Transcript Presentation Package

chapter
16
Children and
Adolescents in
Sport and
Exercise
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 Rate
of 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 Thickness
in 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 Strength
of 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 Responses
to 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 Responses
to 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 Adaptations
to 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 Adaptations
to 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 Adaptations
to 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 Adaptations
to 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
Data from the President’s Council on Physical Fitness and Sports, 1985.
(continued)
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)