Children and Adolescents in Sport and Exercise

16

Children and Adolescents in Sport and Exercise

chapter

Learning Objectives

• Understand the differences between the terms growth, development, and maturation

• Examine the processes by which bone, muscle, and fat mass increase in size with growth from birth to adulthood, how this relates to changes and rates of change in height and weight, and how these processes differ between boys and girls

• Learn how the nervous system matures as children grow, and how this might influence the development of balance, agility, and coordination

(continued)

Learning Objectives (continued)

• Find out how strength increases with age and what factors influence these changes

• Discover how children can safely improve their strength • Learn how training improves aerobic and anaerobic

capacities in prepubescent children• To understand how these physiological changes with

growth affect the motor ability and sports performance of the child and adolescent

• Examine the differences between children and adults with respect to thermoregulation

Terminology

Growth: an increase in the size of the body and/or of its parts

Development: the differentiation of cells along specialized lines of function (functional changes with growth)

Maturation: the process of taking on an adult form and becoming fully functional (defined by the system)• 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; sexual reproduction becomes possible

Adolescence: puberty to completion of growth and development

Changes With Age in the Rateof Increase in Height (cm/yr)

Height and Weight

Key Points• Growth in height is very rapid during the first 2 years of

life, with a child reaching 50% of adult stature by age 2• The rate of growth is slower throughout childhood until a

marked increase occurs near puberty• The peak rate of height growth occurs at age 12 in girls

and 14 in boys • Full height is typically achieved at age 16 in girls and 18

in boys• Growth in weight follows the same trend as height, with

the peak rate of weight increase occurring at age 12.5 in girls and 14.5 in boys

Bone Growth

• During growth, bone develops from cartilage• Fibrous membranes and cartilage transform into bone

by ossification• Growth is completed when cartilage cells stop growing

and growth plates are replaced by bone (by early 20s)• Requires a rich blood supply to deliver essential

nutrients• Calcium is required to build and maintain bone strength• Weight-bearing exercise increases bone width and

density

Muscle Growth

• In boys, skeletal muscle mass increases from 25% of body weight at birth to ~40-45% in young men

• Muscle development rate peaks at puberty in boys corresponding with a ~10-fold increase in testosterone

• In girls, skeletal muscle mass increases from 25% of body weight at birth to ~30-35% in young women

• Muscle development is slower in girls• Muscle development primarily results from hypertrophy

of existing fibers• Muscle length increases with bone growth due to an

increase in sarcomeres (added at the junction between the muscle and the tendon)

Growth and Fat Storage

• Fat cells form and fat deposition starts in fetal development

• Fat is stored by increasing the size and number of fat cells

• Fat storage depends on– Diet– Exercise habits – Heredity

• At birth, 10-12% of total body weight is fat• At maturity, fat content averages 15% of total body weight

in males and 25% in females

Changes in Skinfold Thicknessin Boys and Girls

Data from NHANES-I, National Center for Health Statistics.

Changes in Percent Fat, Fat Mass,and Fat-Free Mass for Females and Males

From Birth to 20 Years of Age

Reprinted, by permission, from R.M. Malina, C. Bouchard, and O. Bar-Or, 2004, Growth, maturation, and physical activity, 2nd ed. (Champaign, IL: Human Kinetics), 114.

Nervous System

• As children grow, they develop better balance, agility, and coordination

• Myelination of the nerve fibers must be completed for fast reactions and skilled movement

• Myelination of the cerebral cortex occurs most rapidly in childhood but continues well beyond puberty

• The full development of reaction speed and motor skills depends on completion of the myelination process

Tissue Growth and Development

Key Points• Muscle mass increases steadily along with weight

gain from birth through adolescence• In boys, the rate of muscle mass increase peaks at

puberty, when testosterone production increases dramatically. Girls do not experience this sharp increase in muscle mass

• Muscle mass increases result primarily from fiber hypertrophy with little or no hyperplasia

• Muscle mass peaks in girls between ages 18-20, and in boys between 18-25, although it can be further increased through diet and exercise

(continued)

Tissue Growth and Development (continued)

Key Points• Fat cells increase in size and number throughout life• The amount of fat accumulation depends on diet,

exercise habits, and heredity• At maturity, the body’s fat content averages 15% in

men and 25% in women• Balance, agility, and coordination improve as

children’s nervous systems develop• Myelination of nerve fibers must be completed before

fast reactions and skilled movements are fully developed

Strength

• Improves as muscle mass increases• Peak strength occurs ~ age 20 in women• Peak strength occurs between ages 20-30 in men

Gains With Age in Leg Strengthof Young Boys Followed Longitudinally

Over 12 Years

Data from H.H. Clarke, 1971, Physical and motor tests in the Medford boys' growth study (Englewood Cliffs, NJ: Prentice-Hall).

Changes in Strength With Developmental Status in Boys and Girls

Reprinted, by permission, from K. Froberg and O. Lammert, 1996, “Development of muscle strength during childhood.” In The child and adolescent athlete (London: Blackwell Publishing Company), 28.

Cardiovascular and Respiratory Function

Resting and Submaximal Exercise• Resting blood pressure is lower in children (proportional

to body size), but progressively increases during the late teen years

• Blood flow to active muscles per unit volume of muscle is greater

• Stroke volume is lower in children (smaller hearts)• Heart rate responses for a given absolute submaximal

work rate is higher (to compensate for reduced SV)• Cardiac output is somewhat lower for a given absolute

work rate• (a-v)O2 differences are increased to compensate for a

lower cardiac output

Submaximal (a) Heart Rate, (b) Stroke Volume, (c) Cardiac Output and (d) (a-v)O2 Difference in a

12-Year-Old Boy and Fully Mature Man

Cardiovascular and Respiratory Function

Maximal Exercise• Maximal heart rate is higher• Lower maximal cardiac output• Oxygen delivery limits performance

Lung Function• All lung volumes increase until growth is complete

• VEmax increases with age until physical maturity.

Physiological Responsesto Acute Exercise

Key Points• Strength improves and muscle mass increases

with age• Gains in strength with growth also depend on

neural maturation • Blood pressure is directly related to body size• Blood pressure is lower both at rest and during

exercise in children vs. adults• During both submaximal and maximal exercise, a

child’s smaller heart and blood volume result in a lower stroke volume vs. adults

• A child’s heart rate is higher than an adult’s for the same rate of work or VO2 (continued)

.

Physiological Responsesto Acute Exercise (continued)

Key Points• A child's cardiac output is less than an adult’s• During submaximal exercise there is an increase

in (a-v)O2 difference to ensure adequate oxygen delivery

• At maximal work rates, oxygen delivery limits performance

• Lung volumes increase until physical maturity, primarily because of increasing body size

• Until physical maturity, maximal ventilatory capacity and maximal expiratory ventilation increase in direct proportion to the increase in body size

Metabolic Function in Children

Aerobic Capacity• In boys, VO2max peaks between ages 17-21

• In girls, VO2max peaks between ages 12-15

• Little difference between children and adults if normalized for differences in body size

Running Economy• Economy is lower in children and improves with growth

(increased stride frequency)

.

.

Changes in Maximal Oxygen Uptake With Age

Metabolic Function in Children

Anaerobic Capacity• Ability to perform anaerobic activities is limited• Resting concentrations of ATP and PCr are similar to

adults• Children have lower glycolytic capacities• Lower lactate concentrations in the blood and muscle at

maximal and supramaximal rates of work • Children have lower RERs during maximal exercise

bouts, suggesting less lactate production• Anaerobic mean and peak power outputs are lower in

children, even when scaled for body mass

Optimal Peak Power Output (Anaerobic Power) Adjusted for Body Mass in

Preteenagers, Teenagers, and Adults

Data from A.M.C. Santos et al., 2002, "Age- and sex-related differences in optimal peak power," Pediatric Exercise Science 14: 202-212.

Development of Aerobic and Anaerobic Characteristics in Boys and Girls

Ages 9 to 16 Years

Adapted, by permission, from O. Bar-Or, 1983, Pediatric sports medicine for the practitioner: From physiologic principles to clinical applications (New York: Springer-Verlag).

Physiological Adaptationsto Training: Body Composition

With both resistance and aerobic training, boys and girls will:– ↓ Weight and fat mass– ↑ Fat-free mass (attenuated compared with

adolescents and adults)– ↑ Bone growth (bone mineral density)

Physiological Adaptationsto Training: Strength

With resistance training, boys and girls will:– ↑ Muscle strength (dependent on volume and

intensity of training)– Strength gains are accomplished without much

change in muscle size, indicating increased motor unit activation (neural adaptations)

(continued)

(continued)

Physiological Adaptationsto Training: Aerobic Capacity

Following aerobic training, children have:– Small ↑ in aerobic capacity in prepubescent children– Larger ↑ in aerobic capacity in adolescents– Differences appear to depend on heart growth

Physiological Adaptationsto Training: Anaerobic Capacity

Following anaerobic training, children have:– ↑ Resting concentrations of PCr, ATP, and glycogen– ↑ Phosphofructokinase activity– ↑ Maximal blood lactate concentrations

Physiological Adaptations to Training

Key Points• Body composition changes with training in children and

adolescents are similar to those seen in adults• The risk of injury from resistance training in young

athletes is relatively low• Strength gains are achieved from resistance training in

children primarily from neurological adaptations with little change in muscle size

• Aerobic training in preadolescents does not alter VO2max as much as would be expected from the training stimulus (heart size)

(continued)

.

Physiological Adaptations to Training (continued)

Key Points• Endurance performance improves with aerobic training

in preadolescents • A child’s anaerobic capacity increases with anaerobic

training• In general, growth and maturation rates and processes

are probably not altered significantly by training

Motor Ability and Sport Performance

Performance generally increases with age– Development of neuromuscular and endocrine

systems– Increased activity

Changes in Motor Abilities From the Ages of 6-17 Years

(continued)Data from the President’s Council on Physical Fitness and Sports, 1985.

Changes in Motor Abilities From the Ages of 6-17 Years (continued)

Data from the President’s Council on Physical Fitness and Sports, 1985.

U.S. National Record Performances for Boys and Girls

Thermal Stress and Children

• Children rely more on convection and radiation, which are enhanced through greater peripheral vasodilation

• Evaporative heat loss is lower because of reduced sweat rates

• Children have greater ratios of surface area to mass• Acclimatization to heat is slower in boys than in adult

men (no data available for girls)• Conductive heat loss is greater in the cold, increasing

risk for hypothermia• Exercising in extreme temperatures should be minimized

(limited data available)