300
UNIT 2
Covering, Support, and Movement of the Body
9
Levels of CP and ATP don’t change much during prolonged
exercise because ATP is generated at the same rate as it is used—
a “pay as you go” system. Compared to anaerobic energy pro-
duction, aerobic generation of ATP is relatively slow, but the
ATP harvest is enormous.
Muscle Fatigue
Muscle fatigue
is a state of
physiological inability to contract
even though the muscle still may be receiving stimuli. Although
many factors appear to contribute to fatigue, its specific causes
are not fully understood. Most experimental evidence indicates
that fatigue is due to a problem in excitation-contraction cou-
pling or, in rare cases, problems at the neuromuscular junction.
Availability of ATP declines during contraction, but it is abnor-
mal for a muscle to totally run out of ATP. So, lack of ATP is not
a fatigue-producing factor in moderate exercise.
Several ionic imbalances contribute to muscle fatigue. As ac-
tion potentials are transmitted, potassium is lost from the mus-
cle cells, and accumulates in the fluids of the T tubules. ±is
ionic change disturbs the membrane potential of the muscle
cells and halts Ca
2
1
release from the SR.
±eoretically, in short-duration exercise, an accumulation
of inorganic phosphate (P
i
) from CP and ATP breakdown may
interfere with calcium release from the SR. Alternatively, it may
interfere with the release of P
i
from myosin and thus hamper my-
osin’s power strokes. Lactic acid has long been assumed to be a
major cause of fatigue, and excessive intracellular accumulation
of lactic acid (which causes the muscles to ache) raises the con-
centration of H
1
and alters contractile proteins. However, pH is
As exercise begins, muscle glycogen provides most of the
fuel. Shortly thereaFer, bloodborne glucose, pyruvic acid from
glycolysis, and free fatty acids are the major sources of fuels.
AFer about 30 minutes, fatty acids become the major energy
fuels. Aerobic respiration provides a high yield of ATP (about
32 ATP per glucose), but it is slow because of its many steps and
it requires continuous delivery of oxygen and nutrient fuels to
keep it going.
Energy Systems Used During Sports
Which pathways pre-
dominate during exercise? As long as a muscle cell has enough
oxygen, it will form ATP by the aerobic pathway. When ATP
demands are within the capacity of the aerobic pathway, light
to moderate muscular activity can continue for several hours
in well-conditioned individuals
(Figure 9.20)
. However, when
exercise demands begin to exceed the ability of the muscle cells
to carry out the necessary reactions quickly enough, anaerobic
pathways begin to contribute more and more of the total ATP
generated. ±e length of time a muscle can continue to contract
using aerobic pathways is called
aerobic endurance
, and the
point at which muscle metabolism converts to anaerobic gly-
colysis is called
anaerobic threshold
.
Activities that require a surge of power but last only a few sec-
onds, such as weight liFing, diving, and sprinting, rely entirely
on ATP and CP stores. ±e more on-and-off or burstlike activi-
ties of tennis, soccer, and a 100-meter swim appear to be fueled
almost entirely by anaerobic glycolysis (²igure 9.20). Prolonged
activities such as marathon runs and jogging, where endurance
rather than power is the goal, depend mainly on aerobic respira-
tion using both glucose and fatty acids as fuels.
Short-duration exercise
Prolonged-duration exercise
6 seconds
10 seconds
30–40 seconds
End of exercise
Hours
ATP stored in
muscles is
used first.
ATP is formed from
creatine phosphate
and ADP (direct
phosphorylation).
Glycogen stored in muscles is broken down to glucose,
which is oxidized to generate ATP (anaerobic pathway).
ATP is generated by breakdown
of several nutrient energy fuels by
aerobic pathway.
Figure 9.20
Comparison of energy sources used during short-duration exercise
and prolonged-duration exercise.
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