Thermal changes during muscle contraction
Out of total energy liberated by the hydrolysis of ATP in a contracting muscle, only 40-45% can be converted into work energy and the rest is evolved as heat; i.e. the efficiency of muscle contraction is about 45%. It is recorded by using thermopile. Thermopile has plates of antimony and bismuth connected to galvanometer. In a single twitch 0.001° to 0.005° C temperature produced.
Total heat Produced = Wt. of muscle (gm) × Sp. Heat × Extent of temperature
Fenn effect
When a work is performed by the muscle and energy is required. Heat generation by a muscle is directly proportional to its work performance due to increase hydrolysis of ATP. This Of the total energy liberated from ATP, the muscle utilises 40 to 50% energy in performing work and the rest is liberated as heat to maintain body temperature.
The liberated heat is classified as resting heat and Initial heat.
Resting heat
Used for biochemical reactions during resting state
Initial heat
Initial heat is produced both at initiation and during the course of muscle contraction. It is dependent on aerobic change in the muscle due to breakdown of ATP and creatine PO4 and formation of lactic acid.
Initial heat further divided into-
Activation heat: produced due to Ca transport associated with initiation of contraction
Shortening heat: Liberated by sliding reaction, muscle lifts load and does external work.
Maintenance heat: Heat liberated in a muscle that is stimulated but does not lead to physical work (isometric contraction); it is proportional to duration of contraction and tension developed during contraction.
Recovery heat: Heat liberated after contraction of muscle fiber. It is due to pumping of Ca++ back into the tubules and re-synthesis of ATP for next cycle.
Effect of temperature in the muscle
An increase of temperature shortens the latent and contraction periods whereas relaxation is not much affected. A decrease of temperature has the opposite effect causing increased latent and contraction period and prolonged relaxation period.
When temperature exceeds a few degrees above 40 ° C, irreversible changes take place in the sarcoplasm. There is a minimum temperature (near 00c) is required for muscle contraction, below which the muscle fibers fail to respond to stimulation; but the loss of excitation is reversible.