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The following changes are found to accompany any muscular contraction: 1. Mechanical Changes 2. Thermal Changes 3. Electrical Changes.
1. Mechanical Changes:
During, contraction, the muscle fibre shortens in length, increases in thickness, but the total volume remains same (or slightly increases). The muscular contraction may be isometric when the length of the muscle fibres remains constant but the tension increases, or isotonic when the muscle becomes shorter and thicker.
2. Thermal Changes:
During muscular contraction, heat is produced in the muscle, which has been measured with the help of an instrument called Thermopile. Heat is produced in two stages – (a) initial heat, which occurs at the onset of contraction, (b) recovery heat or delayed heat, which occurs, following the contraction.
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The initial heat is the rapid outburst of heat, whereas the recovery heat is slow and prolonged production of heat. The recovery heat is equal to the total energy set free during muscular contraction. In the recovery process the chemical changes, which is responsible for the initial liberation of energy, are reversed.
A.V. Hill had shown that the heat produced in a single twitch of a frog’s muscle may be divided into three stages:
i. The Heat of Activation:
The heat of activation occurs immediately after stimulation and diminishes while the contraction proceeds. The heat production occurs about ten milliseconds after stimulation.
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ii. The Heat of Shortening:
This is the second stage of heat liberation and depends upon the amount of shortening -ax, where x denotes the amount of shortening and a is the constant (multiplication of constant and variable),
iii. Recovery Heat:
There is slow production of heat during recovery process in the presence of oxygen. The heat of shortening begins later. There is no heat of relaxation.
3. Electrical Changes:
Instruments used for the purpose are capillary electrometer, Einthoven’s string galvanometer or cathode ray oscillograph. The electrical changes are of the same nature as seen in the nerve. The cut surface or the stimulated spot is negative to the rest of the muscle fibre.
When the injured and the uninjured areas are connected, current of injury will be found. By repeating the same experiments as in nerves current of action with diphasic variation and current of injury with monophasic variation can be found. Mechanical contraction starts when the electrical change attains its maximum, which takes place very early during the latent period.
The action potential as well as the wave of mechanical contraction is propagated at the same rate (3-4 metres per second). Under no condition the electrical wave (excitation process) can be propagated without the accompanying wave of contraction. Hence, the electrical wave can be regarded as the index of the contraction wave. Cold slows and warmth hastens both these waves.
Electromyography:
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Electromyography is a study of the action potential in human skeletal muscle. As a part, the electrical current generated is transmitted to the outer surface of the body. The changes in action potential can be studied by either putting electrodes at the surface of active muscle area or inserting them directly into the muscle concerned.
The potential can be recorded by cathode ray oscillograph. The study has a great clinical value in the diagnosis of different types of neuromuscular functional impairment in which either the rate or rhythm of nerve impulse is influenced.
