The problem with Functional Electrical Stimulation
Functional Electrical Stimulation devices, or FES for short, is a very powerful tool in the hands of a capable therapist but is sadly still poorly understood and less often seen in clinical practice than it deserves. I think there are a number of reasons for this.
The first problem is perhaps the fact that using electrical energy for therapeutic effect has a long and sometimes colourful history. This may have led to the whole idea of using electricity as being associated with “quackery”. In the early 19th Century, the application of electricity in an attempt to bring the dead back to life probably didn’t help. Then along came Mary Shelley’s book, Frankenstein, in which a body made of spare parts was activated by electricity. The Shelleys were already intrigued by the use of electricity to animate limbs and it’s been suggested they heard tales of the alchemist Johann Konrad Dippel, a controversial figure rumored to have robbed graves and experimented on corpses at Frankenstein Castle.
Another problem has perhaps been the fact that FES requires some technical as well as clinical know-how. It should be obvious that the nature and form of the electrical energy should be well understood as this is necessary for safe and effective application. I do meet clinicians who use FES as a “black box” - in other words just turning the unit on and pushing the buttons suggested in the manual, without knowing anything about the actual stimulation parameters. This is not helped by the fact that there is no consistent use of terminology and practical guidance is often lacking about evidence supporting it’s use.
It’s not that there is no literature on the topic. There is supportive as well as negative literature in abundance. The literature can be contradictory and confusing. It might look as if the “evidence” supporting the use of FES is confusing or weak but it can’t be judged in the same way as we would judge a medicine; FES is a tool that needs to be configured to the specific situation.
The choice problem
A quick web search for functional electrical stimulation devices will produce millions of links and numerous device suggestions with vast differences in price. It’s no wonder there is confusion about which devices are suitable for a particular situation.
Stimulators that are certified medical devices, are generally more expensive than those marketed for sports and fitness applications. Of course medical devices are subject to rigourous design and safety testing and this is the reason for the relatively high costs. These costs are even getting higher as the medical device regulations world wide are tending to become even more rigorous now. This in fact, is becoming a serious barrier to innovation in this market. There is no doubt that lower-priced products would allow their adoption by more clients who at present simply cannot afford to use these methods.
Certified medical devices should be our first choice for therapy applications due to the inherent safety offered, and indeed the products we deal with such as the RehaMove 2, the Stimulette RISE and Stimulette Edition 5 are approved medical devices. The RehaMove 2 supporting neuromuscular stimulation appolications such as FES Cycling, the RISE unit specialising in denervated muscle and the Edition 5 offers a range of applications including pain management. All are electrical stimulation devices but each with different clinical application profiles depending on their design.
Clients will sometimes ask whether a particular product they saw on-line would work for them. This might well be a product that seems to offer the same specification and lower price than a medical device they were offered. The only way forward is to weigh up the relative risk and understand the consequences of particular choices.
Seeing a product on-line at an attractive price is tempting but we should be wary of unverified specifications, claims and testimonials. Safety is the first imperative. I remember years ago being accused of profiteering in the sale of a stimulator by a person who felt that the unit could not have more than £20 worth of components. Of course this view neglected the true cost of bringing something to market that must include the development of integrated software and exhaustive safety checks as well as performance testing.
The essence of FES
All FES units will consist of some kind of pulse generator that must ultimately interact with the body via electrodes. Typically, the electrodes are placed on the surface of the skin and are, in that sense are non-invasive. Pulse generators produce their effects by nature of the specific form of energy they deliver. The type of energy delivered by units such as the RehaMove 2 is biphasic rectangular pulses. The descriptive form of these pulses we could refer to as the stimulation parameters; in other words, technical descriptions of the pulse forms.
The majority of stimulators like the RehaMove 2 will deliver “charge balanced”, bipolar, rectangular waveforms and offer a means to vary the waveform in real time. Charged balanced means that there is no Direct current (DC) aspect to the waveform. There is a specific application that uses DC but this is not commonly seen and should certainly be avoided in general purpose FES units.
The three basic stimulation parameters are pulsewidth, current and frequency. Devices such as the RehaMove 2 will deliver pulses with a maximum pulsewidth of 500 microseconds, a maximum current of 130 mA and a typical maximum frequency of 50 Hz. These parameters require the lower motor neurons to be intact. In using such a device, an electrical fiueld is generated under the electrodes that depolarises the cell membrance of nerve cells in the region. If a critial level of energy is reached, the action potentials are generated and a muscle contraction occurs. The art and craft of FES recognises that by varying the stimulation parameters and overall intensity you can influence the strength of contraction as well as the types of fibres activated and their endurance capacity.
Meanwhile, the RISE stimulator can produce a bipolar, rectangular waveform too. The difference in stimulation parameters is significant though. The RISE unit may need to deliver pulsewidths of 200 ms, frequency of less than 2 Hz and currents of up to 250 mA. This stimulator and these particular stimulation parameters are designed for use with denervated muscle where the lower motor neurons are disrupted. This stimulator works by acting directly on the muscle fibres rather than the motor nerves.
The lesson here is that by changing the stimulation parameters we can produce radically different results. The challenge is knowing what parameters are necessary for a particular application and finding devices that can deliver these.
There are a number of text books that adequately describe the technical principles of electrotherapy. After all, the electronic components may have changed over the years but the principles have not. What has been missing is a guide for clinicians that is both thorough and practical.
I was pleased to see the arrival of new book which I recommend as a thorough and practical guide for clinicians that want to know more about this topic. It draws on the experience of clinicians and engineers who work with FES each day.
“Functional Electrical Stimulation in Neurorehabilitation - Synergy Effects of Technology and Therapy”
Edited by Thomas Schick, Springer ISBN 978-3-030-90122-6