Low and medium Frequency Electrotherapy - Implox

2.3.1 Howson
Howson (7) states that it is best to use very short phase times for stimulation of the type II and type III nerve fibres, as
well as for stimulation of the type I (Aα) motor neurons (Fig. 3).
Fig. 3.
The strength/duration curves of the different types of nerve fibre (Howson, 1978, from Li and Bak).
The strength/duration curves of the different types of nerve fibre show that with phase times shorter than 200 μs it is
possible to stimulate the sensory and/or motor nerves without stimulating the non-myelinated fibres (pain).
In other words, with such short phase times it is possible to select a relatively high amplitude without stimulating the
thin nerve fibres. There is thus a wide amplitude range. On the other hand, with longer phase times the different
strength/duration curves lie so close together that a small increase in amplitude can result in stimulation of the thin
nerve fibres. In this case, there is a narrow amplitude range.
2.3.2 Lullies
From investigations by Lullies (18,19) it is possible to draw certain conclusions regarding the conditions that an
alternating current must meet for selective stimulation of the thick nerve fibres. These conditions are:
• a ‘relatively’ low current;
• a ‘relatively’ high frequency (above 3 Hz).
Although the frequency of the medium-frequency alternating currents in interferential therapy differs from the optimum
frequency, these currents are still able to stimulate the thick nerve fibres.
Fig. 4.
The amplitude of a sinusoidal alternating current plotted against frequency for type I (Aa) nerve fibres (myelinated
motor neurons) and type IV (C) (non-myelinated autonomic) fibres of the sciatic nerve of a frog.
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