ENERGY SOURCES FOR MUSCLE
CONTRACTION
Objectives
1.
2.
3.
4.
5.
Energy used
Energy produced
Oxygen debt
Muscle fiber types
Muscle fatigue
ATP
• Muscle cells convert chemical energy to
mechanical energy.
• ATP is the energy source used for this
conversion.
• The ATP pool in skeletal muscle is small and
capable of supporting only a few contractions
if not replenished.
Creatine Phosphate
• Immediate high-energy source for
replenishing the ATP supply
• The enzyme creatine phosphokinase (CPK)
catalyzes the reaction
Carbohydrates
• Muscle cells contain glycogen,
• Glycogen provide glucose for oxidative
phosphorylation and glycolysis
Fatty Acids and Triglycerides
• Fatty acids represent an important source of
energy for muscle cells during prolonged
exercise.
• Muscle cells contain fatty acids but can also
take up fatty acids from blood.
• In addition, muscle cells can store
triglycerides, which can be hydrolyzed when
needed to produce fatty acids
Metabolic Adaptations Allow Contraction to
Continue With an Inadequate Oxygen Supply
• Glycolytic (anaerobic) metabolism can provide
energy for sudden, rapid, and forceful
contractions of some muscles.
• In most muscles, especially under conditions
of rest or moderate exercise, the supply of
oxygen is adequate for aerobic metabolism
(fed by fatty acids and by the end products of
glycolysis) to supply the energy needs of the
contractile system.
Muscle tissue has two sources of oxygen
(1) oxygen that diffuses into muscle fibers from
the blood and
(2) oxygen released by myoglobin within muscle
fibers.
Depletion of aerobic energy capacity
This results from two effects:
(1) the oxygen debt and
(2) depletion of the glycogen stores of the
muscles
Oxygen debt
• The body normally contains about 2 liters of
stored oxygen that can be used for aerobic
metabolism even without breathing any new
oxygen.
• This stored oxygen consists of the following:
(1) 0.5 liter in the air of the lungs, (2) 0.25 liter
dissolved in the body fluids, (3) 1 liter
combined with the hemoglobin of the blood,
and (4) 0.3 liter stored in the muscle fibers
them-selves, combined mainly with myoglobin
• After the exercise is over, this stored oxygen
must be replenished by breathing extra
amounts of oxygen over and above the
normal requirements.
• In addition, about 9 liters more oxygen must
be consumed to provide for reconstituting
both the phosphagen system and the lactic
acid system.
• All this extra oxygen that must be “repaid,”
about 11.5 liters, is called the oxygen debt.
Measuring Maximal Oxygen Uptake Is the most
Common Method of Quantifying Dynamic Exercise
Oxygen debt
If the energy demands of exercise cannot be met by
oxidative phosphorylation, an oxygen debt is incurred.
After completion of exercise, respiration remains above
the resting level in order to "repay" this oxygen debt.
The extra oxygen consumption during this recovery
phase is used to restore metabolite levels (such as
creatine phosphate and ATP) and to metabolize the
lactate generated by glycolysis. The increased cardiac and
respiratory work during recovery also contributes to the
increased oxygen consumption seen at this time and
explains why more O2 has to be "repaid" than was
"borrowed."
TYPES OF SKELETAL
MUSCLE FIBERS
Slow oxidative fibers
Fast glycolytic fibers
Myoglobin content
Large
Small
Mitochondria
Many
Few
Capillaries
Many
Few
Colour
Red
White (Pale)
Capacity for generating ATP
and method used
High capacity, aerobic
respiration
Low capacity, anaerobic
respiration
Glycogen stores
Low
High
Creatine Kinase
Lowest amount
Highest amount
Contraction velocity
Slow
Fast
Fatigue resistance
High
Low
Location where fibers are
abundant
Postural muscles such as
those of the neck
Upper limb muscles
Primary function of the
fibers
Maintaining posture and
aerobic endurance activities
Rapid intense movements of
short duration
Classification of Fiber Types in Skeletal
Muscles
Examples:
• Type I Red fibers: in postural muscles
– Large myoglobin content and many mitochondria
• Type IIa Red fibers: in muscles needed for
activities like middle distance running,
swimming, etc.
– Large myoglobin content and many mitochondria
• Type IIb White fibers: needed for activities like
sprinting
• Low myoglobin content and few mitochondria
Muscle fatigue.
• The inability of a muscle to maintain force of
contraction after prolonged activity .
• Causesinadequate release of calcium ions from the SR.
Depletion of creatine phosphate
 insufficient oxygen,
depletion of glycogen and other nutrients,
 build up of lactic acid and ADP,
failure of action potentials in the motor neuron
to release enough acetylcholine.
Q. Fast twitch muscle fiber differ from slow
twitch muscles in that former:
a. have low ATPase activity
b. are adapted for posture maintaining
contraction
c. contains more sarcoplasmic reticulum with
high glycogen content
d. resistant to fatigue