116 Chapter 6FOCUS ON HEALTHmaking the most of muscles
Muscle cells adapt to the activity demanded of them.
When severe nerve damage or prolonged bed rest prevents
a muscle from being used, the muscle will rapidly begin
to waste away, or atrophy (a-trow-fee). Over time, affected
muscles can lose up to three-fourths of their mass, with a
corresponding loss of strength. It is more common for the
skeletal muscles of a sedentary person to stay basically
healthy, but to be less able to respond to physical demands
in the same way that well-worked muscles can.
the best way to maintain or improve the work capacity
of your muscles is to exercise them—that is, to increase
the demands on muscle fibers to contract. to increase
muscle endurance (how long a
muscle can sustain contractions),
nothing beats regular aerobic
exercise— activities such as brisk
walking, biking, jogging, swimming,
and aerobics classes (Figure 6.19).
aerobic exercise works muscles at
a rate at which the body can keep
them supplied with oxygen. It affects
muscle fibers in several ways:- there is an increase in the number and the size of
mitochondria, the organelles that make atp. - the number of blood capillaries supplying muscle tissue
increases. this increased blood supply brings more
oxygen and nutrients to the muscle tissue and removes
metabolic wastes more efficiently. - Muscle tissues contain more of the oxygen-binding
pigment myoglobin.
In general, exercise affects a muscle enzyme called LpL
(lipoprotein lipase). LpL allows a muscle to take up fatty
acids and triglycerides from the blood (see Section 11.12).
a low level of LpL is associated with increased risk of
cardiovascular disease. recent studies show that people
who sit for long periods tend to have low LpL, even if they
exercise at other times. experts recommend that people
who must sit for long periods take frequent short breaks to
stand up or walk, because activity that puts weight on leg
muscles encourages the steady production of LpL.
together, these changes produce skeletal muscles that
are more efficient metabolically, that maintain their tone,
and that can work longer without becoming fatigued.
Strength training involves intense, short- duration
exercise, such as weight lifting. It affects fast muscle fibers,
which form more myofibrils and make more of the enzymes
used in glycolysis (which forms some atp). these changes
translate into whole muscles that are larger and stronger,
but such bulging muscles fatigue rapidly so they don’t
have much endurance. Fitness experts generally
recommend a workout plan that combines strength
training and aerobic workouts.
Starting at about age 30,
the tension, or physical force,
a person’s muscles can muster
begins to decrease. this
means that, once you enter
your fourth decade of life, you
may exercise just as long and
intensely as a younger person
but your muscles cannot
adapt to the workouts to the
same extent. even so, being
physically active is extremely
beneficial. aerobic exercise
improves your endurance and
blood circulation, and even
modest strength training slows the loss of skeletal muscle
tissue that is an inevitable part of aging.© pio3/Shutterstock.comFigure 6.19 Physical activity is important for muscle health throughout life. A Aerobic exercise builds endurance and improves
overall muscle function. b Regularly sitting for long periods is associated with an increased risk of cardiovascular disease.b
© Prod-akszyn/Shutterstock.comA
© iStockphoto.com/nullplus6.8
aerobic exercise Exercise
that works muscles at a rate
that does not exceed the
body’s ability to keep them
supplied with oxygen (in
blood).
strength training Intense,
short-duration exercise that
produces larger, stronger
skeletal muscles.
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