FES: Whats the Buzz

FES Buzzing on!

Picture 1: The informative text book of Faghri, Garstang, & Kida (2009).

For the information throughout out next blog, we would like to acknowledge Faghri, Garstang, & Kida (2009) for their very interesting book ‘Spinal Cord injuries: Management and Rehabilitation’. Chapter 17 of this book is entirely dedicated to FES and is written in an informative and interesting manner. For those wanting to learn more on this topic you can find an online or hard copy at the Griffith University Library. We have added a picture of the front cover for easy identification if you’re looking for it. Now let’s look at some detail of FES.

There are two basic areas of variability when it comes to FES application. Firstly, the parameters within the stimulation delivered and secondly, the variation of delivery method. The principle factors that that affect the muscular response to FES (Stimulation variables) include: the manipulation of the current via wave amplitude, frequency, pulse width and waveform manipulation. Table 2 outlines the key points to consider when setting stimulus parameters. The other important stimulation variable is the duty cycle. This is the ratio of 'on' time to the total cycle of stimulation which influences the fatigue rate of the muscle being stimulated. The patient’s tolerance to the electrical stimulation also needs to be considered when setting the parameters mentioned (Faghri, Garstang, & Kida, 2009).

Stimulation parameter	Overview
Waveform	A symmetrical biphasic AC current results in a good tetanic muscle response with minimal skin irritation
Amplitude	Direct effect on the sensory and motor unit response. The greater the amplitude the greater the motor unit recruitment.
Frequency	The rate of electrical pulses being delivered to the muscle stimulated therefore influences the temporal summation and rate of fatigue of muscle.
Pulse Width	Width of pulse needs to be enough to exceed excitability threshold of the motor neuron but functional considerations of fatigue are also important to consider.

Table 2: Key points to consider when setting stimulus parameters of FES (Faghri, Garstang, & Kida, 2009)

All powered up but how to make it go?

The second area of variability we mentioned earlier is how the user controls the delivery of FES. Table 3 outlines the various delivery methods via an open or closed loop electrical stimulation to the lower limb.

Parameter	Variables
Open loop	·         The most basic control type is the user controlled open loop system ·         Patient controlled delivery of a pre-set stimulation level regardless of the muscle feedback. ·         Open loop signals are not modified unless manually adjusted by the patient or therapist ·          Application of this would be a cyclical open loop system used to strengthen weak lower limb muscles
Closed Loop	This is a more complicated delivery of current that is firstly initiated by the user then modified on a feedback system that accounts for muscle fatigue and  the force of the required action Adjustment of current delivery can occur without the user knowing via sensor feedback located on the muscle of interest or the point force supplied by the muscle Application examples  of closed loop systems include gait motion retraining and chronic hemiplegia of the dorsiflexors

Table 3 This table shows a comparison between open and closed loop delivery of FSE(Faghri, Garstang, & Kida, 2009).

The final point on current delivery we would like to mention is the external or internal delivery of FES. External delivery FES is simple and involves electrodes placed directly on the skin. This is a nonspecific application and can potentially result in muscle activation and fatigue of non-target muscles (Elder & Spetzler, 2003). Internal electrodes are more specific to external as they can be planted directly into a muscle or adjacent to a specific nerve (Faghri, Garstang, & Kida, 2009). This allows for action potentials to be generated in the nerve that directly innervates the muscle of interest, or direct stimulation of excitation contraction within the muscle itself (Faghri, Garstang, & Kida, 2009), (Elder & Spetzler, 2003). The diagram 1 below highlights how external electrodes can be placed on the body for gait motion retraining.  

http://www.mpi-magdeburg.mpg.de/research/projects/1119/1133/1133GAIT/index.en.html?pp=1

But of course, with anything electrical, there is always some form of risk and contraindications that must be followed for the safe and effective use of FES. The absolute contraindications are: pacemakers, pregnancy, stimulation near the carotid sinus and over malignant tumours. As FES does produce muscle contractions other relative contraindications include: osteoporosis, contractures of limbs, hyper or hypotension and risks of venous thrombi. There are some other risks associated with FES including spacticity and burns risks. A complete list of these precautions can be found in Faghri, Garstang, & Kida (2009) text we discussed at the start of this blog.

This concludes our second posting of FES on the lower limb. So far we have outlined the basic principles for you that govern FES and how its is used on the lower limb. Over the next few blogs we aim to bring you some exciting detail on clinical applications of this amazing treatment.

Till then, stay safe.

References:

Elder, W., & Spetzler, R. F. (2003). Functional Electrical Stimulation (FES). Encyclopedia of the Neurological Sciences (pp. 401-403). New York: Academic Press.

Faghri, P. D., Garstang, S. V., & Kida, S. (2009). Functional Electrical Stimulation (pp. 407-429)