s7pe-31002-ca
12/20/05
11:17 PM
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Page 362
Muscles grow and heal.
An infant’s muscles cannot do very much. A newborn cannot lift and
hold its head up. The neck muscles are not strong enough to support
the baby’s heavy head. For the first few months of life, a baby needs
extra support until these muscles grow strong. The rest of the skeletal
muscles also have to develop and strengthen. During infancy and childhood and into adolescence, humans develop muscular coordination and
become more graceful in their movements.
Stretching the muscles
before exercise can help
prevent injury.
When you exercise regularly, your
muscles may get bigger. However, the
number of muscle fibers does not increase.
Muscles increase in size and strength
with some types of exercise, because
the thickness of the muscle fibers and
surrounding tissues increases.
You may have experienced sore muscles
during or after exercise. During exercise, a
chemical called lactic acid can build up in
the muscles. This chemical can make your
muscles cramp or ache. When the muscle
cells do not receive enough oxygen for
cellular respiration, they switch to
fermentation for energy. Fermentation
causes the buildup of lactic acid.
The muscle soreness you may feel a day or so after exercise can be
caused by damage to the muscle fibers. The muscle fibers are sometimes stretched too much or even torn. Such injuries take time to heal,
because the body must remove or repair any injured cells.
check your re
What causes muscles to ache?
KEY CONCEPTS
CRITICAL THINKING
1. What are the three main
functions of the muscular
system? (7.5.c)
4. Apply You are exercising and
you begin to feel hot. Explain
what is happening in your
muscles.
2. Make a rough outline of a
human body and label where
you could find each of the
three types of muscles. (7.5.c)
5. Analyze Describe what happens in your neck muscles
when you nod your head.
3. Explain how muscles, joints,
and tendons produce movement. (7.5.c)
362 Unit 3: Structure and Function in Living Systems
CHALLENGE
6. Infer The digestive system
breaks down food and transports materials. How are the
short length and slow movement of smooth muscle tissues
in the stomach and intestines
related to the functions of
these organs?
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