A. D. G. Baxter-Jones

A longitudinal analysis of sex differences in bone mineral accrual in healthy 8-19-year-old boys and girls

Background : Although early in life there is little discernible difference in bone mass between boys and girls, at puberty sex differences are observed. It is uncertain if these differences represent differences in bone mass or just differences in anthropometric dimensions. Aim : The study aimed to identify whether sex independently affects bone mineral content (BMC) accrual in growing boys and girls. Three sites are investigated: total body (TB), femoral neck (FN) and lumbar spine (LS). Subjects and methods : 85 boys and 67 girls were assessed annually for seven consecutive years. BMC was assessed by dual energy X-ray absorptiometry (DXA). Biological age was defined as years from age at peak height velocity (PHV). Data were analysed using a hierarchical (random effects) modelling approach. Results : When biological age, body size and body composition were controlled, boys had statistically significantly higher TB and FN BMC at all maturity levels ( p < 0.05). No independent sex differences were found at the LS ( p > 0.05). Conclusion : Although a statistical significant sex effect is observed, it is less than the error of the measurement, and thus sex difference are debatable. In general, sex difference are explained by anthropometric difference.

The influence of physical activity on lean mass accrual during adolescence: A longitudinal analysis

During childhood, physical activity is likely the most important modifiable factor for the development of lean mass. However, the effects of normal growth and maturation must be controlled. To distinguish effects of physical activity from normal growth, longitudinal data are required. One hundred nine boys and one hundred thirteen girls, participating in the Saskatchewan Pediatric Bone Mineral Accrual Study, were repeatedly assessed for 6 yr. Age at entry was 8-15 yr. Stature, body mass, and physical activity were assessed biannually. Body composition was assessed annually by dual-energy X-ray absorptiometry. Physical activity was determined using the physical activity questionnaires for children and adolescence. Biological age was defined as years from age of peak height velocity. Data were analyzed using multilevel random-effects models. In boys, it was found that physical activity had a significant time-dependent effect on lean mass accrual of the total body (484.7+/-157.1 g), arms (69.6+/-27.2 g), legs (197.7+/-60.5 g), and trunk (249.1+/-91.4 g) (P<0.05). Although the physical activity effects were similar in the girls (total body: 306.9+/-96.6 g, arms: 31.4+/-15.5 g, legs: 162.9+/-40.0 g, and trunk: 119.6+/-58.2 g; P<0.05), boys for the same level of activity accrued, depending on the site, between 21 and 120% more absolute lean mass (g). In conclusion, habitual physical activity had a significant independent influence on the growth of lean body mass during adolescence, once biological maturity and stature were controlled.

Dancing for bone health: a 3-year longitudinal study of bone mineral accrual across puberty in female non-elite dancers and controls

IntroductionWeight-bearing exercise during growth enhances peak bone mass. However, the window of opportunity for optimizing positive effects of exercise on peak bone mass remains to be fully defined. Ballet dancing provides a model of mechanical loading patterns required to site-specifically modulate bone.MethodsWe assessed the effects of ballet dancing on bone mineral accrual in female non-elite dancers and normally active controls for 3 years across puberty. We recruited 82 ballet dancers and 61 controls age 8–11 years at baseline. Participants were measured over 3 consecutive years; however, the overlap in ages allowed analysis of the groups across 8–14 years of age. We annually assessed bone mineral content (BMC) at the total body (TB), including upper and lower limb regions, and biannually assessed BMC at the proximal femur and lumbar spine (LS) using dual x-ray absorptiometry (DXA). We derived TB lean mass and fat mass from DXA TB scans. Anthropometry, exercise levels, and calcium intake were also measured biannually. Maturational age was determined by age at peak height velocity (PHV). A multilevel regression model was used to determine the independent effects of body size, body composition, maturation, exercise levels, and calcium intake at each measurement occasion.ResultsWhen adjusted for growth and maturation, dancers had significantly greater BMC at the TB, lower limbs, femoral neck (FN), and LS than controls. Excepting the FN region, these differences became apparent at 1 year post-PHV, or the peripubertal years, and by 2 years post-PHV the differences represented a cumulative advantage in dancers of 0.6–1.3% (p<0.05) greater BMC than controls. At the FN, dancers had 4% (p<0.05) greater BMC than controls in prepuberty and maintained this advantage throughout the pubertal years.ConclusionsResults from this novel population provide evidence for modest site-specific and maturity-specific effects of mechanical loading on bone.

Limitations to the use of secondary sex characteristics for gender comparisons

Background: To control for the confounding effect of maturation many researchers use secondary sex characteristics to compare individuals within and between genders. However, this assumption presumes that the timing and tempo of secondary sex characteristics is identical in both genders. Aim: The study investigated the timing and relationships between sexual and somatic maturation indices between and within genders. Subjects and methods: Eighty three boys and 75 girls, aged between 8 and 15 years at study entry, were measured every 6 months for 6 consecutive years. Sexual maturation was assessed through pubic hair, facial hair and axillary hair development in boys, and pubic hair development and menarcheal status in girls. Somatic maturation was assessed through age at peak height velocity (PHV). Results: Low to moderate correlations (r = 0.30–0.55, p<0.05) existed between age of PHV and age of reaching each pubic hair stage. The majority of boys reached PHV in pubic hair stage 4 (79.2%). The majority of girls reached PHV in pubic hair stage 3 (42.5%) and pubic hair stage 4 (47.5%). Conclusion: Boys and girls differ in the timing and tempo of somatic and sexual maturity. Thus boys and girls should not be aligned on secondary sex characteristics when controlling for the confounding effects of maturity.

The relationship between body composition and onset of menarche

Background: A late age of menarche in elite adolescent athletes is frequently attributed to low body fat/weight. If a critical body weight/fat is necessary for menarche, a reduction in the variability of these parameters would be observed at menarche compared to times before and after this event. Aim: The study determined the variability in body mass (BM), per cent body fat (%BF) and total body fat (TBF) of girls between −2 and +2 years from menarche. Methods: Participants were part of the Saskatchewan Pediatric Bone Mineral Accrual Study (1991–1998). Body composition was assessed using dual X-ray absorptiometry (DXA). An individuals data was retained for analysis if they had at least four DXA scans between −3 and +3 years from menarche. Values were interpolated from the cubic spline at whole years between −2 and +2 years from menarche. Coefficient of variation (CV) was used to assess variability. Results/Conclusion: Data on 61 girls were retained for analysis. The range in BM, TBF and %BF at menarche was 50.10 kg, 35050.06 g and 31.61%, respectively. Reductions in variability of body fat and weight were not apparent at menarche, which does not provide support for the hypothesis that a critical body weight/fat is required for menarche.

The relationship between girls’ (8–14 years) physical activity and maternal education

Background: Mothers with higher levels of education are more likely to engage in health-promoting behaviour; thus, it is intuitive that a childs physical activity would be positively related to maternal education. The literature on this area, however, is inconclusive and may be due to the methods used to assess physical activity (i.e. the use of aggregated and self-reported physical activity that may not reflect the true and detailed variation of physical activity). Aim: To profile the physical activity behaviours of girls with mothers of differing educational attainment. Subjects and methods: 77 girls (grades 4–8) wore an Actical accelerometer for 7 days. Minutes spent sedentary and in light, moderate and vigorous physical activity per day over 7 days, 5 weekdays, 2 weekends, and 1 h commuting period to and from school of girls of University educated (UE) and non-UE mothers were analysed. Results: After controlling for confounders, girls with UE mothers were more likely to participate in vigorous physical activity at the weekend and moderate-to-vigorous physical activity during the morning commute to school. Conclusions: Research investigating the relationship between maternal education and childs physical activity should explore, at higher resolution, when activity is accumulated, in particular separating weekday and weekend physical activity.

Tracking of aerobic fitness from adolescence to mid-adulthood

Abstract Background: Although adults’ aerobic fitness is known to be correlated with cardiovascular disease risk, the longitudinal relationship with adolescent aerobic fitness is poorly described. Aim: To longitudinally investigate the relationship between aerobic fitness during adolescence and adulthood. Subjects and methods: Participants (207 boys, 149 girls) aged 7–17 years performed annual measures of VO2peak. In adulthood (40 and 50 years), 78 individuals (59 males and 18 females) were reassessed. Serial height measurements were used to estimate age at peak height velocity (APHV). During adolescence, VO2peak was measured via a treadmill test to voluntary exhaustion; adult VO2peak was assessed using submaximal predictive tests. Correlations were tested using Spearman’s rho. ANCOVA was used to assess adult VO2peak group differences based off APHV VO2peak groupings (low, average or high). Results: When sexes were pooled, moderate tracking existed from 2 years prior to APHV to APHV and APHV to 2 years after APHV (0.46, p < 0.001 and 0.35, p < 0.01, respectively). Correlations between APHV and adult values were low when sexes were pooled (p < 0.05). Comparisons of aggregated sexes revealed the low adolescent VO2peak group had lower values in adulthood relative to other groups (p < 0.05). Conclusion: Aerobic fitness has a low tracking between APHV and adulthood.

Is Boys’ Physical Activity in Childhood associated with being Overweight in Mid-Adulthood?A Longitudinal Study Spanning 35 Years

The purpose of this study was to investigate the influence of childhood physical activity (PA) on subsequent adult body mass. Using longitudinal data from the Saskatchewan Growth and Development Study (1964 – 1973), PA, height, body mass and percent body fat were assessed on 207 males (age 7 years at study entry). Subjects were categorized into activity groups based on annual composite activity assessments. 58 participants returned during adulthood (between 39-41 years of age) when similar anthropometric measurements were taken. Childhood PA groupings were significantly related to adult BMI, but not body fat. The more active child had a greater BMI in adulthood (P<0.05). The results from this study indicate that the active child, in this sample, is not protected against overweight/obesity in adulthood.

The effects of exercise on linear growth and maturation in female novice dancers and controls: a longitudinal study

Whether physical activity during the adolescent growth spurt adversely influences maturation and growth remains in dispute. However, it is unclear whether this is simply due to selection of genetically late maturing individuals. The aim of this longitudinal study was to assess effects of dancing on linear growth and maturation of young girls progressing through puberty. Eighty-two novice ballet dancers and 61 controls aged 8-11 yrs at baseline were assessed every six months for three years. Maturational age was determined by age at peak height velocity (PHV). Body dimensions were measured by anthropometry and exercise levels, nutritional intake, and age at menarche by questionnaires. At baseline, dancers had attended dance classes for 1-9 yrs (mean 5.4 yrs) with total weekly dance hours during the study ranging from 1-16. There was no difference in nutritional intake between groups, except calcium, which was greater in dancers. Controls had significantly greater height velocity than dancers one year before age at PHV, however there was no difference between groups in PHV attained. There were no group differences in the velocity of growth in sitting height or leg length. Age at menarche was 7 months later in dancers than controls (p 6.5yrs) or weekly dance hours (>7hrs/wk) on growth velocities. The results suggest that moderate levels of dance training do not affect linear growth and maturational development. Thus children should not be discouraged from dance participation on the basis of potential growth delays.