The prepubertal years: a uniquely opportune stage of growth when the skeleton is most responsive to exercise?

High parity is associated with increased bone size and strength

Some, but not all, studies report an association between decreased hip fracture risk and high parity despite similar bone mineral density (BMD). Our hypothesis was that bone size, a major determinant of bone strength, is greater in women with high parity compared with low parity or nulliparous women. A cross-sectional study of 168 Hutterite women aged 40-80 years was conducted. BMD, bone mineral content (BMC) and bone area of the total body (TB), hip, femoral neck (FN), and lumbar spine (LS) were measured, as well as bone geometry at the 4% and 20% distal radius and bending strength at 20% radius. Diet and activity recall and strength measurements were obtained. Of the 168 women, 42 (25%) were nulliparous while the remaining women reported 1 to 16 births (median=6). Of the 126 parous women, 122 (97%) breast-fed their infants (range 1.5-24 months). Hip, FN and LS BMD were not associated with either parity or months of breast-feeding. TB BMC and bone area (both, p<0.05) and FN bone area (p<0.01) were associated with parity. FN bone area was 4% greater in women with 7+ vs 1-4 children. Torsional bending strength, which includes structural and material bone properties, at the 20% distal radius was greater with higher parity (p=0.01). No bone measure was associated with average months of breast-feeding. High parity is associated with increased radial torsional bending strength and femoral neck size. The greater femoral neck size, without higher BMD, may explain the reduced hip fracture risk among women with high parity previously reported in some studies.

"Bounce at the Bell": a novel program of short bouts of exercise improves proximal femur bone mass in early pubertal children

OBJECTIVES

To examine the effects of a simple and inexpensive physical activity intervention on change in bone mass and structure in school aged children.

METHODS

Fifty one children (n = 23 boys and 28 girls; mean age 10.1 years) participated in "Bounce at the Bell" which consisted of 10 counter-movement jumps 3x per day (total approximately 3 min/day). Controls were 71 matched children who followed usual school practice. We assessed dietary calcium, physical activity, physical performance, and anthropometry in September and after 8 months of intervention (June). We measured bone mineral content (BMC) and bone area at the lumbar spine, total body, and proximal femur. Proximal femur scans were also analysed for bone geometry and structural strength using the hip structural analysis program. Lean and fat mass (g) were also calculated.

RESULTS

Groups were similar at baseline and did not differ in weight, height, total body, lumbar spine, proximal femur, or femoral neck BMC. Control children had a greater increase in adjusted total body BMC (1.4%). Intervention children gained significantly more BMC at the total proximal femur (2%) and the intertrochanteric region (27%). Change in bone structural parameters did not differ between groups.

CONCLUSIONS

This novel, easily implemented exercise program, took only a few minutes each day and enhanced bone mass at the weight bearing proximal femur in early pubertal children. A large, randomised study of boys and girls should be undertaken powered to test the effectiveness of Bounce at the Bell in children at different stages of maturity, and in boys and girls independently.

Initial years of recreational artistic gymnastics training improves lumbar spine bone mineral accrual in 4- to 8-year-old females

Gymnasts' bone mineral characteristics are generally not known before starting their sport. Prepubertal females who enrolled in beginning artistic gymnastics (n = 65) had lower bone mineral than controls (n = 78). However, 2 years of gymnastics participation versus no participation led to a significantly greater accrual of forearm bone area and lumbar spine areal BMD.

INTRODUCTION

The skeletal response to exercise in children compared with adults is heightened because of the high bone turnover rate and the ability of bone to change its size and shape. Whereas child gymnasts generally have greater rates of bone mineral accrual compared with nongymnasts, it is unknown if some of these skeletal advantages are present before the onset of training or are caused entirely by training.

MATERIALS AND METHODS

Changes in bone area (BA; cm2), BMC (g), and areal BMD (aBMD; g/cm2) over 24 months were examined in prepubertal females, 4-8 years of age, who selected to perform recreational gymnastics (GYM; n = 65), nongymnastic activities, or no organized activity (CON; n = 78). Participants had essentially no lifetime history of organized athletic participation (< 12 weeks). Pubertal maturation was assessed annually by a physician. Total body, lumbar spine, total proximal femur, and forearm BA, BMC, and aBMD were measured every 6 months using DXA (Hologic QDR-1000W). Independent samples t-tests determined baseline group differences. Nonlinear mixed effects models were used to model 24-month changes in bone data. In subset analyses, high-level gymnasts advancing to competition (HLG; n = 9) were compared with low-level nonadvancing gymnasts (LLG; n = 56).

RESULTS

At baseline, GYM were shorter, lighter, and had lower BA, BMC, and aBMD compared with CON (p < 0.05), whereas HLG did not differ significantly in these measurements compared with LLG (p > 0.05). Controlling for differences in race, baseline measures of body mass, height, and calcium intake, and change in breast development beyond stage II at 24 months, GYM had greater long-term (asymptotic) mean responses for total body aBMD and forearm BMC (p < 0.04) and greater rates of increase in the mean responses of lumbar spine aBMD and forearm BA compared with CON over 24 months. Over time, forearm BA increased to a greater extent in HLG compared with LLG (p < 0.01).

CONCLUSIONS

Females participating in recreational gymnastics initiated during childhood have enhanced bone mineral gains at the total body, lumbar spine, and forearm over 24 months. Higher-level training promotes additional gains in forearm BA.

Health-enhancing physical activity and sedentary behaviour in children and adolescents

We provide a wide-ranging review of health-related physical activity in children and adolescents using a behavioural epidemiology framework. In contrast to many other reviews, we highlight issues associated with true sedentary behaviours alongside physically active behaviours. Specifically, we review the evidence concerning the links between physical activity and cardiovascular disease, overweight and obesity, psychosocial measures, type II diabetes, and skeletal health. Although the evidence is unconvincing at times, several factors lead to the conclusion that promoting physical activity in youth is desirable. A review of the prevalence of physical activity and sedentary behaviours shows that many young people are active, but this declines with age. A substantial number are not adequately active for health benefits and current trends in juvenile obesity are a cause for concern. Prevalence data on sedentary behaviours are less extensive but suggest that total media use by young people has not changed greatly in recent years. Most children and adolescents do not exceed recommended daily hours of TV viewing. Physical activity is unrelated to TV viewing. We also identified the key determinants of physical activity in this age group, highlighting demographic, biological, psychological, behavioural, social and environmental determinants. Interventions were considered for school, family and community environments. Finally, policy recommendations are offered for the education, governmental, sport and recreation, health, and mass media sectors.

Bone outcomes and technical measurement issues of bone health among children and adolescents: considerations for nutrition and physical activity intervention trials

Weight-bearing physical activity and calcium intake are two important behavioral influences for bone health. Physical-activity and calcium-intake intervention trials with youth have been implemented to evaluate their efficacy and effectiveness to decrease the risk for subsequent osteoporosis and fractures. Technical aspects of bone measurement have not been routinely reported in published trials of youth, even though they can have an impact on study findings and interpretation. This paper provides an overview of the outcome variables reported, and technical issues--such as software and bone detection, growth, and movement--that affect bone mass measurements among children and adolescents. It describes the implications of these issues for the interpretation of intervention effects observed in intervention trials, and provides recommendations for future research.

Enhanced bone mass and physical fitness in young female handball players

This study evaluates the effect of physical activity on the bone content (BMC) and density (BMD) in 51 girls (14.2+/-0.4 yr). Twenty-four were placed in the handball group as they have been playing handball for at least 1 year (3.9+/-0.4). The other 27 who did not perform in any kind of regular physical activity other than that programmed during the compulsory physical education courses comprised the control group. Bone mass and areal density were measured by dual-energy X-ray absorptiometry (DXA). The maximal leg extension isometric force in the squat position with knees bent at 90 degrees and the peak force, mean power, and height jumped during vertical squat jump were assessed with a force plate. Additionally, 30-m run (running speed) and 300-m run (as an estimate of anaerobic capacity) tests were also performed. Maximal aerobic capacity was estimated using the 20-m shuttle-run tests. Compared to the controls, handballers attained better results in the physical fitness tests and had a 6% and 11% higher total body and right upper extremity lean mass (all P<0.05). The handballers showed enhanced BMC and BMD in the lumbar spine, pelvic region, and lower extremity (all P<0.05). They also showed greater BMC in the whole body and enhanced BMD in the right upper extremity and femoral neck than the control subjects (all P<0.05). As expected, total lean mass strongly correlated with total and regional BMC and BMD (r=0.79-0.91 P<0.001). Interestingly, 300-m running speed correlated with BMC and BMD variables (r=0.59-0.67 and r=0.60-0.70, respectively; all P<0.001). Multiple regression analysis showed that the 30-m running speed test, combined with the height and body mass, has also predictive value for whole-body BMC and BMD (R=0.93 and R=0.90, P<0.001). In conclusion, handball participation is associated with improved physical fitness, increased lean and bone masses, and enhanced axial and appendicular BMD in young girls. The combination of anthropometric and fitness-related variables may be used to detect girls with potentially reduced bone mass.

Physical Activity and Obesity in Canadian Women

HEALTH ISSUE: Overweight and obesity have been recognized as major public health concern in Canada and throughout the world. Lack of physical activity, through its impact on energy balance, has been identified as an important modifiable risk factor for obesity. Physical activity and obesity are also important risk factors for a variety of chronic diseases. This chapter provides an overview of the current state of physical activity and overweight/obesity among Canadian women. KEY FINDINGS: For all ages combined more women (57%) than men (50%) are physically inactive (energy expenditure <1.5 KKD). Physical activity increases as income adequacy and educational level decrease. Physical inactivity also varies by ethnicity. The prevalence of both overweight (BMI 25.0 - 29.9 kg/m2) and obese (BMI >/= 30 kg/m2) Canadian women has increased 7% since 1985. Obesity increases with age and is highest among women reporting low and lower middle incomes and lower levels of education. The prevalence of obesity is highest among Aboriginal women and men (28% and 22% respectively). DATA GAPS AND RECOMMENDATIONS: There is currently no surveillance system in Canada to monitor the level of physical activity among children, those performing activity at work, at school or in the home. There is a gap in the knowledge surrounding socio-cultural and ecological determinants of physical activity and obesity and the associations of these to chronic disease among women and minority populations. Multi-sectoral policy interventions that act to decrease the broad systemic barriers to physical activity and healthy weights among all women are needed.

Lifestyle factors and the development of bone mass and bone strength in young women

OBJECTIVE

To evaluate the contributions of adolescent calcium intake, oral contraceptive use, and exercise on bone mass and bone strength.Study design Eighty white women participated in 10 years of the Penn State Young Women's Health Study, a longitudinal study of community participants. We measured bone mineral mass (g), density (BMD, g/cm(2)), and body composition from dual energy x-ray absorptiometry and estimated proximal femur section modulus (bone bending strength). Calcium intake was determined from 45 days of prospective food records at regular intervals between the ages of 12 and 22 years. Exercise history and oral contraceptive use were assessed by questionnaire.

RESULTS

Daily calcium intakes between the ages of 12 and 22 years ranged from 500 to 1900 mg/d and were not significantly associated with bone gain or bone strength. Oral contraceptive use during adolescence was not correlated with bone or body composition measurements. Femoral neck BMD did not change from 17 to 22 years of age, but section modulus increased 3% (P <.05). Only exercise during adolescence was significantly associated with increased BMD and bone bending strength.

CONCLUSIONS

Adolescent lifestyle patterns can influence young adult bone strength. Our data suggest that exercise is the predominant lifestyle determinant of bone strength for this cohort.

Bone response to jumping is site-specific in children: a randomized trial

Skeletal loading during growth may be one way of increasing bone mass early in life. We hypothesized that children randomized to a jumping program (25 jumps/day from a 45-cm box, 5 days/week for 12 weeks) would have greater increases in hip and spine bone mineral content (BMC) and 4% distal tibia volumetric bone density than children randomized to the control group. Our secondary hypothesis was that jumping would not be as beneficial among peripubertal children as compared to prepubertal or pubertal children due to the relatively high growth rate that occurs during the peripubertal period. Fifty-four children (31 girls) ages 3-5, 7-8, 11-12, and 15-18 years were enrolled. We performed bone, anthropometric, and force plate measurements at baseline and 12 weeks. Twenty-four-hour diet recall and Tanner's self-report of pubertal development were completed at baseline. Jumpers had a lower calcium intake than nonjumpers at baseline (965 +/- 403 vs 1295 +/- 465 mg/day, P < 0.01), but the groups were otherwise similar. Overall, jumpers had greater increases in total body BMC (45.0 +/- 4.9 vs 29.4 +/- 5.3 g, P = 0.03) and regional dual energy x-ray absorptiometry leg BMC (19.8 +/- 2.6 vs 11.5 +/- 2.8 g, P = 0.03) than nonjumpers at all pubertal stages. However, the 4% distal tibia bone response to jumping appeared to be modified by pubertal stage, with the greatest bone benefit from jumping observed in pubertal children (interaction of jumping group by pubertal stage, P < 0.05, for both BMC and volumetric BMD). A similar pattern was observed for spine BMC (interaction, P = 0.10). We conclude that skeletal loading increases total body and leg BMC in children, but may not have a positive effect at sites that are predominantly trabecular bone during periods of rapid growth (i.e., peripubertal period).

Differences in bone density, body composition, physical activity, and diet between child gymnasts and untrained children 7-8 years of age

Strategies that enhance the acquisition of bone mass may be protective against osteoporosis. BMD was compared in 20 artistic gymnasts (10 boys; 10 girls) and 20 untrained children ages 7-8 years. Higher regional values of BMD were observed in female gymnasts than untrained girls. If retained to adulthood, this higher BMD may protect skeletal integrity in later life. Strategies that enhance the acquisition of bone mass in children may assist with the prevention of osteoporosis. This study explored the effects of regular high-impact and weight-bearing activity before the age of 7 years on total and regional bone mineral density (BMD). Twenty artistic gymnasts (10 boys and 10 girls) and 20 untrained children, 7-8 years of age, were recruited. The untrained children were matched to gymnasts by sex, height, weight, and age. Female gymnasts trained 8-10 h per week and had trained regularly for 3-4 years. Male gymnasts trained 4-6 h per week and had trained for 1-2 years. Measurements of bone mineral density were made using DXA for total body BMD (TBBMD); lumbar spine, both areal (aSBMD) and volumetric (vSBMD); total spine; pelvis; arms; and legs. Significant mean differences (8-10%) in aSBMD, vSBMD, arm BMD, and TBBMD were observed between female gymnasts and untrained girls (p < 0.05: aSBMD, vSBMD, and TBBMD body mass (BM); p < 0.01: arm BMD). A nonsignificant trend toward a higher TBBMD/BM and arm BMD was observed in male gymnasts compared with untrained boys. Trends toward a higher BMD within the pelvis, legs, and total spine were also observed in gymnasts. There were no differences in total and regional BMD between untrained boys and untrained girls. The results suggest that gymnastics training before the age of 7 years enhances the acquisition of bone mass at selected skeletal sites. The magnitude of this enhancement seems to be linked to the cumulative volume of such training. If retained during adolescence and young adulthood, a surfeit of bone acquired through high-impact and weight-bearing activity in early childhood may protect skeletal integrity in later life.

Randomized trial of physical activity and calcium supplementation on bone mineral content in 3- to 5-year-old children

A meta-analysis of adult exercise studies and an infant activity trial show a possible interaction between physical activity and calcium intake on bone. This randomized trial of activity and calcium supplementation was conducted in 239 children aged 3-5 years (178 completed). Children were randomized to participate in either gross motor or fine motor activities for 30 minutes/day, 5 days per week for 12 months. Within each group, children received either calcium (1000 mg/day) or placebo. Total body and regional bone mineral content by DXA and 20% distal tibia measurements by peripheral quantitative computed tomography (pQCT) were obtained at 0 and 12 months. Three-day diet records and 48-h accelerometer readings were obtained at 0, 6, and 12 months. Higher activity levels were observed in gross motor versus fine motor activity groups, and calcium intake was greater in calcium versus placebo (1354 +/- 301 vs. 940 +/- 258 mg/day, p < 0.001). Main effects of activity and calcium group were not significant for total body bone mineral content or leg bone mineral content by DXA. However, the difference in leg bone mineral content gain between gross motor and fine motor was more pronounced in children receiving calcium versus placebo (interaction, p = 0.05). Children in the gross motor group had greater tibia periosteal and endosteal circumferences by pQCT compared with children in the fine motor group at study completion (p < 0.05). There was a significant interaction (both p < or = 0.02) between supplement and activity groups in both cortical thickness and cortical area: among children receiving placebo, thickness and area were smaller with gross motor activity compared with fine motor activity, but among children receiving calcium, thickness and area were larger with gross motor activity. These findings indicate that calcium intake modifies the bone response to activity in young children.

Is there a critical period for bone response to weight-bearing exercise in children and adolescents? a systematic review

This systematic review examines and compares the bone mineral changes in children and adolescents, as measured by dual energy x ray absorptiometry, reported in exercise intervention studies. The effects of hormonal factors and growth on bone mineral change during puberty are examined, and the possibility of a critical period during which bone is especially adaptable to exercise is discussed.

Bone mineral response to a 7-month randomized controlled, school-based jumping intervention in 121 prepubertal boys: associations with ethnicity and body mass index

We examined the effects of a 7-month jumping intervention (10 minutes, 3 times per week) on bone mineral gain in prepubertal Asian and white boys (10.3+/-0.6 years, 36.0+/-9.2 kg) at 14 schools randomized to control (n = 60) and intervention (n = 61) groups. Intervention and control groups had similar mean baseline and change in height, weight, lean mass and fat mass, baseline areal bone mineral density (aBMD; g/cm2), bone mineral content (BMC; g; dual-energy X-ray absorptiometry [DXA], QDR 4500W), and similar average physical activity and calcium intakes. Over 7 months, the intervention group gained more total body (TB) BMC (1.6%,p < 0.01) and proximal femur (PF) aBMD (1%, p < 0.05) than the control group after adjusting for age, baseline weight, change in height, and loaded physical activity. We also investigated the 41 Asian and 50 white boys (10.2+/-0.6 years and 31.9+/-4.4 kg) who were below the 75th percentile (19.4 kg/m2) of the cohort mean for baseline body mass index (BMI). Boys in the intervention group gained significantly more TB and lumbar spine (LS) BMC, PF aBMD, and trochanteric (TR) aBMD (+ approximately2%) than boys in the control group (adjusted for baseline weight, final Tanner stage, change in height, and loaded physical activity). Bone changes were similar between Asians and whites. Finally, we compared the boys in the control group (n = 16) and the boys in the intervention group (n = 14) whose baseline BMI fell in the highest quartile (10.5+/-0.6 years and 49.1+/-8.2 kg). Seven-month bone changes (adjusted as aforementioned) were similar in the control and intervention groups. In summary, jumping exercise augmented bone mineral accrual at several regions equally in prepubertal Asian and white boys of average or low BMI, and intervention effects on bone mineral were undetectable in high BMI prepubertal boys.