Application of Faradic Stimulation

Faradic stimulation may be divided into four main classes:

• Transcutaneous
  
• Percutaneous
  
• Subcutaneous - via peripheral root nerves
  
• Subcutaneous - via spinal root nerves

These represent the depth that the electrode is placed at. Generally as the electrode gets closer to the nerve/muscle, then so the intensity required to initiate nerve depolarisation is reduced, thus less energy is required for the action. Equally important is the fact that that it is possible to be much more specific about which nerves, and therefore to some extent which motor units, are stimulated as the electrodes get closer to the nerve/muscle.

Transcutaneous Stimulation

Transcutaneous stimulation takes place on the skin via surface electrodes. The intensity needed varies greatly with the muscle, its condition, the stimulation pulsewidth used and the contraction required, but in some cases, eg when stimulating the quadriceps to effect standing, then amplitudes up to around 150 mA are not uncommon. The best results are obtained by removing hairs from under the electrodes (through clipping NOT shaving) and using a conducting jelly between skin and electrode.

Typical Electrode Positions for Quadricep Stimulation (-ve over motor point)
(c) David Ewins 1990

Typical Electrode Positions for Peroneal Stimulation -withdrawal reflex
(c) David Ewins 1990

Apart from the high energy required to maintain stimulation, the other major drawback with surface stimulation is that fine movements (as required to produce function in the hand) can be difficult to produce and reproduce. Monophasic or charge balanced pulse types may be used.

Percutaneous Stimulation

The electrical stimulation is transmitted through the skin via a wire or needle into the muscle belly. Flexible wires are most often used as the electrodes as they enable the subject to move around more. This type of stimulation is generally used for research only as it is not considered ethical in a clinical situation. MSc students, of course are expendable :-) and therefore suitable experimental subjects.

Subcutaneous Stimulation

Via peripheral root nerves
Electrode cuffs around nerve bundles are sutured to the muscle epimyseum.

Via spinal root nerves
Electrodes are implanted around spinal root nerves specific to the stimulation needed, eg sacral roots for bladder stimulation, and possibly lumbar roots for standing/walking.