The short duration interrupted direct current with pulse duration 0.1 – 1 ms and frequency in between 50 – 100 Hz, used for the stimulation of innervated muscles is called faradic type of current. The current produced by faradic coil is called multivibrator current (producing tetanic contraction).
The process of giving relaxation to the tetanic contraction produced by faradic current is called surging.
i. Trapezoidal surging:
The impulses increases and decreases gradually forming trapezoidal shape
ii. Triangular surging:
The impulses increases and decreases gradually forming triangular shape
iii. Saw Tooth:
Physiological effects of faradic type of current (low frequency current):
It stimulates the intact nerve by producing change in the permeability of the cell membrane altering the testing potential of the membrane. When the resting potential of the cell membrane reaches the excitatory level, the muscle supplied by the nerve contracts. Along with motor supply the current also produces sensory stimulation of nerve.
The skin provides 3200Ω and epidermis 1000 Ω, deep fascia, fats having different ions. When it passes through them it may cause chemical burn. To reduce this effect, the resistance is reduced to appropriate measures.
a) Sensory effects (sensory nerve stimulation):
By applying the faradic type of current, there is mild prickling sensation. There is also mild erythema (redness) of the skin due to vasodilatation of superficial vessels.
b) Motor effects (motor nerve stimulation):
The faradic current stimulates the motor nerve, if the faradic current is of sufficient intensity having 50 Hz frequency and 0.1 – 1ms shortest duration it will produce tetanic contraction. The tetanic contraction causes muscle fatigue. So to avoid this current is surged to allow muscular relaxation.
c) Effect on muscle contraction:
Faradic current causes muscle contraction similar to the normal muscle. Contraction and relaxation of muscles produce pumping action which increases blood supply to the muscles as a result of increased demand and supply of oxygen and nutrition.
Sufficient muscular contraction and relaxation causes pumping effect on venous and lymphatic vessels which leads to removal of waste products.
d) Effect on denervated muscles:
For the stimulation of denervated muscles, the impulse required is more than 1 ms which can not be tolerated by the patient. So, faradic current is short duration current with sufficient intensity, which cannot stimulate the denervated muscles.
Indications of faradic type of current:
1) Facilitation of muscle contraction:
When the muscle is unable or difficult to contract then we use stimulation by faradic current. The muscle contraction is integrated by complex process throughout the spinal cord level or higher center.
Excitation of the small efferent nerve causes contraction of intrafusal muscle fibers.
Stretching of muscle spindle stimulates the primary nerve endings and sends information to the anterior horn cells of the spinal cord.
When contraction of agonist occurs, at the mean time relaxation of antagonist occurs.
2) Re – education of muscle action:
Inability to contract a muscle voluntarily may be as a result of prolong disuse and incorrect pattern. The nerve has the power of accommodation, the current which rises or falls suddenly is more effective and initiating an impulse than one which changes slowly.
3) Stimulation of nerve:
When a sensory nerve is stimulated, the downward traveling impulse has no effect while the upward traveling impulse is appreciated to reach the conscious level of brain.
When the motor nerve is stimulated, the upward traveling impulse in unable to pass the 1st synapse (N/N) while the downward traveling impulse passes to the muscles causing contraction
4) Effect of frequency stimulation:
When a single stimulus is applied per second, it will produce a sudden brisk contraction followed by immediate relaxation. Increasing the frequency to 20 Hz or above will produce tetanic contraction because there is no time for relaxation.
5) Strength of contraction:
It depends on number of motor units activated and rate of change of current. If the intensity of current rises suddenly, there will be no time for accommodation which results in sudden brisk contraction but if the current rises slowly, greater time will be required for accommodation i.e. trapezoidal, triangular current.
6) Oedema reduction:
Alternative contraction and relaxation (pumping action) of the muscle causes lymphatic and venous drainage (return) so reduces the oedema. This process is called faradism under pressure.