Low and medium Frequency Electrotherapy - Implox

5 MUSCLE STRENGTHENING WITH ALTERNATING
CURRENTS
5.1 Introduction
This form of muscle stimulation can only be applied if the peripheral nervous system is intact.
In physiotherapy, the objectives tend to be optimalization of a nonpathological situation, rather than alleviation of a
pathology. An exception is the prevention of atrophy, e.g. in immobilization.
The therapeutic objectives include:
• restoration of the sensation of muscle tension following surgery or trauma;
• increasing the muscle strength to improve the (active) stability of a joint;
• maintaining the condition of a muscle (prevention of atrophy).
Before giving an overview of the various types of alternating current that can be used, we shall first consider some
kinesiological aspects.
5.2 Kinesiological aspects
In physiotherapy it has become usual to refer to tonic and/or phasic musculature. It would, in fact, be better to refer to
tonic and/or phasic motor units. In general, the classification used is that propsed by Janda (8) . Janda’s greatest
contribution was to describe the clinical behaviour of the musculature.
However, Janda’s classification is not accurate on all points. Posture (antigravity) muscles should be tonic, and tend
towards shortening. According to Janda’s classification, many typically postural muscles, such as the ascending and
horizontal parts of the trapezius, belong to the phasic musculature. Furthermore, it has been found in practice that
phasic musculature is also capable of shortening.
From other investigations (9, 14, 15) it appears that the distribution of fibre types in the muscles does not correspond to
Janda’s views, and that it can vary considerably from one individual to another. Johnson, in particular, showed in an
autopsy study (9) carried out in six men within 24 hours of death that there are marked differences between the
distribution of fibre types in the muscles of different individuals (Table 1).
With one exception, all muscles in the human body are composed of a mixture of fibre types. Thus, there appears to
be a difference between the muscle fibre distribution in a particular muscle and its clinical behaviour as described by
Janda.
In general it can be stated that, in a movement, the tonic motor units are the first to come into action. The phasic
motor units only become active if additional strength is required (15) . In rapid movements, the phasic motor units may
be activated earlier than the tonic motor units. According to Kluo and Clamann (15) , this phenomenon is most marked
in synergistic muscles with different fibre distributions.
Electrical stimulation over a long period can lead to a change in the muscle fibre distribution (6) . The change appears
to be mainly dependent on the frequency with which the motor neuron is depolarized by the electric current. This
effect should be taken into account in lengthy treatments. The change in the muscle fibre distribution is reversible. In
other words, the muscle fibre distribution will adapt the function if the muscle is used.
Tonic motor units
Red muscle fibres
Phylogenetically older
Better vascularization
Innervation by Aa 2 neurons
Tetanic frequency 20-30 Hz
Slowly tiring
Static
Phasic motor units
White muscle fibres
Phylogenetically younger
Poorer vascularization
Innervation by Aa 1 neurons
Tetanic frequency 50-150 Hz
Rapidly tiring
Dynamic
Table 2.
Characteristics of tonic and phasic motor units (8) .
28