2. Muscle Contraction
Isometric contraction Isotonic contraction
Length of the
muscle
Remains same Shortening of the
muscle
Tension ↑ during the
contraction
No change
Mechanism Sarcomere which
shorten do so by
stretching those
which do not
Shortening of individual
sarcomeres adds up to
the shortening of the
whole muscle
External work No external work
down
Work down
Example Trying to lift heavy
weights (when the
weights are not
actually lifted)
Lifting of weights
3.
4.
5.
6.
7. Energetics of Muscle Contraction
when a muscle contracts against a load, it performs
work
energy is transferred from the muscle to the
external load to lift an object
energy required to perform the work is derived
from the chemical reactions in the muscle cells
during contraction
8. muscle contraction depends on energy supplied by
ATP
required to actuate the walk-along mechanism
pumping Ca++ from sarcoplasm into SR
pumping Na+ & K+
concentration of ATP in muscle fiber is sufficient to
maintain full contraction for only 1 to 2 s
ADP is rephosphorylated to form new ATP within
another fraction of a second
There are several sources of the energy for this
rephosphorylation.
9. Phosphocreatine
phosphocreatine is instantly cleaved, & its released
energy causes bonding of a new phosphate ion to
ADP to reconstitute the ATP
the total amount of phosphocreatine in the muscle
fiber is also very little (only about five times as
great as the ATP)
the combined energy of both the stored ATP & the
phosphocreatine in the muscle is capable of
causing maximal muscle contraction for only 5 to 8
seconds
10. Glycogen
breakdown of glycogen to pyruvic acid & lactic
acid liberates energy that is used to convert ADP to
ATP
ATP can be used directly to energize additional
muscle contraction & also to re-form stores of
phosphocreatine
importance of glycolysis mechanism is
glycolytic reactions can occur even in the absence of O2
rate of formation of ATP is about 2.5 times as rapid as
ATP formation in response to cellular foodstuffs reacting
with O2
glycolysis also loses its capability to sustain
maximum muscle contraction after about 1 min
11. Oxidative metabolism
means combining O2 with the end products of
glycolysis & with various other cellular foodstuffs
to liberate ATP
↑ 95 % of all energy used by muscles for sustained,
long term contraction is derived from this source
for extremely long-term maximal muscle activity
the greatest proportion of energy comes from fats,
but for periods of 2 to 4 hours, as much as one half
of the energy can come from stored carbohydrates
12. Thermal Changes
the energy expenditure of muscle differ markedly
at rest as compared to that during activity
although an unstimulated muscle produces heat,
heat production increases during & immediately
after contraction
13. Resting heat
heat produced in unstimulated muscle & reflects energy
metabolism in the resting muscle
a resting muscle needs energy for the vital processes of
life, especially for operating the sodium pump to
maintain the resting membrane potential
Activation heat
heat produced in stimulated muscle before shortening
energy spent on release of calcium form the terminal
cisternae, binding of calcium to troponin
14. Shortening heat
heat associated with shortening
energy spent on the ratchet mechanism involving myosin
cross bridges & the active sites on actin filaments
Maintenance heat
heat produced during tetanus
partly made up of the activation heat associate with each
stimulus & partly of the heat produced due to actin-
myosin interaction
15. Relaxation heat
relaxation heat is associated with relaxation
energy expenditure associated with uptake of calcium by
the terminal cisternae
Recovery heat or delayed heat
additional heat spent over & above the resting heat after
contraction & relaxation are over
this is due to restoration of the resting state
16. References
Textbook of Medical Physiology, 12/E Guyton &
Hall
Understanding Medical Physiology, 4/E Bijlani &
Manjunatha
