Bone Density


Regular physical activity is also essential for the development and maintenance of bone mineral density. Te osteogenic force of physical activity is related to the local and specific mechanical loading of the bone. Te tensile and compressive forces of specific resistance or strength-producing programs can promote bone development. According to the National Strength and Conditioning Association position paper, when properly performed and supervised, a comprehensive strength and conditioning program is safe to initi- ate in children as young as 5 to 6 years old, although more typically a strength program begins at age 12.9 Additionally, numerous sport-specific studies have demonstrated targeted areas of bone development


based on the loading forces. Studies with gymnasts show improved bone density in wrists and the proximal portion of the femur given the nature of the sport.10,11


Although research results vary, consistent load-bear-


ing exercise in the prepubertal phase of adolescence can promote an increase in bone accrual of 0.6% to 1.7% per year.12


In a review article by Hind, prepubertal children who exercised regularly experienced


mean percent increases of 0.9% to 4.9% in bone parameters over 6 months in early pubertal development.13 Less bone accrual was realized in late adolescence or young adulthood, indicating the role of exercise in bone accrual during childhood and early puberty.8


A longitudinal study of soccer players demonstrated


that the osteogenic benefits of exercise in adolescents can have a positive impact on bone health aſter they complete their competitive years. Tirty years aſter they stopped playing, former soccer players had a higher bone mineral density and larger bone mass compared with sedentary counterparts, indicating that playing sports during adolescence may have long-lasting health benefits.14


In a review article by Tenforde


and Fredericson, the loading of bone in sports can be characterized by high impact (gymnastics, hurdling), loading impact (soccer, basketball), and nonimpact (swimming, cycling) sports.15


Higher impact loading


sports result in a favorable improvement in bone geometry, whereas nonimpact sports are either neutral or detrimental to bone accrual. Optimal bone development during childhood and adolescence is predicated on energy availability and a normal hormonal milieu.


Physical Activity and Obesity


Multiple studies have highlighted the known benefits of physical fitness and nutrition on health, including lower body fat, improved lipids, lower blood pressure, and improved insulin sensitivity in youth.7


Over the


past 25 years, the rate of obesity in US youth has doubled, and the rate in adolescents is even more alarm- ing. Te incidence of type 2 diabetes is increasing faster in the adolescent population than in adults, with an increase of 30.5% from 2001 to 2009.16


Sports and physical activity are an integral component of obesity


and type 2 diabetes prevention. Type 2 diabetes is oſten characterized by peripheral insulin resistance and the inability of the pancreatic β cells to maintain euglycemia in the fasted and postprandial state. Approx- imately 90% of the glucose uptake is in skeletal muscle in a healthy individual. However, a consequence of obesity is the reduction of this peripheral uptake of glucose to approximately 50% of baseline with a fur- ther reduction in those with type 2 diabetes.17


been shown to be directly related to insulin resistance in the pediatric population.18


Increased visceral fat distribution and hepatic steatosis have Te prevailing mech-


anism for the development of type 2 diabetes has historically centered on peripheral insulin resistance. Newer models have highlighted the role of mitochondrial dysfunction as an additional mechanism. Mito- chondrial dysfunction is characterized by alterations in oxidative phosphorylation, enzymatic defects, and lipid metabolism. Cardiorespiratory fitness is an indirect measure of oxidative phosphorylation, and exer- cise also plays a unique role in the improvement of mitochondrial function and quality. In mitochondrial dysfunction, lipid oxidation is impaired, and fatty acid metabolites accumulate, contributing to insulin resistance. Although the exact pathophysiology of type 2 diabetes in youth is yet to be completely eluci- dated, it is clear that consistent, vigorous exercise (> 80% maximal heart rate19


) can improve the markers


of insulin sensitivity, irrespective of changes in body composition. Te Diabetes Prevention Program has demonstrated that increased activity (150 minutes per week) and weight loss (5% to 7% of initial weight) in adults can reduce the development of diabetes in high-risk individuals, but less is known about chil- dren and teens. In children, increased adiposity, including increased visceral fat, has been linked with the


CHAPTER 12: CHILD AND ADOLESCENT ATHLETES 239


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