This assignment involves collecting information from multiple chapters and putting the information together into a personalized plan for your overall health! You will be required to write a paper that includes the following: – Go to the National Institute of Health BMI calculation website listed in the “Links” portion of the Unit 2 page. Complete your BMI calculation. Measure your waist circumference in inches (with only light clothing on). How do your calculations fit into the healthy recommendations described in Ch. 6? Are you surprised at all by your measurements? Do you believe your BMI is an accurate description of your overall health?- Calculate your Target Heart Rate Zone to determine what level of intensity you should exercise at to achieve your goals. Please refer to your lecture notes and Unit 2 resources to determine how to calculate this. Make sure you include the recommended parameters of exercise intensity based on the information in Chapter 3. – Develop a goal for your overall health/fitness that follows the “SMART” goal-setting recommendations. You must address each of the five areas of the SMART acronym in describing your goal to me. Why are you setting this goal? What is your desired outcome? What importance does it have to you? (Contact me if you do not understand any of the five portions of the acronym). – Tell me how you will apply the FITT principle to your workouts. Some of you may want to work on your hamstring flexibility, others may be trying to achieve greater power in your vertical jumps, etc… Your FITT descriptions should be specific to your individual goal. You must describe to me what the optimal frequency, intensity, time, and type of exercise would be relative to your fitness objectives. Do not simply state that your are going to do strength training 2-3 days per week. Tell me what exercises you will do in the gym (bench press, plyometrics, lower body machines) for what muscle groups (quadriceps, biceps). How can you progress your FITT plan over the next few weeks/months to make improvements toward your goal? – This is your chance to set a REAL plan for yourself, whether you are a high-level athlete or an inactive person, please take this assignment seriously and reflect on what you specifically want to work on for your own physical fitness.https://youtu.be/IxADLxiqHuc
chapter 3
PHYSICAL DEVELOPMENT
AND BIOLOGICAL AGING
chapter outline
1 Body Growth and Change
3 Sleep
Learning Goal 1 Discuss major changes in the
body through the life span.
Learning Goal 3 Summarize how sleep
patterns change as people develop.
Patterns of Growth
Height and Weight in Infancy and Childhood
Puberty
Early Adulthood
Middle Adulthood
Late Adulthood
Why Do We Sleep?
Infancy
Childhood
Adolescence and Emerging Adulthood
Adulthood and Aging
2 The Brain
4 Longevity and Biological
Aging
Learning Goal 2 Describe how the brain
changes through the life span.
Learning Goal 4 Explain longevity and the
biological aspects of aging.
The Neuroconstructivist View
Brain Physiology
Infancy
Childhood
Adolescence
Adulthood and Aging
Life Expectancy and Life Span
Centenarians
Biological Theories of Aging
©Adam Smith/The Image Bank/Getty Images
preview
Think about how much you have changed physically and will continue to change as you age. We
come into this life as small beings. But we grow very rapidly in infancy, more slowly in childhood,
and once again more rapidly during puberty, and then experience another slowdown. Eventually
we decline, but many older adults are still physically robust. In this chapter, we explore changes
in body growth, the brain, and sleep across the life span. We also examine longevity and evaluate
some fascinating theories about why we age, and we explore both physical and physiological
aspects of development.
1 Body Growth and Change
Patterns of
Growth
Height and Weight in
Infancy and Childhood
LG1
Discuss major changes in the body through the life span.
Puberty
Early
Adulthood
Middle
Adulthood
Late
Adulthood
In life’s long journey, we go through many bodily changes. We grow up, we grow out, we
shrink. The very visible changes in height and weight are accompanied by less visible ones
in bones, lungs, and every other organ of the body. These changes will help shape how we
think about ourselves, how other people think about us, and what we are capable of thinking,
doing, and feeling. Are there strict timelines for these changes? Are they set in our genes?
Let’s begin by studying some basic patterns of growth and then trace bodily changes from
the time we are infants through the time we are older adults.
PATTERNS OF GROWTH
Two key patterns of growth are the cephalocaudal and proximodistal patterns. The cephalocaudal pattern is the sequence in which the fastest growth in the human body occurs at the
top, with the head. Physical growth in size, weight, and feature differentiation gradually works
its way down from the top to the bottom (for example, neck, shoulders, middle trunk, and so
on). This same pattern occurs in the head area, because the top parts of the head—the eyes
and brain—grow faster than the lower parts, such as the jaw. During prenatal development
and early infancy, the head constitutes an extraordinarily large proportion of the total body
(see Figure 1).
In most cases, sensory and motor development proceeds according to the cephalocaudal pattern. For example, infants see objects before they can control their torso, and
they can use their hands long before they can crawl or walk. However, one study contradicted the cephalocaudal pattern by finding that infants reached for toys with their feet
before using their hands (Galloway & Thelen, 2004). In this study, infants on average
first contacted the toy with their feet when they were 12 weeks old and with their hands
when they were 16 weeks old. Thus, contrary to long-standing beliefs, early leg movements can be precisely controlled, some aspects of development that involve reaching do
not involve lengthy practice, and early motor behaviors don’t always develop in a strict
cephalocaudal pattern.
The proximodistal pattern is the growth sequence that starts at the center of the body
and moves toward the extremities. An example is the early maturation of muscular control of
the trunk and arms, compared with that of the hands and fingers. Further, infants use the
whole hand as a unit before they can control several fingers.
HEIGHT AND WEIGHT IN INFANCY AND CHILDHOOD
Height and weight increase rapidly in infancy, then take a slower course during the childhood years.
SECTION 2
cephalocaudal pattern The sequence in
which the fastest growth occurs at the top of
the body—the head—with physical growth in
size, weight, and feature differentiation
gradually working from top to bottom.
proximodistal pattern The sequence in
which growth starts at the center of the body
and moves toward the extremities.
Biological Processes, Physical Development, and Health
87
1/2
1/3
1/4
1/5
1/6
1/7
1/8
2 months
5 months
Newborn
2
6
12
25
Fetal age
Years
FIGURE 1
CHANGES IN PROPORTIONS OF THE HUMAN BODY DURING GROWTH. As individuals develop from infancy through adulthood, one of the most
noticeable physical changes is that the head becomes smaller in relation to the rest of the body. The fractions listed refer to head size as a proportion of
total body length at different ages.
Infancy The average North American newborn is 20 inches long and weighs 7½ pounds.
Ninety-five percent of full-term newborns are 18 to 22 inches long and weigh between 5½
and 10 pounds.
In the first several days of life, most newborns lose 5 to 7 percent of their body weight.
Once infants adjust to sucking, swallowing, and digesting, they grow rapidly, gaining an average of 5 to 6 ounces per week during the first month. Typically they have doubled
their birth weight by the age of 4 months and have nearly tripled it by their first
birthday. Infants grow about 3/4 inch per month during the first year, increasing
their birth length by about 40 percent by their first birthday.
Infants’ rate of growth slows considerably in the second year of life (Hockenberry, Wilson, & Rodgers, 2017; Kliegman & others, 2016). By 2 years of age,
infants weigh approximately 26 to 32 pounds, having gained a quarter to half a
pound per month during the second year; at age 2 they have reached about onefifth of their adult weight. The average 2-year-old is 32 to 35 inches tall, which
is nearly one-half of adult height.
The bodies of 5-year-olds and 2-year-olds are different
from one another. The 5-year-old not only is taller and
heavier, but also has a longer trunk and legs than the
2-year-old. What might be some other physical differences
between 2- and 5-year-olds?
©Michael Hitoshi/Getty Images RF
88
CHAPTER 3
Early Childhood What is the overall growth rate like in early childhood?
As the preschool child grows older, the percentage of increase in height and
weight decreases with each additional year (Kliegman & others, 2016). Girls are
only slightly smaller and lighter than boys during these years. Both boys and girls
slim down as the trunks of their bodies lengthen. Although their heads are still
somewhat large for their bodies, by the end of the preschool years most children
have lost their top-heavy look. Body fat also shows a slow, steady decline during
the preschool years. Girls have more fatty tissue than boys; boys have more muscle tissue (McMahon & Stryjewski, 2012).
Growth patterns vary individually (Hockenberry, Wilson, & Rodgers, 2017;
Kliegman & others, 2016). Think back to your preschool years. This was probably the first time you noticed that some children were taller than you, some
shorter; some were fatter, some thinner; some were stronger, some weaker.
Much of the variation is due to heredity, but environmental experiences are also
involved. A review of the height and weight of children around the world concluded that two important contributors to height differences are ethnic origin
and nutrition (Meredith, 1978).
Why are some children unusually short? The culprits are congenital factors
(genetic or prenatal problems), growth hormone deficiency, a physical problem
Physical Development and Biological Aging
that develops in childhood, maternal smoking during pregnancy, or an emotional
difficulty (Hay & others, 2017; Krebs & others, 2016). A recent study
of children born small for gestational age or short stature revealed that
five years of growth hormone treatment in childhood was linked to an
increase to close to average height (Ross & others, 2015). Also, a
recent review concluded that accurate assessment of growth hormone deficiency is difficult and that many children diagnosed with
growth hormone deficiency re-test normal later in childhood
(Murray, Dattani, & Clayton, 2016).
In sum, the main factors that contribute to children’s height
are genetic influences, ethnic origin, and nutrition.
Middle and Late Childhood The period of middle and
PUBERTY
Puberty is a brain-neuroendocrine process occurring primarily in early adolescence that provides stimulation for the rapid physical changes that take place during this period of development (Susman & Dorn, 2013). In this section, we explore a number of puberty’s physical
changes and its psychological accompaniments.
Sexual Maturation, Height, and Weight
Think back to the onset of your puberty.
Of the striking changes that were taking place in your body, what was the first to occur?
Researchers have found that male pubertal characteristics typically develop in this order:
increase in penis and testicle size, appearance of straight pubic hair, minor voice change, first
ejaculation (which usually occurs through masturbation or a wet dream), appearance of curly
pubic hair, onset of maximum growth in height and weight, growth of hair in armpits, more
detectable voice changes, and, finally, growth of facial hair.
What is the order of appearance of physical changes in females? First, for most girls,
their breasts enlarge or pubic hair appears. Later, hair appears in the armpits. As these changes
occur, the female grows in height and her hips become wider than her shoulders.
Menarche—a girl’s first menstruation—comes rather late in the pubertal cycle. Initially, her menstrual cycles may be highly irregular. For the first several years, she may not
ovulate every menstrual cycle; some girls do not ovulate at all until a year or two after
menstruation begins.
Marked weight gains coincide with the onset of puberty. During early adolescence, girls
tend to outweigh boys, but by about age 14 boys begin to surpass girls. Similarly, at the
beginning of the adolescent period, girls tend to be as tall as or taller than boys of their age,
but by the end of the middle school years most boys have caught up or, in many cases, surpassed girls in height.
As indicated in Figure 2, the growth spurt occurs approximately two years earlier for girls
than for boys. The mean age at the beginning of the growth spurt in girls is 9; for boys, it is 11.
The peak rate of pubertal change occurs at 11½ years for girls and 13½ years for boys. During
their growth spurt, girls increase in height about 3½ inches per year, boys about 4 inches.
SECTION 2
Height gain (inches/year)
late childhood involves slow, consistent growth (Kliegman &
others, 2016). This is a period of calm before the rapid growth
spurt of adolescence.
During the elementary school years, children grow an average of 2 to 3 inches a year. At the age of 8, the average girl and
the average boy are 4 feet 2 inches tall. During the middle and
late childhood years, children gain about 5 to 7 pounds a year.
What characterizes children’s physical growth in middle and late childhood?
The average 8-year-old girl and the average 8-year-old boy ©RubberBall Productions/Getty Images RF
weigh 56 pounds. The weight increase is due mainly to increases
in the size of the skeletal and muscular systems, as well as the size of some body organs.
Muscle mass and strength gradually increase as “baby fat” decreases in middle and late child5.0
hood (Kliegman & others, 2016).
4.5
Changes in proportions are among the most pronounced physical changes in middle and
4.0
late childhood. Head circumference, waist circumference, and leg length decrease in relation
to body height (Hockenberry, Wilson, & Rodgers, 2017).
3.5
Females
3.0
Males
2.5
2.0
1.5
1.0
0.5
0
2
4
6 8 10 12
Age (years)
14
16 18
FIGURE 2
PUBERTAL GROWTH SPURT. On average,
the peak of the growth spurt during puberty
occurs two years earlier for girls (11½) than
for boys (13½). How are hormones related to
the growth spurt and to the difference
between the average height of adolescent
boys and that of girls?
Source: Tanner, J.M., et al., “Standards from
Birth to Maturity for Height, Weight, Height
Velocity: British Children in 1965,” Archives of
Diseases in Childhood vol. 41, no. 219, 1966,
p. 454–471.
puberty A brain-neuroendocrine process
occurring primarily in early adolescence that
provides stimulation for the rapid physical
changes that occur in this period of
development.
menarche A girl’s first menstrual period.
Biological Processes, Physical Development, and Health
89
Hypothalamus: A structure in the brain that interacts with the
pituitary gland to monitor the bodily regulation of hormones.
Pituitary: This master gland produces hormones that stimulate
other glands. It also influences growth by producing growth
hormones; it sends gonadotropins to the testes and ovaries and
a thyroid-stimulating hormone to the thyroid gland. It sends a
hormone to the adrenal gland as well.
Thyroid gland: It interacts with the pituitary gland to influence
growth.
Adrenal gland: It interacts with the pituitary gland and likely
plays a role in pubertal development, but less is known
about its function than about sex glands. Recent research,
however, suggests it may be involved in adolescent
behavior, particularly for boys.
The gonads, or sex glands: These consist of the testes in
males and the ovaries in females. The sex glands are strongly
involved in the appearance of secondary sex characteristics,
such as facial hair in males and breast development in
females. The general class of hormones called estrogens is
dominant in females, while androgens are dominant in males.
More specifically, testosterone in males and estradiol in
females are key hormones in pubertal development.
FIGURE 3
THE MAJOR ENDOCRINE GLANDS
INVOLVED IN PUBERTAL CHANGE
hormones Powerful chemical substances
secreted by the endocrine glands and carried
through the body by the bloodstream.
hypothalamus A structure in the brain that is
involved with eating and sexual behavior.
pituitary gland An important endocrine gland
that controls growth and regulates the activity
of other glands.
gonads The sex glands, which are the testes
in males and the ovaries in females.
gonadotropins Hormones that stimulate the
testes or ovaries.
testosterone A hormone associated in boys
with the development of the genitals,
increased height, and voice changes.
estradiol A hormone associated in girls with
breast, uterine, and skeletal development.
90
CHAPTER 3
Hormonal Changes Behind the first whisker in boys and the widening of hips in girls
is a flood of hormones, powerful chemical substances secreted by the endocrine glands and
carried through the body by the bloodstream (Herting & Sowell, 2017). The endocrine system’s role in puberty involves the interaction of the hypothalamus, the pituitary gland, and
the gonads (see Figure 3). The hypothalamus, a structure in the brain, is involved with eating
and sexual behavior. The pituitary gland, an important endocrine gland, controls growth and
regulates other glands; among these, the gonads—the testes in males, the ovaries in females—
are particularly important in giving rise to pubertal changes in the body.
How do the gonads, or sex glands, work? The pituitary gland sends a signal via gonadotropins (hormones that stimulate the testes or ovaries) to the appropriate gland to manufacture
hormones. These hormones give rise to such changes as the production of sperm in males
and menstruation and the release of eggs from the ovaries in females. The pituitary gland,
through interaction with the hypothalamus, detects when the optimal level of hormones is
reached and maintains it with additional gonadotropin secretion (Susman & Dorn, 2013). Not
only does the pituitary gland release gonadotropins that stimulate the testes and ovaries, but
through interaction with the hypothalamus the pituitary gland also secretes hormones that
either directly lead to growth and skeletal maturation or produce growth effects through
interaction with the thyroid gland, located at the base of the throat.
The concentrations of certain hormones increase dramatically during adolescence (Piekarski & others, 2017). The concentrations of two key hormones increase in puberty, and the
changes are very different in boys and girls:
? Testosterone is a hormone associated in boys with the development of genitals,
increased height, and deepening of the voice.
? Estradiol is a type of estrogen associated in girls with breast, uterine, and skeletal
development.
A recent study documented the growth of the pituitary gland in adolescence and found
that its volume was linked to circulating blood levels of estradiol and testosterone (Wong &
Physical Development and Biological Aging
others, 2014). In one study, testosterone levels increased eighteenfold in boys but only twofold
in girls during puberty; estradiol increased eightfold in girls but only twofold in boys (Nottelmann & others, 1987). Thus, both testosterone and estradiol are present in the hormonal makeup
of both boys and girls, but testosterone dominates in male pubertal development, estradiol in
female pubertal development (Richmond & Rogol, 2007). A recent study of 9- to 17-year-old
boys found that testosterone levels peaked at 17 years of age (Khairullah & others, 2014).
The same influx of hormones that grows hair on a male’s chest and increases the fatty
tissue in a female’s breasts may also contribute to psychological development in adolescence
(Wang & others, 2017). In one study of boys and girls ranging in age from 9 to 14, a higher
concentration of testosterone was present in boys who rated themselves as more socially competent (Nottelmann & others, 1987). However, a recent research review concluded that there
is insufficient quality research to confirm that changing testosterone levels during puberty are
linked to mood and behavior in adolescent males (Duke, Balzer, & Steinbeck, 2014).
Hormonal effects by themselves do not account for adolescent psychological development
(Graber, 2008). For example, in one study, social factors were much better predictors of young
adolescent girls’ depression and anger than hormonal factors (Brooks-Gunn & Warren, 1989).
Behavior and moods also can affect hormones. Stress, eating patterns, exercise, sexual activity, tension, and depression can activate or suppress various aspects of the hormonal system.
In sum, the hormone-behavior link is complex (Susman & Dorn, 2013).
Timing and Variations in Puberty In the United States—where children mature up
to a year earlier than children in European countries—the average age of menarche has declined
significantly since the mid-nineteenth century. Fortunately, however, we are unlikely to see pubescent toddlers, since what has happened in the past century is likely the result of improved nutrition
and health, and the rate of decline in age of onset of puberty has slowed considerably in the
last several decades. However, some researchers recently have found that the onset of puberty is
still occurring earlier in girls and boys (Herman-Giddens & others, 2012; McBride, 2013).
Is age of pubertal onset linked to how tall boys and girls will be toward the end of
adolescence? A recent study found that for girls, earlier onset of menarche, breast development, and growth spurt were linked to shorter height at 18 years of age; however, for boys,
earlier age of growth spurt and slower progression through puberty were associated with being
taller at 18 years of age (Yousefi & others, 2013).
Why do the changes of puberty occur when they do, and how can variations in their timing
be explained? The basic genetic program for puberty is wired into the species (Dvornyk &
Waqar-ul-Haq, 2012). However, nutrition, health, family stress, and other environmental factors
also affect puberty’s timing (Susman & Dorn, 2013; Villamor & Jansen, 2016). A cross-cultural
study of 48,000 girls in 29 countries found that childhood obesity was linked to early puberty
(Currie & others, 2012). A recent study found that child sexual abuse was linked to earlier
pubertal onset (Noll & others, 2017).
For most boys, the pubertal sequence may begin as early as age 10 or as
late as 13½, and it may end as early as age 13 or as late as 17. Thus, the normal
range is wide enough that, given two boys of the same chronological age, one
might complete the pubertal sequence before the other one has begun it. For
girls, menarche is considered within the normal range if it appears between the
ages of 9 and 15.
Psychological Accompaniments of Puberty
What are some
links between puberty and psychological characteristics? How do early and
late maturation influence adolescents’ psychological development?
Body Image
One psychological aspect of puberty is certain for both
boys and girls: Adolescents are preoccupied with their bodies (SeninCalderon & others, 2017; Solomon-Krakus & others, 2017). At this age you
may have looked in the mirror on a daily, and sometimes even hourly, basis
to see if you could detect anything different about your changing body. Preoccupation with one’s body image is strong throughout adolescence but it is
especially acute during puberty, a time when adolescents are more dissatisfied
with their bodies than in late adolescence. A recent study found that an
increase in Facebook friends across two years in adolescence was linked to
an enhanced motivation to be thin (Tiggemann & Slater, 2017).
SECTION 2
Adolescents show a strong preoccupation with their changing
bodies and develop images of what their bodies are like. Why
might adolescent males have more positive body images than
adolescent females?
©age fotostock/SuperStock
Biological Processes, Physical Development, and Health
91
Gender Differences Gender differences characterize adolescents’ perceptions
of their bodies (Hoffman & Warschburger, 2017). Girls tend to have more negative
body images, which to some extent may be due to media portrayals of the attractiveness of being thin while the percentage of girls’ body fat is increasing during puberty
(Benowitz-Fredericks & others, 2012). One study found that both boys’ and girls’
body images became more positive as they moved from the beginning to the end of
adolescence (Holsen, Carlson Jones, & Skogbrott Birkeland, 2012).
Early and Late Maturation
Did you enter puberty early, late, or on time?
When adolescents mature earlier or later than their peers, they may have different
experiences and perceive themselves differently (Lee & others, 2017; Wang & others, 2017). A recent study found that in the early high school years, late-maturing
boys had a more negative body image than early-maturing boys (de Guzman &
Nishina, 2014). Similarly, in the Berkeley Longitudinal Study conducted half a
What are some outcomes of early and late maturation in
century ago, early-maturing boys perceived themselves more positively and had
adolescence?
more successful peer relations than did late-maturing boys (Jones, 1965). The find©Fuse/Getty Images RF
ings for early-maturing girls were similar but not as strong as for boys. When the late-maturing
boys were in their thirties, however, they had developed a more positive identity than the
early-maturing boys had (Peskin, 1967). Perhaps the late-maturing boys had had more time to
explore life’s options, or perhaps the early-maturing boys continued to focus on their physical
status instead of paying attention to career development and achievement.
An increasing number of researchers have found that early maturation increases girls’ vulnerability to a number of problems (Graber, 2013; Hamilton & others, 2014). Early-maturing girls
developmental connection
are more likely to smoke, drink, be depressed, have an eating disorder, struggle for earlier indeSexuality
pendence from their parents, and have older friends (Negriff, Susman, & Trickett, 2011; Verhoef
Early sexual experience is one of a
& others, 2014). Researchers have found that early-maturing girls tend to engage in sexual intercourse earlier and have more unstable sexual relationships (Moore, Harden, & Mendle, 2014).
number of risk factors in adolescent
For example, in a recent Korean study, early menarche was associated with risky sexual behavior
development. Connect to “Gender
in females (Cheong & others, 2015). Another study found that early-maturing girls’ higher level
and Sexuality.”
of internalizing problems (depression, for example) was linked to their heightened sensitivity to
interpersonal stress (Natsuaki & others, 2010). A recent study found that early maturation predicted a stable higher level of depression for adolescent girls (Rudolph & others, 2014). And
early-maturing girls are more likely to drop out of high school and to cohabit and marry at younger
ages (Cavanagh, 2009). Apparently as a result of their social and cognitive immaturity, combined
with early physical development, early-maturing girls are easily lured into problem behaviors, not
recognizing how these behaviors might affect their development. Thus, early-maturing adolescents, especially girls, require earlier risk education efforts related to sexual development, risky
behaviors, relationships, and Internet safety than their on-time peers (Susman & Dorn, 2013).
In sum, early maturation often has more favorable outcomes for boys than for girls,
especially in early adolescence. However, late maturation may be more favorable for boys,
especially in terms of identity and career development. Research increasingly has found that
early-maturing girls are vulnerable to a number of problems.
EARLY ADULTHOOD
After the dramatic physical changes of puberty, the years of early adulthood might seem to
be an uneventful time in the body’s history. Physical changes during these years may be
subtle, but they do continue.
Height remains rather constant during early adulthood. Peak functioning of the body’s joints
usually occurs in the twenties. Many individuals also reach a peak of muscle tone and strength
in their late teens and twenties (Candow & Chilibeck, 2005). However, these attributes may begin
to decline in the thirties. Sagging chins and protruding abdomens may also appear for the first
time. Muscles start to have less elasticity, and aches may appear in places not felt before.
Most of us reach our peak levels of physical performance before the age of 30, often
between the ages of 19 and 26. This peak of physical performance occurs not only for the
average young adult, but for outstanding athletes as well. Different types of athletes, however,
reach their peak performances at different ages. Most swimmers and gymnasts peak in their
late teens. Golfers and marathon runners tend to peak in their late twenties. In other areas of
athletics, peak performance often occurs in the early to mid-twenties. However, in recent
92
CHAPTER 3
Physical Development and Biological Aging
years, some highly conditioned athletes—such as Dana Torres (Olympic swimming) and Tom
Brady (football)—have stretched the upper age limits of award-winning performances.
MIDDLE ADULTHOOD
Like the changes of early adulthood, midlife physical changes are usually gradual. Although
everyone experiences some physical change due to aging in middle adulthood, the rates of
aging vary considerably from one individual to another. Genetic makeup and lifestyle factors
play important roles in whether and when chronic diseases will appear (Koenig, Lincoln, &
Garg, 2017; Nasef, Mehta, & Ferguson, 2017; Theendakara & others, 2016). Middle age is a
window through which we can glimpse later life while there is still time to engage in preventive
behaviors and influence the course of aging (Lachman, Teshale, & Agrigoroaei, 2015).
Physical Appearance Individuals lose height in middle age, and many gain weight
(Haftenberger & others, 2016; Yang & others, 2017). On average, from 30 to 50 years of age, men
lose about 1/2 inch in height, then lose another 1/2 inch from 50 to 70 years of age (Hoyer &
Roodin, 2009). The height loss for women can be as much as 2 inches from 25 to 75 years of age.
Note that there are large variations in the extent to which individuals become shorter with aging.
The decrease in height is due to bone loss in the vertebrae. On average, body fat accounts for about
10 percent of body weight in adolescence; it makes up 20 percent or more in middle age.
Noticeable signs of aging usually are apparent by the forties or fifties. The skin begins to
wrinkle and sag because of a loss of fat and collagen in underlying tissues (Miyawaki & others, 2016). Small, localized areas of pigmentation in the skin produce aging spots, especially
in areas that are exposed to sunlight, such as the hands and face. A twin study found that twins
who had been smoking longer were more likely to have more sagging facial skin and wrinkles,
especially in the middle and lower portion of the face (Okada & others, 2013). The hair thins
and grays because of a lower replacement rate and a decline in melanin production.
Since a youthful appearance is valued in many cultures, many Americans strive to make
themselves look younger. Undergoing cosmetic surgery, dyeing hair, purchasing wigs, enrolling in
weight reduction programs, participating in exercise regimens, and taking heavy doses of vitamins
are common in middle age. Baby boomers have shown a strong interest in plastic surgery and
Botox, which may reflect their desire to take control of the aging process (Solish & others, 2016).
Strength, Joints, and Bones
The term sarcopenia is given to age-related loss of lean
muscle mass and strength (Bianchi & others, 2016; Marzetti & others, 2017). After age 50, muscle
loss occurs at a rate of approximately 1 to 2 percent per year. A loss of strength especially occurs
in the back and legs. Obesity is a risk factor for sarcopenia (Cruz-Jentoft & others, 2017). Recently,
researchers began using the term “sarcopenic obesity” in reference to individuals who have sarcopenia and are obese (Ma & others, 2016; Yang & others, 2017). One study linked sarcopenic
obesity to hypertension (Park & others, 2013). Also, in