How Does FES work? 1) In the previous section, we looked at what FES is. In this section we add a little more detail about how FES works. Let's take a look first of all at an overview of FES cycling with the RehaMove System. 2) We have two main systems that Interface with the user. The first is a motorised bike and the second system is the FES unit or stimulator. Electrodes are placed on leg muscles for example and these electrodes are used to deliver electrical energy to the muscle groups. As we discussed in the previous lesson, stimulation must be synchronised with the movement of the bike’s pedals. Consequently, the FES unit continuously monitors pedal position, cycling speed and the power being generated by the user. This information is used by the FES unit to modulate the stimulation been delivered to the muscles. Depending on the exercise goals and the training level of the user, the intensity of stimulation can be varied by the stimulation programme. 3) As we saw earlier there are three things that have to be controlled. These are the stimulation frequency, current and pulse width. These have to be controlled for each muscle involved in the exercise program. We look at how to do this in detail in lessons to come. 4) Typically we choose one of three electrode shapes and sizes to suit the particular application. The two rectangular electrodes are typically used with larger muscles such as the quadriceps or hamstrings. The oval electrodes are most often used with the upper limbs. 5) This diagram shows the most common electrode positions but of course we are not limited to these. 6) The image on left shows the oval electrodes being used for wrist extension exercise with a custom stimulation program. The middle and right hand images, show a situation when FES is being used for shoulder stabilisation exercise post-Stroke. Notice how the oval electrodes are being used to specifically target muscles that affect scapula position, whilst large rectangular electrodes are being used to target the larger muscles of the Latissimus Dorsi. Page 1 of 2
Derek Jones
Quite often with FES cycling, electrode positioning will not be absolutely critical because we will be working with large muscle groups. What we will do, is aim for consistent placement. 7) Up until now we have described the three parameters which deliver the stimulation to the muscles, but of course that energy must somehow pass between two electrodes placed on the muscles. As the energy flows, it needs to exceed a threshold level necessary to activate the motor nerves deep within the target muscle. Sufficient energy must be delivered to ensure that the nerve is activated and what is referred to as the action potential is generated. 8) The schematic diagram on the left, illustrates that there are various tissues beneath the electrodes. These include the epidermis, dermis and target muscle. In order for the energy to activate the nerve it must pass through these tissues. An engineer would model this situation as shown on the right. In terms of the flow of electrical energy, the skin and the subcutaneous tissues represent resistance to the current flow - represented here by resistors and capacitors. 9) For our purposes, we just need to realise that the skin and subcutaneous tissue represent barriers to the flow of this energy. The resistance within tissues to the stimulation varies with the thickness of fat covering the muscle, the age and quality of electrodes being used and even the thickness of any hair beneath the electrodes. Skin creams can also cause problems by increasing resistance to the flow of energy. We can say that high frequencies and shorter pulse widths penetrate the tissues more easily. 10) A final point we can make is that electrical stimulation can and will alter the properties of muscle. We can then expect that the nature of the energy we need to generate a muscle contraction will change over time. We are dealing with living tissue and the muscle and tissue properties are indeed shaped by the stimulation in a number of ways. Over time muscle can grow in size and it’s structure will benefit from the stimulation.
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Derek Jones