298
UNIT 2
Covering, Support, and Movement of the Body
9
immediately after he grabs the bar? As his body begins to
move upward toward the bar? When his body begins to
approach the mat?
For answers, see Appendix H.
Muscle Metabolism
Describe three ways in which ATP is regenerated during
skeletal muscle contraction.
Define EPOC and muscle fatigue. List possible causes of
muscle fatigue.
Providing Energy for Contraction
As a muscle contracts, ATP supplies the energy to move and
detach cross bridges, operate the calcium pump in the SR, and
return Na
1
and K
1
to the cell exterior and interior respectively
afer excitation-contraction coupling. Surprisingly, muscles store
very limited reserves oF ATP—4 to 6 seconds’ worth at most, just
enough to get you going. Because ATP is the
only
energy source
used directly For contractile activities, it must be regenerated as
Fast as it is broken down iF contraction is to continue.
±ortunately, afer ATP is hydrolyzed to ADP and inorganic
phosphate in muscle fibers, it is regenerated within a Fraction oF
a second by one or more oF the three pathways summarized in
Figure 9.19
: (1) direct phosphorylation oF ADP by creatine phos-
phate, (2) anaerobic, glycolysis, which converts glucose to lactic
acid, and (3) aerobic respiration. All body cells use glycolysis and
aerobic respiration to produce ATP, so we touch on them here but
describe them in detail later, in Chapter 24.
Direct Phosphorylation of ADP by Creatine Phosphate (Fig-
ure 9.19a)
As we begin to exercise vigorously, the demand For
ATP soars and consumes the ATP stored in working muscles
within a Few twitches. ²en
creatine phosphate (CP)
(kre
9
ah-
tin), a unique high-energy molecule stored in muscles, is tapped
to regenerate ATP while the metabolic pathways adjust to the
suddenly higher demand For ATP.
Coupling CP with ADP transFers energy and a phosphate
group From CP to ADP to Form ATP almost instantly:
creatine
kinase
Creatine phosphate
1
ADP
____
→ creatine
1
ATP
Muscle cells store two to three times more CP than ATP. ²e
CP-ADP reaction, catalyzed by the enzyme
creatine kinase
, is
so efficient that the amount oF ATP in muscle cells changes very
little during the initial period oF contraction.
Together, stored ATP and CP provide For maximum muscle
power For about 15 seconds—long enough to energize a 100-meter
dash (slightly longer iF the activity is less vigorous). ²e coupled
reaction is readily reversible, and to keep CP “on tap,” CP reserves
are replenished during periods oF rest or inactivity.
Anaerobic Pathway: Glycolysis and Lactic Acid Formation
(Figure 9.19b)
As stored ATP and CP are exhausted, more
ATP is generated by breaking down (catabolizing) glucose ob-
tained From the blood or glycogen stored in the muscle. ²e
initial phase oF glucose breakdown is
glycolysis
(gli-kol
9
ĭ-sis;
but it may be that the muscle stretching that occurs during eccen-
tric contractions causes microtears in the muscles.
Biceps curls provide a simple example oF how concentric and
eccentric contractions work together in our everyday activities.
When you flex your elbow to raise this textbook to your shoul-
der, the biceps muscle in your arm is contracting concentrically.
When you straighten your arm to return the book to the desk-
top, the isotonic contraction oF your biceps is eccentric. Basi-
cally, eccentric contractions put the body in position to contract
concentrically. All jumping and throwing activities involve both
types oF contraction.
In
isometric contractions
(
metric
5
measure), tension may
build to the muscle’s peak tension-producing capacity, but the mus-
cle
neither shortens nor lengthens
(±igure 9.18b). Isometric contrac-
tions occur when a muscle attempts to move a load that is greater
than the Force (tension) the muscle is able to develop—think oF try-
ing to lif a piano single-handedly. Muscles contract isometrically
when they act primarily to maintain upright posture or to hold
joints stationary while movements occur at other joints.
Consider a knee bend. When you squat For a Few seconds, the
quadriceps muscles oF your anterior thigh contract isometrically to
hold your knee in the flexed position. When you start to rise to the
upright position, they continue to contract isometrically until their
tension exceeds the load (weight oF your upper body). At that point
muscle shortening (concentric contraction) begins. So the quadri-
ceps contractile sequence For a deep knee bend From start to fin-
ish is (1) flex knee (eccentric), (2) hold squat position (isometric),
(3) extend knee (isometric, then concentric). OF course, this list
does not even begin to consider the isometric contractions oF the
posterior thigh muscles or oF the trunk muscles that maintain a
relatively erect trunk posture during this movement.
Electrochemical and mechanical events occurring within a
muscle are identical in both isotonic and isometric contractions.
However, the results are different. In isotonic contractions, the
thin filaments slide. In isometric contractions, the cross bridges
generate Force but do
not
move the thin filaments, so there is no
change in the banding pattern From that oF the resting state. (You
could say that they are “spinning their wheels” on the same actin
binding sites.)
Muscle Tone
Skeletal muscles are described as voluntary, but even relaxed
muscles are almost always slightly contracted, a phenomenon
called
muscle tone
. Muscle tone is due to spinal reflexes that
activate first one group oF motor units and then another in re-
sponse to activated stretch receptors in the muscles. Muscle
tone does not produce active movements, but it keeps the mus-
cles firm, healthy, and ready to respond to stimulation. Skeletal
muscle tone also helps stabilize joints and maintain posture.
Check Your Understanding
10.
What is a motor unit?
11.
What is happening in the muscle during the latent period of
a twitch contraction?
12.
Jay is competing in a chin-up competition. What type of
muscle contractions are occurring in his biceps muscles
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