1) Muscle contraction requires energy supplied by ATP, which is generated through several metabolic pathways in the muscle cell, including phosphocreatine, glycogen breakdown, and oxidative metabolism. 2) As muscle contracts, it produces heat from various sources like activation, shortening, maintenance, relaxation, and recovery. 3) The different metabolic pathways sustain muscle contraction for varying durations, with phosphocreatine and glycolysis sustaining maximal contraction for only a few seconds to a minute, while oxidative metabolism provides energy for sustained, long-term contraction.

1. RK Goit, Lecturer
Department of Physiology

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

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

17. Thank You