1. Glycogen stored in muscles and liver, triglycerides in adipose tissue, and free fatty acids are the main fuel sources for prolonged exercise. 2. Catecholamines released early in exercise stimulate lipolysis, releasing free fatty acids from fat cells. As exercise continues, growth hormone increases fatty acid availability, reducing reliance on glycogen and glucose. 3. Glycogenolysis and gluconeogenesis in the liver release glucose into the bloodstream to fuel exercising muscles through Cori cycle metabolism.

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2. Type I and 11 muscle fibers can regenerate ATP to some
extent byglycolysis . . TP .
3. However, glycolysis is insufficient in generating A m
comparison to aerobic regeneration of ATP.
4. Also, glycolysis accumulates protons in muscle cell
that decreases intracellular pH and inhibits force gen-
eration. Hence, anaerobic metabolism impairs muscle
activity.
Triglycerides and Fatty Acids
Though storage of 02 in the body is negligible, sufficient
reserve of fuel is available to sustain exercise for a longer
duration. The stored energy is derived from triglycerides in
the adipose tissue and intramuscular stores of fatty acids.
In prolonged exercise, free fatty acids (FFA) account for
approximately 60% of total supply of fuel to the muscle.
1. Catecholamines secreted early in exercise stimulates
hormone-sensitive lipase in adipocytes, which converts
trigfycerides to free fatty acids.
2. Whenexerciseisprolonged,increasedsecretionofgrowth
hormone mobilizesmore FFA. FFA is oxidized byexercising
muscle,and increasedavailability offatty acids decreases
the demand of muscle glycogenolysis and glucose
oxidation.
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2. uscle and Liver Glycogens
Muscle glycogen is another source of energy. Norma lly,
300-400 g of glycogen ca n be stored in muscles, which at the
time of need, can provide about 1500 kcal of energy.
1. Glycogen is also stored in liver, which can accommodate
about 100 g of glycogen that can supp ly about 400 kca l.
2 · However, hepatic glycogen is of little use for acute muscular
contractions as it is not made available immediately for the
musclecells.
3. Nevertheless,as exercise continues, glycogen is released
from liver for utilization by the exerc ising musc le.
Glucose
Glucose utilized by muscle during exercise decreases blood
glucose level, which is balanced by release of glucose from
Iiver.
1
. In anaerobic exercise, ca rbohydrates break dow n to
form acetate that enters blood to reach f iver, where it is
converted to glucose through gluconeogenes is. Glucose
released from liver fina lly reaches muscle. This completes
the Cori cycle.

3. iu t i uLl r
't
m bolism released
u I ( lucose) by liver
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n u .
f lu o
lu n
m inly due to activation
n i from lactic acid and
.lf n in
1
ntribut , p i lly when exercise is
• u imm di t ourc s of fuel are ATP and
phosphocrea.t1
·ne th t re available inthe musc.
le.
n t imm di t fuel source is glycogen stor d 1n the
• 1 u cl . Glycogenolysis that liberates glucose 1s also a
ur e of energy. .
4. Lactic acid released from muscle also supplies energy.
s. In prolonged exercise,liver glycogen that releases glucose
into blood by glycogenolysis becomes the fuel source for
the exercising muscle (Application Box 179.2).
6. When exercise reaches its steady state, the oxidation of
glucose released from muscle and liver glycogen accounts
for about 50% of the energy supply, and rest of energy
source is the oxidation of fatty acid.
r. r:r!j H•Ta --.: i
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Carbohydrate diet priortoexercise postponesfatigue inrunners:The
timeofonsetoffatigueduringexercise largelydependsontheamount
ofglycogenstoredintheskeletalmuscles,especiallyinmusclesofleg
priorto beginning ofexercise. Ina normalwell-built individual, muscle
glycogen storage averages about 15-18 g/kg ofmuscle, which isreduced
toab ut3 g/kgofmuscleaftera marathon running.To avoid earlyonset
offatrgue due to muscle glycogen depletion, the runner should have
moremusc eglycogenatthebeginningofrunning.Asdietisthe main
source ofth1s glycogen, consumption of carbohydrate-rich diet few days
or at least a day prior to the competition will increase resting muscle
glycogen anddelaysthe onset off t;,.... '"