Learning Objectives • Find out what age, height, and weight reach its peak rate of growth in boys and girls • Learn what changes occur with maximal submaximal HR and pulmonary function and with growth • Discover how growth affects SV and Q at fixed rates of work • Find out what absolute aerobic and cardiorespiratory endurance capacity increases from age 6 to age 20
Learning Objectives • Learn how training improves aerobic and anaerobic capacities prepubescent children • Discover how children can improve their strength safely • Review the effects of physical activity and regular training on a child’s growth and maturation • Examine the differences between children and adults in respect to thermoregulation
Terminology • Growth- an increase in the size of the body or its parts • Development- the functional changes that occur with growth • Maturation- process of taking on adult form and function • Measure or expressed in different ways: • Chronological age • Skeletal age • Stage of sexual maturation
Phases of Growth and Development • Infancy- first year of life • Childhood- age 1 to puberty • Puberty- development of secondary sex characteristics and capability of sexual reproduction • Usually 8-12 yrs of age • Adolescence- puberty to completion of growth and development
Bone Growth • Complete when cartilage cells stop growing and epiphyseal plates are replaced by bone • (by early 20s; varies from pre-teens to mid-20s; 2-3 yrs earlier in girls) • Requires rich blood supply to deliver essential nutrients
Bone Growth • Requires calcium to build and maintain strength • vitamin D promotes calcium absorption from the small intestine during digestion • Growth slows when blood calcium levels are too low • can lead to osteoporosis later in life • Helped by gravity-resisting exercise • loads the bone; affecting bone width, density, and strength (problems with astronauts)
Bone Injuries and Growth • Fractures of the epiphyseal plate: • Disrupts the blood supply • Disrupts growth, which leads to limb length discrepancies • Traumatic epiphysitis • Inflammation of epiphyseal plate from overuse (pitchers) • Can lead to separation of epiphysis • If caught early, can be treated without permanent damage
Muscle Growth • Growth in diameter results primarily from hypertrophy of existing fibers • Due to increase in myofilaments and myofibrils caused by loading of the muscles • Muscle length increases with bone growth • Due to increase in the number of sarcomeres in series
Muscle Growth • Boys’ muscle mass peaks at about 50% of body weight at 18 to 25 years • Girls’ muscle mass peaks at about 40% of body weight at 16 to 20 years
Growth and Fat Storage • Fat (triglycerides) stored starting at birth • Fat is stored by increasing the size and number of fat cells • Cells can only increase to a certain maximum volume and then new cells are formed • Fat storage depends on diet, exercise habits, and heredity • At maturity, fat content averages about 15% in males and about 25% in females
Tissue Growth and Development • Girls mature physiologically about 2 years earlier than boys • Balance, agility, and coordination improve as children’s nervous systems develop.
Tissue Growth and Development • Myelination of neurons in the cerebral cortex—which speeds the transmission of impulses in those neurons—is necessary before fast reactions and skills are fully developed. • Not usually completed until during adolescence.
Physiological Responses to Exercise • Strength increases • Gains in strength with growth also depend on neural maturation because neuromuscular control is limited until myelination is complete- usually around sexual maturity. • Blood volume, heart size (stroke volume), and blood pressure increase
Physiological Responses to Exercise • Resting heart rate decreases • Aerobic and anaerobic capacities and running economy increase • Lung volume and peak flow increase
Strength Gain in Age These data are for boys only.
Composite Strength Changes With Development PHV= Peak Height Velocity
Submaximal Exercise and Growth • Blood pressure: • Lower in children but progressively increases to adult levels in later teens • Larger body size results in higher blood pressure
Submaximal Exercise and Growth • Cardiovascular function at a given oxygen uptake: • Smaller heart size and total blood volume of children result in a lower SV • HR response is higher to compensate for the lower SV • Lower cardiac output than adults • Therefore, a higher a-vO2 difference than adults
Maximal Exercise and Growth • HRmax is higher in children but decreases linearly with age. • Maximal SV and Qmax are lower in children than in adults. • Lower oxygen delivery capacity (blood volume and pump capacity) limits performance at high absolute rates of work.
Lung Function and Growth • As body size increases, lung size and lung function increase. • Lung volumes and peak flow rates increase until growth is complete. • VEmax increases with age until physical maturity, then begins to decrease with age. • Boys' absolute lung volumes and peak flow rate values are higher than girls' absolute values due to girls’ smaller body size.
Aerobic Capacity in Children • VO2max (L/min) peaks around age 17 to 21 in males, then decreases linearly with age. • VO2max (L/min) has been shown to peak around age 12 to 15 in females • Decrease after age 15 may be due to females tending to reduce physical activity • Absolute VO2max (L/min) is lower in children than adults at similar training levels.
Aerobic Capacity in Children • When VO2max is expressed relative to body weight, there is little difference in aerobic capacity between adults and children, thus, additional muscle mass increases maximal oxygen consumption. • Relative to body weight, running economy is lower in children compared to adults.
Changes in VO2max With Age Relative to body weight (e.g. ml/kg/min) Absolute (e.g. L/min)
Anaerobic Capacity in Children • Ability to perform anaerobic activities is lower than in adults • Glycolytic capacity (e.g., glycolytic enzyme levels) is lower • Produce less lactate and cannot attain as high RER values during maximal exercise as adults • Anaerobic mean and peak power outputs are lower than in adults, even when scaled for body mass
Resistance Training in Preadolescence • May protect against injury and help build bones • Improves motor skill coordination • Increases strength largely through increased neural activation of motor units • Causes little change in muscle size (e.g., little hypertrophy) and is considered safe if not overdone
Theoretical Model for Strength Development for Boys What about for girls?
Training the Young Athlete • Training programs for children should be conservative to reduce the risk of injury, overtraining, and loss of interest in the sport. • An appropriate resistance training program is relatively safe for children. • Aerobic training improves cardiorespiratory endurance performance in children, but the changes in VO2max are less than expected.
Training the Young Athlete • Anaerobic capacity increases with anaerobic training. • Regular training typically results in decreased total body fat, increased fat-free mass, and increased total body mass. • Generally, training does not appear to significantly alter growth and maturation rates.
Motor Ability and Sport Performance • Motor ability in boys generally increases for the first 18 years of life • In girls, it tends to plateau around puberty. • Sports performance improves dramatically through childhood and adolescence.
400-m freestyle Age Group National Swimming Records 100-m freestyle
100-m run 1,500-m run Age Group U.S. National Track Records
Thermal Stress and Children • Evaporative heat loss is lower due to less sweat produced by sweat glands. • Acclimatization to heat is slower in boys than adult men • This presumably is also true in girls.
Thermal Stress and Children • Conductive heat loss and gain is greater because of child’s greater ratio of body surface area to mass • Increasing risk for hypothermia in cold environments and hyperthermia in extremely hot environments (e.g., when environmental temperature is higher than body temperature) • Exercising in extreme temperatures, both hot and cold, should be minimized in children.