts in mobility, and changing subsistence strategies. Although there is no doubt that mechanical loading leaves its mark on the skeleton, less is known about whether individuals differ in their skeletal responses to exercise. For example, the potential effects of hormones or growth factors on bone-strain interactions are largely unexplored. If the hormonal background can increase or decrease the effects of exercise on skeletal robusticity, then the same mechanical loads might cause different degrees of bone response in different individuals. Here I focus on the role of the hormone estrogen in modulating exercise-induced changes in human bone thickness.

Bone mineral density of adolescent female tennis players and nontennis players

The purpose of this study was to determine differences in bone mineral density (BMD) among adolescent female tennis players (TPs) and nontennis players (NTPs) and to assess body composition as a predictor variable of BMD. Nineteen female TPs and 19 female NTPs, ages 14 to 18 years, participated in this study. Lumbar spine, total hip, femoral neck, forearms BMD, and body composition were assessed using dual-energy X-ray absorptiometry (DXA). Lumbar spine and total hip BMD measurements for TP were greater than NTP. However, these differences were not statistically significant (P = 0.37 and 0.12, resp.). TP had significantly greater femoral neck BMD than NTPs (P = 0.02). This difference might play an important role in preventing osteoporosis and decreasing the risk of fractures at the hip later in life.

The Lichfield bone study: the skeletal response to exercise in healthy young men

The skeletal response to short-term exercise training remains poorly described. We thus studied the lower limb skeletal response of 723 Caucasian male army recruits to a 12-wk training regime. Femoral bone volume was assessed using magnetic resonance imaging, bone ultrastructure by quantitative ultrasound (QUS), and bone mineral density (BMD) using dual-energy X-ray absorptiometry (DXA) of the hip. Left hip BMD increased with training (mean ± SD: 0.85 ± 3.24, 2.93 ± 4.85, and 1.89 ± 2.85% for femoral neck, Ward's area, and total hip, respectively; all P < 0.001). Left calcaneal broadband ultrasound attenuation rose 3.57 ± 0.5% (P < 0.001), and left and right femoral cortical volume by 1.09 ± 4.05 and 0.71 ± 4.05%, respectively (P = 0.0001 and 0.003), largely through the rise in periosteal volume (0.78 ± 3.14 and 0.59 ± 2.58% for right and left, respectively, P < 0.001) with endosteal volumes unchanged. Before training, DXA and QUS measures were independent of limb dominance. However, the dominant femur had higher periosteal (25,991.49 vs. 2,5572 mm(3), P < 0.001), endosteal (6,063.33 vs. 5,983.12 mm(3), P = 0.001), and cortical volumes (19,928 vs. 19,589.56 mm(3), P = 0.001). Changes in DXA, QUS, and magnetic resonance imaging measures were independent of limb dominance. We show, for the first time, that short-term exercise training in young men is associated not only with a rise in human femoral BMD, but also in femoral bone volume, the latter largely through a periosteal response.

Obstacles in the optimization of bone health outcomes in the female athlete triad

Maintaining low body weight for the sake of performance and aesthetic purposes is a common feature among young girls and women who exercise on a regular basis, including elite, college and high-school athletes, members of fitness centres, and recreational exercisers. High energy expenditure without adequate compensation in energy intake leads to an energy deficiency, which may ultimately affect reproductive function and bone health. The combination of low energy availability, menstrual disturbances and low bone mineral density is referred to as the 'female athlete triad'. Not all athletes seek medical assistance in response to the absence of menstruation for 3 or more months as some believe that long-term amenorrhoea is not harmful. Indeed, many women may not seek medical attention until they sustain a stress fracture. This review investigates current issues, controversies and strategies in the clinical management of bone health concerns related to the female athlete triad. Current recommendations focus on either increasing energy intake or decreasing energy expenditure, as this approach remains the most efficient strategy to prevent further bone health complications. However, convincing the athlete to increase energy availability can be extremely challenging. Oral contraceptive therapy seems to be a common strategy chosen by many physicians to address bone health issues in young women with amenorrhoea, although there is little evidence that this strategy improves bone mineral density in this population. Assessment of bone health itself is difficult due to the limitations of dual-energy X-ray absorptiometry (DXA) to estimate bone strength. Understanding how bone strength is affected by low energy availability, weight gain and resumption of menses requires further investigations using 3-dimensional bone imaging techniques in order to improve the clinical management of the female athlete triad.

Sex differences in chondrocyte maturation in the mandibular condyle from a decreased occlusal loading model

Temporomandibular joint disorders (TMDs) predominantly afflict women of childbearing age. Defects in mechanical loading-induced temporomandibular joint (TMJ) remodeling are believed to be a major etiological factor in the development of TMD. The goal of this study was to determine if there are sex differences in CD-1 and C57BL/6 mice exposed to a decreased occlusal loading TMJ remodeling model. Male and female CD-1 and C57BL/6 mice, 21 days old, were each divided into two groups. They were fed either a normal pellet diet (normal loading) or a soft diet and had their incisors trimmed out of occlusion (decreased occlusal loading) for 4 weeks. The mandibular condylar cartilage was evaluated by histology, and the subchondral bone was evaluated by micro-CT analysis. Gene expression from both was evaluated by real-time PCR analysis. In both strains and sexes of mice, decreased occlusal loading caused similar effects in the subchondral bone, decreases in bone volume and total volume compared with their normal loading controls. However, in both strains, decreased occlusal loading caused a significant decrease in the expression of collagen type II (Col2) and Sox9 only in female mice, but not in male mice, compared with their normal loading controls. Decreased occlusal loading causes decreased bone volume in both sexes and a decrease in early chondrocyte maturation exclusively in female mice.

Skeletal health of children and adolescents with inflammatory bowel disease

Current evidence points to suboptimal bone health in children and adolescents with inflammatory bowel disease (IBD) when compared with their healthy peers. This compromise is evident from diagnosis. The clinical consequences and long-term outcome of this finding are still unknown. The mechanism of suboptimal bone health in children and adolescents with IBD lays mainly in reduced bone formation, but also reduced bone resorption, processes necessary for bone growth. Factors contributing to this derangement are inflammation, delayed growth and puberty, lean mass deficits, and use of glucocorticoids. We recognize that evidence is sparse on the topic of bone health in children and adolescents with IBD. In this clinical guideline, based on current evidence, we provide recommendations on screening and monitoring bone health in children and adolescents with IBD, including modalities to achieve this and their limitations; monitoring of parameters of growth, pubertal development, and reasons for concern; evaluation of vitamin D status and vitamin D and calcium intake; exercise; and nutritional support. We also report on the current evidence of the effect of biologics on bone health in children and adolescents with IBD, as well as the role of bone active medications such as bisphosphonates. Finally, we summarize the existing numerous gaps in knowledge and potential subjects for future research endeavors.

Effects of repetitive loading on the growth-induced changes in bone mass and cortical bone geometry: a 12-month study in pre/peri- and postmenarcheal tennis players

Pre- and early puberty may be the most opportune time to strengthen the female skeleton, but there are few longitudinal data to support this claim. Competitive female premenarcheal (pre/peri, n = 13) and postmenarcheal (post, n = 32) tennis players aged 10 to 17 years were followed over 12 months. The osteogenic response to loading was studied by comparing the playing and nonplaying humeri for dual-energy X-ray absorptiometry (DXA) bone mineral content (BMC) and magnetic resonance imaging (MRI) total bone area (ToA), medullary area (MedA), cortical area (CoA), and muscle area (MCSA) at the humerus. Over 12 months, growth-induced gains (nonplaying arm) in BMC, ToA, and CoA were greater in pre/peri (10% to 19%, p < .001) than in post (3% to 5%, p < .05 to .001) players. At baseline, BMC, ToA, CoA, and MCSA were 8% to 18% greater in the playing versus nonplaying arms in pre/peri and post players (all p < .001); MedA was smaller in the playing versus nonplaying arms in post only players (p < .05). When comparing the annual gains in the playing arm relative to changes in the nonplaying arm, the increases in ToA and CoA were greater in pre/peri than post players (all p < .05). The smaller the side-to-side differences in BMC and CoA at baseline, the larger the exercise benefits at 12 months (r = -0.39 to -0.48, p < .01). The exercise-induced change in MCSA was predictive of the exercise benefits in BMC in pre/peri players only (p < .05). In conclusion, both pre/peri- and postmenarcheal tennis players showed significant exercise-induced skeletal benefits within a year, with greater benefits in cortical bone geometry in pre/perimenarcheal girls.

Bone geometry and strength adaptations to physical constraints inherent in different sports: comparison between elite female soccer players and swimmers

Sports training characterized by impacts or weight-bearing activity is well known to induce osteogenic effects on the skeleton. Less is known about the potential effects on bone strength and geometry, especially in female adolescent athletes. The aim of this study was to investigate hip geometry in adolescent soccer players and swimmers compared to normal values that stemmed from a control group. This study included 26 swimmers (SWIM; 15.9 ± 2 years) and 32 soccer players (SOC; 16.2 ± 0.7 years), matched in body height and weight. A group of 15 age-matched controls served for the calculation of hip parameter Z-scores. Body composition and bone mineral density (BMD) were assessed by dual-energy X-ray absorptiometry (DXA). DXA scans were analyzed at the femoral neck by the hip structure analysis (HSA) program to calculate the cross-sectional area (CSA), cortical dimensions (inner endocortical diameter, ED; outer width and thickness, ACT), the centroid (CMP), cross-sectional moment of inertia (CSMI), section modulus (Z), and buckling ratio (BR) at the narrow neck (NN), intertrochanteric (IT), and femoral shaft (FS) sites. Specific BMDs were significantly higher in soccer players compared with swimmers. At all bone sites, every parameter reflecting strength (CSMI, Z, BR) favored soccer players. In contrast, swimmers had hip structural analysis (HSA) Z-scores below the normal values of the controls, thus denoting weaker bone in swimmers. In conclusion, this study suggests an influence of training practice not only on BMD values but also on bone geometry parameters. Sports with high impacts are likely to improve bone strength and bone geometry. Moreover, this study does not support the argument that female swimmers can be considered sedentary subjects regarding bone characteristics.

Associations of physical activity duration, frequency, and load with volumetric BMD, geometry, and bone strength in young girls

SUMMARY

More efficacious physical activity (PA) prescriptions for optimal bone development are needed. This study showed that PA duration, frequency, and load were all independently associated with bone parameters in young girls. Increased PA duration, frequency, and load are all important osteogenic stimuli that should be incorporated into future PA interventions.

INTRODUCTION

This study evaluated the associations of physical activity (PA) duration, frequency, load, and their interaction (total PA score = duration × frequency × load) with volumetric bone mineral density, geometry, and indices of bone strength in young girls.

METHODS

Four hundred sixty-five girls (aged 8-13 years) completed a past year physical activity questionnaire (PYPAQ) which inquires about the frequency (days per week) and duration (average minutes per session) of leisure-time PA and sports. Load (peak strain score) values were assigned to each activity based on ground reaction forces. Peripheral quantitative computed tomography was used to assess bone parameters at metaphyseal and diaphyseal sites of the femur and tibia of the non-dominant leg.

RESULTS

Correlations across all skeletal sites between PA duration, frequency, load and periosteal circumference (PC), bone strength index (BSI), and strength-strain index (SSI) were significant (p ≤ 0.05), although low (0.10-0.17). A 2.7-3.7% greater PC across all skeletal sites was associated with a high compared to a low PYPAQ score. Also, a high PYPAQ score was associated with greater BSI (6.5-8.7%) at metaphyseal sites and SSI (7.5-8.1%) at diaphyseal sites of the femur and tibia. The effect of a low PYPAQ score on bone geometric parameters and strength was greater than a high PYPAQ score.

CONCLUSIONS

PA duration, frequency, and load were all associated with bone geometry and strength, although their independent influences were modest and site specific. Low levels of PA may compromise bone development whereas high levels have only a small benefit over more average levels.

Blockade of receptor-activated G(i) signaling in osteoblasts in vivo leads to site-specific increases in cortical and cancellous bone formation

Osteoblasts play a critical role in the maintenance of bone mass through bone formation and regulation of bone resorption. Targeted expression of a constitutively active engineered G(i)-coupled G protein-coupled receptor (GPCR) to osteoblasts in vivo leads to severe osteopenia. However, little is known about the role of endogenous receptor-mediated G(i) signaling in regulating osteoblast function. In this study, we investigated the skeletal effects of blocking G(i)-coupled signaling in osteoblasts in vivo. This was accomplished by transgenic expression of the catalytic subunit of pertussis toxin (PTX) under control of the collagen Iα 2.3-kb promoter. These mice, designated Col1(2.3)(+)/PTX(+), showed increased cortical thickness at the femoral midshaft at 12 weeks of age. This correlated with increased periosteal bone formation associated with expanded mineralizing surface observed in 8-week-old mice of both genders. The cancellous bone phenotype of the Col1(2.3)(+)/PTX(+) mice was sexually dimorphic, with increases in fractional bone volume at the distal femur seen only in females. Similarly, while cancellous bone-formation rates were unchanged in males, they could not be quantified for female Col1(2.3)(+)/PTX(+) mice owing to the disorganized nature of the labeling pattern, which was consistent with rapid formation of woven bone. Alterations in osteoclast activity did not appear to participate in the phenotype. These data demonstrate that G(i)-coupled signaling by GPCRs endogenous to osteoblasts plays a complex role in the regulation of bone formation in a manner that is dependent on both gender and the anatomic site within bone.

Effect of a general school-based physical activity intervention on bone mineral content and density: a cluster-randomized controlled trial

BACKGROUND

Specific physical loading leads to enhanced bone development during childhood. A general physical activity program mimicking a real-life situation was successful at increasing general physical health in children. Yet, it is not clear whether it can equally increase bone mineral mass. We performed a cluster-randomized controlled trial in children of both gender and different pubertal stages to determine whether a school-based physical activity (PA) program during one school-year influences bone mineral content (BMC) and density (BMD), irrespective of gender.

METHODS

Twenty-eight 1st and 5th grade (6-7 and 11-12 year-old) classes were cluster randomized to an intervention (INT, 16 classes, n=297) and control (CON; 12 classes, n=205) group. The intervention consisted of a multi-component PA intervention including daily physical education with at least 10 min of jumping or strength training exercises of various intensities. Measurements included anthropometry, and BMC and BMD of total body, femoral neck, total hip and lumbar spine using dual-energy X-ray absorptiometry (DXA). PA was assessed by accelerometers and Tanner stages by questionnaires. Analyses were performed by a regression model adjusted for gender, baseline height and weight, baseline PA, post-intervention pubertal stage, baseline BMC, and cluster.

RESULTS

275 (72%) of 380 children who initially agreed to have DXA measurements had also post-intervention DXA and PA data. Mean age of prepubertal and pubertal children at baseline was 8.7±2.1 and 11.1±0.6 years, respectively. Compared to CON, children in INT showed statistically significant increases in BMC of total body, femoral neck, and lumbar spine by 5.5%, 5.4% and 4.7% (all p<0.05), respectively, and BMD of total body and lumbar spine by 8.4% and 7.3% (both p<0.01), respectively. There was no gender *group, but a pubertal stage *group interaction consistently favoring prepubertal children.

CONCLUSION

A general school-based PA intervention can increase bone health in elementary school children of both genders, particularly before puberty.

Pediatric inflammatory bowel disease and bone health

Childhood and adolescence are important periods for bone development. Any disease that affects bone health has the potential to affect the bones not only in the short term but also later in life. Bone health abnormalities in patients with inflammatory bowel disease are being increasingly recognized. Screening the at-risk patient is important so that appropriate treatments can be instituted. Treatment options are limited to vitamin D and calcium supplementation, control of underlying disease activity, and appropriate physical activity. The role of bisphosphonates in these patients needs to be better studied, and treatment with bisphosphonates may be considered for some patients in consultation with a bone health expert.

Body Composition of Young Athletes

Absolute and relative components of body mass are a primary focus in studies of body composition. Components change with growth and maturation, requiring care in selecting methods of assessment in children and adolescents. Although sex differences are apparent in fat-free mass (FFM), fat mass (FM), and relative fatness (% Fat) during childhood, they are more clearly defined during adolescence and adulthood. Body composition is one of many determinants of sport performance, but % Fat tends to be the primary focus of discussion in young athletes. The influence of training for sport on bone mineral has received more attention in recent years because of methodological advances in assessment and concern for potential negative effects of altered menstrual function on bone. Relative fatness of young athletes in several sports is summarized, and issues related to sport training and body composition in young athletes are considered. Suggestions for dealing with body composition in young athletes are provided.

Distal radius geometry and skeletal strength indices after peripubertal artistic gymnastics

Development of optimal skeletal strength should decrease adult bone fragility. Nongymnasts (NON): were compared with girls exposed to gymnastics during growth (EX/GYM: ), using peripheral quantitative computed tomography (pQCT) to evaluate postmenarcheal bone geometry, density, and strength. Pre- and perimenarcheal gymnastic loading yields advantages in indices of postmenarcheal bone geometry and skeletal strength.

INTRODUCTION

Two prior studies using pQCT have reported bone density and size advantages in Tanner I/II gymnasts, but none describe gymnasts' bone properties later in adolescence. The current study used pQCT to evaluate whether girls exposed to gymnastics during late childhood growth and perimenarcheal growth exhibited greater indices of distal radius geometry, density, and skeletal strength.

METHODS

Postmenarcheal subjects underwent 4% and 33% distal radius pQCT scans, yielding: 1) vBMD and cross-sectional areas (CSA) (total bone, compartments); 2) polar strength-strain index; 3) index of structural strength in axial compression. Output was compared for EX/GYM: vs. NON: , adjusting for gynecological age and stature (maturity and body size), reporting means, standard errors, and significance.

RESULTS

Sixteen postmenarcheal EX/GYM: (age 16.7 years; gynecological age 3.4 years) and 13 NON: (age 16.2 years; gynecological age 3.6 years) were evaluated. At both diaphysis and metaphysis, EX/GYM: exhibited greater CSA and bone strength indices than NON; EX/GYM: exhibited 79% larger intramedullary CSA than NON: (p < 0.05). EX/GYM: had significantly higher 4% trabecular vBMD; differences were not detected for 4% total vBMD and 33% cortical vBMD.

CONCLUSIONS

Following pre-/perimenarcheal gymnastic exposure, relative to nongymnasts, postmenarcheal EX/GYM: demonstrated greater indices of distal radius geometry and skeletal strength (metaphysis and diaphysis) with greater metaphyseal trabecular vBMD; larger intramedullary cavity size was particularly striking.

Ward's area location, physical activity, and body composition in 8- and 9-year-old boys and girls

Bone strength is the result of its material composition and structural design, particularly bone mass distribution. The purpose of this study was to analyze femoral neck bone mass distribution by Ward's area location and its relationship with physical activity (PA) and body composition in children 8 and 9 years of age. The proximal femur shape was defined by geometric morphometric analysis in 88 participants (48 boys and 40 girls). Using dual-energy X-ray absorptiometry (DXA) images, 18 landmarks were digitized to define the proximal femur shape and to identify Ward's area position. Body weight, lean and fat mass, and bone mineral were assessed by DXA, PA by accelerometry, and bone age by the Tanner-Whitehouse III method. Warps analysis with Thin-Plate Spline software showed that the first axis explained 63% of proximal femur shape variation in boys and 58% in girls. Most of this variation was associated with differences in Ward's area location, from the central zone to the superior aspect of the femoral neck in both genders. Regression analysis demonstrated that body composition explained 4% to 7% of the proximal femur shape variation in girls. In boys, body composition variables explained a similar amount of variance, but moderate plus vigorous PA (MVPA) also accounted for 6% of proximal femur shape variation. In conclusion, proximal femur shape variation in children ages 8 and 9 was due mainly to differences in Ward's area position determined, in part, by body composition in both genders and by MVPA in boys. These variables were positively associated with a central Ward's area and thus with a more balanced femoral neck bone mass distribution.

Targeted exercise against osteoporosis: A systematic review and meta-analysis for optimising bone strength throughout life

BACKGROUND

Exercise is widely recommended to reduce osteoporosis, falls and related fragility fractures, but its effect on whole bone strength has remained inconclusive. The primary purpose of this systematic review and meta-analysis was to evaluate the effects of long-term supervised exercise (> or =6 months) on estimates of lower-extremity bone strength from childhood to older age.

METHODS

We searched four databases (PubMed, Sport Discus, Physical Education Index, and Embase) up to October 2009 and included 10 randomised controlled trials (RCTs) that assessed the effects of exercise training on whole bone strength. We analysed the results by age groups (childhood, adolescence, and young and older adulthood) and compared the changes to habitually active or sedentary controls. To calculate standardized mean differences (SMD; effect size), we used the follow-up values of bone strength measures adjusted for baseline bone values. An inverse variance-weighted random-effects model was used to pool the results across studies.

RESULTS

Our quality analysis revealed that exercise regimens were heterogeneous; some trials were short in duration and small in sample size, and the weekly training doses varied considerably between trials. We found a small and significant exercise effect among pre- and early pubertal boys [SMD, effect size, 0.17 (95% CI, 0.02-0.32)], but not among pubertal girls [-0.01 (-0.18 to 0.17)], adolescent boys [0.10 (-0.75 to 0.95)], adolescent girls [0.21 (-0.53 to 0.97)], premenopausal women [0.00 (-0.43 to 0.44)] or postmenopausal women [0.00 (-0.15 to 0.15)]. Evidence based on per-protocol analyses of individual trials in children and adolescents indicated that programmes incorporating regular weight-bearing exercise can result in 1% to 8% improvements in bone strength at the loaded skeletal sites. In premenopausal women with high exercise compliance, improvements ranging from 0.5% to 2.5% have been reported.

CONCLUSIONS

The findings from our meta-analysis of RCTs indicate that exercise can significantly enhance bone strength at loaded sites in children but not in adults. Since few RCTs were conducted to investigate exercise effects on bone strength, there is still a need for further well-designed, long-term RCTs with adequate sample sizes to quantify the effects of exercise on whole bone strength and its structural determinants throughout life.

Bone and lean mass inter-arm asymmetries in young male tennis players depend on training frequency

Professional tennis players (TP) have marked inter-arm asymmetry in bone mass (BMC) and density (BMD). To determine if this asymmetry is influenced by training frequency and volume, we studied 24 young tennis players (mean age 10.6 years, Tanner 1-2), 17 physically active control boys (CG) and ten male professional tennis players. Young TP were divided into two groups depending on the number of training days per week (TP5: 5 days/week, n = 10; TP2: 2 days/week, n = 14). In young TP, the dominant arm (DA) compared to the non-dominant arm (NDA) had greater lean mass (TP5, 13.3 +/- 2.0% and TP2, 8.3 +/- 1.3%), BMC (TP5, 22.4 +/- 4.1% and TP2, 12.1 +/- 2.2%), bone area (TP5, 15.6 +/- 3.3% and TP2, 7.9 +/- 2.2%) and BMD (TP5, 4.6 +/- 1.5% and TP2, 3.8 +/- 0.6%). Inter-arm asymmetry in lean mass, BMC and bone area was greater in TP5 than TP2, being related to the number of weekly hours devoted to tennis (r = 0.45-52, P < 0.05). No significant differences in lumbar spine or femoral neck BMC or BMD were observed between TP5, TP2 and CG. In professional TP, the DA had 18, 32, 11 and 15% greater lean mass, BMC, bone area and BMD than the NDA. Thus, TP5 had 69% of the inter-arm asymmetry in BMC observed in professional TP and a similar inter-arm asymmetry in bone area, although this comparison may not be generalisable. Young tennis players have increased BMC, bone area and lean mass in dominant arm, which magnitude depends on the number of weekly hours devoted to tennis.

Sarcopenia: etiology, clinical consequences, intervention, and assessment

The aging process is associated with loss of muscle mass and strength and decline in physical functioning. The term sarcopenia is primarily defined as low level of muscle mass resulting from age-related muscle loss, but its definition is often broadened to include the underlying cellular processes involved in skeletal muscle loss as well as their clinical manifestations. The underlying cellular changes involve weakening of factors promoting muscle anabolism and increased expression of inflammatory factors and other agents which contribute to skeletal muscle catabolism. At the cellular level, these molecular processes are manifested in a loss of muscle fiber cross-sectional area, loss of innervation, and adaptive changes in the proportions of slow and fast motor units in muscle tissue. Ultimately, these alterations translate to bulk changes in muscle mass, strength, and function which lead to reduced physical performance, disability, increased risk of fall-related injury, and, often, frailty. In this review, we summarize current understanding of the mechanisms underlying sarcopenia and age-related changes in muscle tissue morphology and function. We also discuss the resulting long-term outcomes in terms of loss of function, which causes increased risk of musculoskeletal injuries and other morbidities, leading to frailty and loss of independence.

Hip fracture prevalence in grandfathers is associated with reduced cortical cross-sectional bone area in their young adult grandsons

CONTEXT

Parent hip fracture prevalence is a known risk factor for osteoporosis. The role of hip fracture prevalence in grandparents on areal bone mineral density (aBMD) and bone size in their grandsons remains unknown.

OBJECTIVE

The objective of the study was to examine whether hip fracture prevalence in grandparents was associated with lower aBMD and reduced cortical bone size in their grandsons.

DESIGN AND SETTING

This was a population-based cohort study in Sweden.

STUDY SUBJECTS

Subjects included 1015 grandsons (18.9 +/- 0.6) (mean +/- sd) and 3688 grandparents.

MAIN OUTCOME MEASURES

aBMD, cortical bone size, volumetric bone mineral density and polar strength strain index of the cortex in the grandsons in relation to hip fracture prevalence in their grandparents were measured.

RESULTS

Grandsons of grandparents with hip fracture (n = 269) had lower aBMD at the total body, radius, and lumbar spine, but not at the hip, as well as reduced cortical cross-sectional area at the radius (P < 0.05) than grandsons of grandparents without hip fracture. Subgroup analysis demonstrated that grandsons of grandfathers with hip fracture (n = 99) had substantially lower aBMD at the lumbar spine (4.9%, P < 0.001) and total femur (4.1%, P = 0.003) and lower cortical cross-sectional area of the radius (4.1%, P < 0.001) and tibia (3.3%, P < 0.011). Adjusting bone variables for grandson age, weight, height, smoking, calcium intake, and physical activity and taking grandparent age at register entry, years in register, and grandparent sex into account strengthened or did not affect these associations.

CONCLUSIONS

Family history of a grandfather with hip fracture was associated with reduced aBMD and cortical bone size in 19-yr-old men, indicating that patient history of hip fracture in a grandfather could be of value when evaluating the risk of low bone mass in men.

Physical activity is the strongest predictor of calcaneal peak bone mass in young Swedish men

SUMMARY

In a highly representative sample of young adult Swedish men (n = 2,384), we demonstrate that physical activity during childhood and adolescence was the strongest predictor of calcaneal bone mineral density (BMD), and that peak bone mass was reached at this site at the age of 18 years.

INTRODUCTION

The purpose of the present study was to determine if physical activity during growth is associated with peak calcaneal BMD in a large, highly representative cohort of young Swedish men.

METHODS

In this study, 2,384 men, 18.3 +/- 0.3 (mean +/- SD) years old, were included from a population attending the mandatory tests for selection to compulsory military service in Sweden. BMD (g/cm(2)) of the calcaneus was measured using dual-energy X-ray absorptiometry. Training habits were investigated using a standardized questionnaire.

RESULTS

Regression analysis (with age, height, weight, smoking, and calcium intake as covariates) demonstrated that history of regular physical activity was the strongest predictor and could explain 10.1% of the variation in BMD (standardized beta = 0.31, p < 0.001). A regression model with quadratic age effect revealed maximum BMD at 18.4 years.

CONCLUSIONS

We found that history of physical activity during growth was the strongest predictor of peak calcaneal BMD in young men.

Androgen receptor disruption increases the osteogenic response to mechanical loading in male mice

In female mice, estrogen receptor-alpha (ERalpha) mediates the anabolic response of bone to mechanical loading. Whether ERalpha plays a similar role in the male skeleton and to what extent androgens and androgen receptor (AR) affect this response in males remain unaddressed. Therefore, we studied the adaptive response of in vivo ulna loading in AR-ERalpha knockout (KO) mice and corresponding male and female single KO and wild-type (WT) littermates using dynamic histomorphometry and immunohistochemistry. Additionally, cultured bone cells from WT and AR KO mice were subjected to mechanical loading by pulsating fluid flow in the presence or absence of testosterone. In contrast with female mice, ERalpha inactivation in male mice had no effect on the response to loading. Interestingly, loading induced significantly more periosteal bone formation in AR KO (+320%) and AR-ERalpha KO mice (+256%) compared with male WT mice (+114%) and had a stronger inhibitory effect on SOST/sclerostin expression in AR KO versus WT mice. In accordance, the fluid flow-induced nitric oxide production was higher in the absence of testosterone in bone cells from WT but not AR KO mice. In conclusion, AR but not ERalpha activation limits the osteogenic response to loading in male mice possibly via an effect on WNT signaling.

Habitual throwing and swimming correspond with upper limb diaphyseal strength and shape in modern human athletes

Variation in upper limb long bone cross-sectional properties may reflect a phenotypically plastic response to habitual loading patterns. Structural differences between limb bones have often been used to infer past behavior from hominin remains; however, few studies have examined direct relationships between behavioral differences and bone structure in humans. To help address this, cross-sectional images (50% length) of the humeri and ulnae of university varsity-level swimmers, cricketers, and controls were captured using peripheral quantitative computed tomography. High levels of humeral robusticity were found in the dominant arms of cricketers, and bilaterally among swimmers, whereas the most gracile humeri were found in both arms of controls, and the nondominant arms of cricketers. In addition, the dominant humeri of cricketers were more circular than controls. The highest levels of ulnar robusticity were also found in the dominant arm of cricketers, and bilaterally amongst swimmers. Bilateral asymmetry in humeral rigidity among cricketers was greater than swimmers and controls, while asymmetry for ulnar rigidity was greater in cricketers than controls. The results suggest that more mechanically loaded upper limb elements--unilaterally or bilaterally--are strengthened relative to less mechanically loaded elements, and that differences in mechanical loading may have a more significant effect on proximal compared to distal limb segments. The more circular humerus in the dominant arm in cricketers may be an adaptation to torsional strain associated with throwing activities. The reported correspondence between habitual activity patterns and upper limb diaphyseal properties may inform future behavioral interpretations involving hominin skeletal remains.

Throwing induces substantial torsional adaptation within the midshaft humerus of male baseball players

Athletes participating in unilateral dominant sports are useful models for investigating skeletal responses to mechanical loading as they provide controlled evidence in the absence of completing a randomized controlled trial. Throwing athletes may be an additional model for this purpose as they overload their dominant upper extremity enabling the contralateral side to act as an internal control and load the bones of the upper extremity purely via the generation of internal (i.e. muscular) forces without superposition of externally applied loads (i.e. impact with an external object). The aim of this study was to investigate upper extremity bone adaptation in throwing athletes and explore factors that predict this adaptation. Two cohorts were recruited-male baseball players (throwers; n=15) and matched controls (controls; n=15). Each subject was assessed for shoulder range and strength, and upper extremity bone mass, structure and estimated strength. Throwers had substantially greater skeletal differences between their dominant and nondominant upper extremities than controls, indicating that throwing induces greater adaptation than induced by habitual loading of the dominant upper extremity. Bone adaptation in throwers was localized to the humerus, with the midshaft humerus in the dominant upper extremity of throwers having enhanced bone mass, structure and estimated strength. The largest effect was for estimated strength of the midshaft humerus which had 30% greater polar moment of inertia (I(P)) in throwers and suggests adaptation to resist torsional loads. The skeletal effect of throwing at the midshaft humerus was influenced by playing position with pitchers and catchers displaying greater dominant-to-nondominant differences than fielders, and was predicted by years throwing and dominant-to-nondominant difference in upper arm lean cross-sectional area. The latter two variables explained 67% of the variance in dominant-to-nondominant differences in I(P). Collectively, these data indicate that throwing induces substantial adaptation within the midshaft humerus. Adaptation was primarily in the direction of torsion which is consistent with biomechanical and injury data suggesting throwing introduces high magnitude torsional forces. As the magnitude of adaptation in throwers was equivalent to that observed in athletes participating in other unilateral dominant sports, throwers represent an alternative model for investigating the skeletal effects of mechanical loading.