motor areasof the frontal lobes generate electro-
chemical impulses that travel along motor nerves to
muscle fibers, causing the muscle fibers to contract.
For a movement to be effective, some muscles must
contract while others relax. When walking, for exam-
ple, antagonistic muscles on the front and back of the
thigh or the lower leg will alternate their contractions
and relaxations, and our steps will be smooth and effi-
cient. This is what we call coordination, and we do not
have to think about making it happen. Coordination
takes place below the level of conscious thought and is
regulated by the cerebellum, which is located below
the occipital lobes of the cerebrum.
MUSCLE TONE
Except during certain stages of sleep, most of our
muscles are in a state of slight contraction; this is what
is known as muscle tone. When sitting upright, for
example, the tone of your neck muscles keeps your
head up, and the tone of your back muscles keeps your
back straight. This is an important function of muscle
tone for human beings, because it helps us to maintain
an upright posture. For a muscle to remain slightly
contracted, only a few of the muscle fibers in that
muscle must contract. Alternate fibers contract so that
the muscle as a whole does not become fatigued. This
is similar to a pianist continuously rippling her fingers
over the keys of the piano—some notes are always
sounding at any given moment, but the notes that are
sounding are always changing. This contraction of
alternate fibers, muscle tone, is also regulated by the
cerebellum of the brain.
Muscle fibers need the energy of ATP(adenosine
triphosphate) in order to contract. When they pro-
duce ATP in the process of cell respiration, muscle
fibers also produce heat. The heat generated by nor-
mal muscle tone is approximately 25% of the total
body heat at rest. During exercise, of course, heat pro-
duction increases significantly.
EXERCISE
Good muscle tone improves coordination. When
muscles are slightly contracted, they can react more
rapidly if and when greater exertion is necessary.
Muscles with poor tone are usually soft and flabby, but
exercise will improve muscle tone.
There are two general types of exercise: isotonic
and isometric. In isotonic exercise, muscles contract
and bring about movement. Jogging, swimming, and
weight lifting are examples. Isotonic exercise improves
muscle tone, muscle strength, and, if done repetitively
against great resistance (as in weight lifting), muscle
size. This type of exercise also improves cardiovascu-
lar and respiratory efficiency, because movement
exerts demands on the heart and respiratory muscles.
If done for 30 minutes or longer, such exercise may be
called aerobic, because it strengthens the heart and res-
piratory muscles as well as the muscles attached to the
skeleton.
Isotonic contractions are of two kinds, concentric
or eccentric. A concentric contractionis the short-
ening of a muscle as it exerts force. An eccentric
contractionis the lengthening of a muscle as it still
exerts force. Imagine lifting a book straight up (or try
it); the triceps brachii contracts and shortens to
straighten the elbow and raise the book, a concentric
contraction. Now imagine slowly lowering the book.
The triceps brachii is still contracting even as it is
lengthening, exerting force to oppose gravity (which
would make the book drop quickly). This is an eccen-
tric contraction.
Isometric exerciseinvolves contraction without
movement. If you put your palms together and push
one hand against the other, you can feel your arm
muscles contracting. If both hands push equally, there
will be no movement; this is isometric contraction.
Such exercises will increase muscle tone and muscle
strength but are not considered aerobic. When the
body is moving, the brain receives sensory informa-
tion about this movement from the joints involved,
and responds with reflexes that increase heart rate and
respiration. Without movement, the brain does not
get this sensory information, and heart rate and
breathing do not increase nearly as much as they
would during an equally strenuous isotonic exercise.
Many of our actions involve both isotonic and iso-
metric contractions. Pulling open a door requires iso-
tonic contractions of arm muscles, but if the door is
then held open for someone else, those contractions
become isometric. Picking up a pencil is isotonic;
holding it in your hand is isometric. Walking uphill
involves concentric isotonic contractions, and may be
quite strenuous. Walking downhill seems easier, but is
no less complex. The eccentric isotonic contractions
involved make each step a precisely aimed and con-140 The Muscular System