Developmental Essentials by IYCA, (08) The Endocrine System and Athletic Development, Notes

Post date: May 8, 2016 3:04:26 PM

Developmental Essentials by IYCA

Chapter 8: The Endocrine System and Athletic Development

• 1. Study questions

     1. Development is a continuum: it happens in stages not ages.

     2. Consider gender differences in hormonal and endocrine systems.

     3. How can one appropriately modify training for gender and developmental differences?

     4. What are the implications of trying to build muscle mass on a prepubescent boy?

     5. Identify potential danger of high level training in a pubescent girl.

     6. Endocrine function and diet choices

     7. Potential hormonal/endocrine effects of overtraining

  2. Introduction -The endocrine system is a collection of individual glands located in specific sites throughout the body. These glands produce one or more hormones that travel through the bloodstream to reach target organs. Hormones are chemical signals that control organs. The amount and timing of release is carefully controlled. The system responds to signals from other hormones and peptides, the central nervous system, the environment, light, sleep, stress, nutrient levels and exercise. This has both direct and indirect effects on athletic performance, controlling metabolism and electrolyte balance as well as hormonal control of growth and maturation.

• • 1. Growth Hormone - Is produced by the pituitary gland (located at base of the brain.) It controls growth and development during childhood and adolescence by stimulation of insulin like growth factor. Insulin like growth factor mobilizes fatty acids from adipose tissue to provide fuel for growth. Growth hormone levels gradually increase throughout childhood and reach stable levels between 12 and 18 years old. It is highest during the adolescent growth spurt, triggered by rising levels of estrogen and testosterone during puberty. By late adolescence growth, hormone production begins to decline and falls steadily through adult life

       1. Growth Hormone and Exercise - Physical activity increases growth hormone. It acts on muscle cell for growth and repair (protein synthesis during and post exercise.) Response to exercise increases with each stage of puberty.

    2. Thyroid Hormone - Produced by the thyroid gland in the neck. It’s main role is to regulate metabolism. It stimulates production of metabolic enzymes in muscle cells. Increases oxygen uptake and energy expenditure. It increases absorption of carbohydrates from the gastrointestinal tract, regulates mobilization of fatty acids from adipose tissue, increases the force of heart / muscle contractions. The thyroid hormone regulates blood pressure, the growth of fast and slow twitch muscle fibers during the fetal period, is essential for normal growth throughout childhood and regulates function of growth hormone during adolescence

• • • 1. The Thyroid Hormone and Exercise - involved in physiological adaptations that lead to aerobic and anaerobic improvements post training -- produces muscle metabolic enzymes and enhances nutrient and oxygen uptake by the muscle. Resting metabolic rate and thyroid hormone levels are higher in trained populations.

• • 3. Calcitonin - the thyroid also produces this hormone which regulates calcium levels in blood and bone

• • • 1. Calcitonin and Exercise - neither strength nor endurance exercise have an effect

• • 4. Parathyroid hormone - adjacent to the thyroid. It regulates calcium and phosphate in blood. When calcium levels become low, the parathyroid hormone stimulates bone to release calcium into the blood. Normal nerve and muscle function depend on tightly controlled blood to calcium levels

• • • 1. Parathyroid Hormone and Exercise - Increases after moderated aerobic and anaerobic exercise. The muscles extract calcium from the blood during exercise.

• • 5. Insulin - produced in islet cells of the the pancreas (located in the abdomen behind upper part of small intestine.) The “hormone of plenty” released in response to a meal. It acts to promote storage of excess carbohydrates, fats and protein. It regulates blood glucose, stimulates transfer of fatty acids from blood to adipose tissue for storage, stimulates protein synthesis and prevents protein breakdown. Insulin ncreases by about 30% during puberty.

• • • 1. Insulin and Exercise - Children demonstrate lower decreases in insulin from exercise. Training reduces insulin levels and insulin sensitivity and decreases insulin secretion. Adequate amounts of carbohydrates are needed for prolonged activity.

• • 6. Glucagon - produced in the pancreas. It acts on the liver to make energy (glucose) available between meals. Glucagon promotes conversion of amino acids into glucose for use as and energy source. It does not change significantly with puberty

• • • 1. Glucagon and Exercise - high intensity increases production of glucagon

• • 7. Prolactin - produced in pituitary gland. It is responsible for stimulating the increase in breast development during pregnancy and milk production afterwards. Prolactin stimulates maternal behavior. In males, the role is unclear. Children have lower heat tolerance. It regulates response to heat stress. Levels increase with puberty

• • • 1. Prolactin and Exercise - causes rise in prolactin levels. Levels increase is greater with each developmental stage. It increases the ability to acclimatize to heat as children progress through puberty. Intense training may cause a decrease in levels for adolescents.

• • 8. Adrenal Gland Hormones - adjacent to the kidneys. Produce catecholamines, the principal of which is epinephrine. It is released in response to the stress of low blood glucose. It increases heart rate, blood pressure, ventilation and blood glucose levels. Higher in boys than girls and reduces significantly with puberty.

• • • 1. Catecholamines and Exercise - stimulates their release

• • 9. Glucocorticoids - regulate metabolism of carbohydrates, protein and fat. They ensure appropriate response to stress. Glucocorticoids tap into stored energy sources in times of need. They have a role in bone mineral metabolism and modulate immune system and inflammatory processes. Cortisol is the main glucocorticoid. It follows the circadian rhythm. Levels do not change significantly throughout life.

• • • 1. Glucocorticoids and exercise - training stimulates cortisol release

• • 10. Mineralocorticoids - play a central role in maintenance of electrolyte balance and blood pressure. Aldosterone is the main mineralocorticoid. It acts on the kidneys, gastrointestinal tract, sweat glands and salivary glands to stimulate absorption of sodium

• • • 1. Mineralocorticoids and Exercise - exercise causes a rise in aldosterone levels same for children as in adults

• • 11. Androgens - byproducts of cortisol synthesis. They include dehydroepiandrosterone (DHEA), dehydroepiandrosterone sulfate (DHEA sulfate) and androstenedione. Levels increase between 6 and 10 years old and rise throughout puberty.

• • • 1. Androgens and Exercise - females demonstrate increase in androgens after training (may be hormonal marker of adaptation.)

• • 12. Testosterone - the male testes are the primary source. Small quantities are produced by conversion of adrenal androgens. Levels increase markedly during puberty stimulating growth of genitalia and secondary sexual characteristics (pubic, facial and axillary hair growth, deepening of voice.) Effects on muscle, bone and skin are secondary. It enhances growth hormone secretion in puberty. Testosterone causes greater bone deposition on outer surface (why bones are stronger in males than females.) Stimulates increase in red blood cell mass (development of increased aerobic capacity)

• • • 1. Testosterone and Exercise - males have greater aerobic capacity, muscle strength and bone density. Increases sweat rate and regulates electrolyte balance. Exercise in prepubertal males and females does not alter levels. Exercise in adolescence increases testosterone levels.

• • 13. Estrogen - in females, the ovaries produce both testosterone and estrogen. Levels vary throughout the menstrual cycle. Estrogen stimulates growth of female genitalia. It changes uterine lining during the menstrual cycle and increases fat mass. In males it is produced by enzymes in target tissues. Estrogen increases during puberty in both boys and girls. It contributes to bone density. It acts on growth plates to stimulate linear bone growth and regulates timing of growth spurts

• • • 1. Estrogen and Exercise - regular exercise regulates estrogen levels.

• • 14. Progesterone - produced by the ovaries. It is released during the latter half of the menstrual cycle. Progesterone prepares the internal lining for pregnancy and stimulates breast development.

• • • 1. Progesterone and Exercise - intense exercise alters levels and can cause irregularities in cycles. Response is greater as females near sexual maturation.

• 3. The Menstrual Cycle and Developing Athletes - the time from the beginning of one flow to the beginning of the next can be affected by race, economic status, heredity, nutrition and culture. 12.3 years average age of first flow (9-17 is considered normal range)

• • 1. The Menstrual Cycle and Exercise - monthly blood loss may cause anemia, lowering aerobic capacity. Appropriate exercise can reduce cramping and severity. However, frequent intense exercise causes irregularities--amenorrhea (6 months without period), oligomenorrhea (9 or fewer periods per year), chronic energy drain (if calories in are less than calories out the body shuts down ovaries) and bone loss which sets the athlete up for the risk of injury in the short term injury and premature osteoporosis in the long term.

• 4. Puberty and the Developing Athlete - Puberty is the transitional period between childhood and adulthood marked by the appearance of secondary sexual characteristics, achievement of reproductive capacity and rapid growth and changes in body composition.

• • 1. Puberty is orchestrated by the endocrine system and may impact athletic development. The onset and rate of progression varies from child to child. It is determined by genetic factors and multiple hormones: growth hormones, thyroid hormones, estrogen and testosterone. Boys begin puberty on average 11.6 years old and girls 11.2. Athletic girls start later.

    2. Prior to puberty, there is no significant physical difference from gender to gender in terms of body composition, strength, endurance, motor skill or injury risk. Body composition changes significantly with puberty. Peak velocity of muscle growth is greater.

       1. In boys, muscle mass goes from 80% to 85-90%, (strength development reaches maximum around 25 years.) In girls, muscle mass goes from 80% to 75%.

       2. Girls reach peak height velocity earlier than boys. On average the growth spurt is 3 to 5 centimeters more in boys than girls and lasts from 24 to 36 months.

       3. Peak weight velocity coincides with height velocity in boys. Girls reach peak weight velocity 6-9 months after.

       4. The shoulders become wider in boys. The upper to lower body ratio changes as the limbs elongate sooner than the trunk.

    3. Puberty and Exercise: The growth spurt affects coordination, center of mass (balance) and influences motor skill development. Girls may grow out of a sport (gymnastics.) The heart and lungs grow and max aerobic capacity increases. Tightness from growth can increase risk of overuse injuries. Hormonal changes affect everything from strength and motor development to nutrition.