An FES Pulse Train
Surface electrical stimulation typically consists of a train of regular monophasic or biphasic pulses which may be described by the following parameters: frequency; amplitude; duration of individual pulses; duration of the pulse train; and rise time for the individual pulses.
Some of these are defined in the FES pulse train shown in below.
An FES Pulse Train
The amplitude and pulse width of the stimulation must be sufficient to meet the threshold of excitability of the stimulated tissue, changes under this level will have no effect as shown in the graphs below.
As the amplitude or pulse width rise the nerve fibres nearest the electrodes and largest in diameter are triggered to threshold and fire.
This continues until all fibres are firing at which point no more increase in force can be obtained, the muscle is said to be saturated.
This increase in recruitment is almost linear between threshold and saturation as shown below:
Effect of Amplitude on Force - Data taken from quadriceps femoris, surface electrodes, pulse duration 300 microseconds, frequency 35 Hz.
Adapted from: Functional Electrical Stimulation: A Practical Clinical Guide (2nd Edition). Benton, Baker et al., 1981 (see bibliography).
Effect of pulse duration on force - Data taken from surface stimulation of the dorsiflexors at a stimulation voltage of 50V and a frequency 30 Hz.
Adapted from: Functional Electrical Stimulation: Standing and Walking after Spinal Cord Injury. Kralj and Bajd, 1989 (see bibliography).
The rate of rise of the pulse can also be important. Too slow a rise time results in changes in the tissue membrane known as accommodation, which gradually elevates the threshold required for the nerve to fire. The pulse used in electrical stimulation do not, in general, allow this effect to occur.
The rate at which the nerve fibres fire is dependent on the frequency of pulse repetition. A single pulse produces a short lived muscle twitch of not more than 250ms. If pulses are repeated more frequently than this the muscle does not have time to relax in-between stimuli and eventually tetanic (continuous) contraction occurs.
Although these look similar to contractions evoked by voluntary stimuli, as voluntary motoneurons are innervated asyncronously, tetany is achieved at much lower rates - 5-25 Hz.
The high rate of synchronous activity in electrical stimulation can cause decreased neural transmitter release. However, the biggest problem is that of muscle fatigue itself. This is because stimulation tends to elicit recruitment of the larger diameter motorneurons (they have a lower threshold), which recruit the faster and more powerful muscle fibres. These fibres, termed type 2 or white, fatigue quicker than the slower, but less powerful type 1 or red muscle fibres. Additonally, with long term paralysis, there can be a transformation of slow fibres to fast ones, which will exacerbate the fatigue problem
This type of fibre recruitment is often a reversal of the normal patterns which, in addition, involve an asynchronous firing which will allow fibres to "rest". Overall, therefore, stimulated muscle will generally fatigue sooner than the same response initiated voluntary.
NOTE: There is a good deal of work being undertaken in muscle condtioning, eg the use of skeletal muscle for cardaic assist (caridac myoplasty), and material on this will be added to these introductory pages at a later date.
As shown in below, the higher the stimulation frequency, the faster the muscle fatigues:
Effect of frequency on fatigue - This data is from an implanted electrode on the peroneal nerve.
From: Functional Electrical Stimulation: A Practical Clinical Guide (2nd Edition). Benton, Baker et al., 1981 (see bibliography).
In practical FES systems frequency is often held at a sufficient level for tetanus. Amplitude and pulse width are varied to control the contraction for the patient's needs. Closed loop systems, in which the stimulation is moderated automatically according to such parmeters as joint angles and foot pressure distribution, are now being developed, but few of these are in routine clinical practice.
Torque does not usually decline linearly with time as was the case in the simulation above. The fatigue observed in the simulations is attributed to three factors:
In the fatigue simulation there is no fatigue until 18 Hz and no increase in fatigue above about 80 Hz. This is not the case in reality.
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