Functional Electrical Stimulation (FES) is a therapeutic intervention where electrical current is delivered to skeletal muscles to elicit a contraction, in a way that ultimately produce functional movements such as reaching and grasping. By training functional task intensively over prolonged time, even severely affected patients may recovery upper limb functionality (further reading at this previous post).
We organized recently a workshop on FES, to elicit the following three movements:
- fine pinch (thumb-index opposition);
- hand opening (finger extension);
- arm protraction (shoulder elevation and elbow extension).
Notably, we used a customized pedal to take control over the FES device (you can view the 'how-to-make' tutorial at this link).
In fact, I want to stress the concept that we need smart controllers to modulate the optimal timing and intensity of the stimulation. The lack of such device on the market pushed me to fabricate it from an expression (music) pedal, which is simply a variable resistor to current flow, proportional to the angle of the pedal.
For the rest of this article I will explain the theoretical basis of FES for upper limb neurorehabilitation, covered in the following five parts:
Part 1. Peripheral and central effects of FES
The first question I may wonder is 'WHY to perform FES in the first place'. It's worth consider that FES is the ONLY therapy that assist movement by eliciting muscle contraction.
Here are three key effects of FES:
- central effect, direct: increased excitability of the primary sensorimotor cortex;
- central effect, indirect: increased motor recovery and normalization of muscle tone, because of enhanced quantity and quality of functional training being performed;
- peripheral effect: increased throphism and blood flow circulation.
For an overview of evidence regarding FES, I suggest to read our article published in 2022 in Frontiers Neurology (download the open-access pdf at this link).
Part 2. Electrode size and positioning
Large electrodes are optimal for large muscle groups, and are more comfortable and effective because of the uniform distribution of the stimulus.
Small electrodes are less comfortable, but very useful for selective recruitment of small muscles (for instance, intrinsic muscles at the level of the hand).
Except for symmetrical biphasic current (see later), pin colour matters, with the black pin (cathode) placed proximally (at the level of the motor point, or entrance of the nerve in the muscle belly) and the red pin (anode) placed distally.
Part 3. Stimulation parameters to consider
The first parameter to take into account is the waveform. We may distinguish between direct and alternate current. Direct current provoked the flow of the electrons in one direction, which results in accumulation of ions and increased risk of discomfort, pain or even skin burns. To avoid this issue, alternate current is preferrable.
Within alternate current, we may consider symmetric current (we wave that goes up has the same width and intensity as the one that goes down) and asymmetric (where one of the two waves is bigger than the other). Therefore, as we mentioned before the colour of the pins doesn't matter if we have symmetric biphasic current, while it matters whether it is asymmetric.
Finally, each wave can be defined by three fundamental parameters: intensity, width/amplitude, and frequency. Intensity is adjusted by increasing the mA (milli-amperes), width is regulated by setting the microseconds of duration (of a certain intensity), and frequency is measured in Hertz. Typical stimulation parameters are:
- for TENS: width: 50-250 microseconds, frequency: 1-20 Hz (low-frequency) and 80-120 Hz (high frequency);
- for EMS: width: 300-450 microseconds, frequency: 50-100 Hz
In theory, TENS is used for treating pain (continuous stimulus), while EMS is to elicit muscle contraction (with different programs of x-seconds ON and x-seconds OFF). As you may notice, the frequency is not that different, while pulse width is higher for EMS, as compared to TENS.
In my personal experience working with pedal-triggered FES I had to use simple devices, where current flowed continuously, being strong enough to elicit muscle contraction, and cause minimal to none discomfort. Therefore, I ended up using TENS devices to provoke muscle contraction, with continuous stimulation at about 100-300 microseconds of pulse width, frequency between 70 and 150 Hz, an analogic potentiometer to finely tune maximal intensity, and of course a pedal in between to control timing and modulate the intensity. What I want to tell you is that TENS can be used to provoke muscle contraction, by just increasing the stimulation parameters. And they are much cheaper than EMS devices (like TENS 7000).
Part 4. Three limitations of FES (and how to overcome them)
Problem 1. Differences between natural versus artificial muscle contraction:
- voluntary: recruitment of small, fatigue resistance fibres first, asynchronous pattern
- artificial (FES): recruitment of large, fast fatiguant fibres first, synchronous pattern
Problem 2. Fatiguability: reduced contraction in response to the same stimulus (phenomenon related to the muscle physiology)
Problem 3. Accomodation: increase in threshold for action potential to occur (phenomenon related to nerve physiology)
All these issues results in sub-optimal contraction in response to a stimulus, and reduced reliability over time. The best way I found to solve this is the adoption of a pedal to tune progressively up and down stimulation intensity. In fact, progressive increase of stimulation intensity gives more time for the recruitment of muscle fibres, and is also used for the recovery of denervated muscles. Furthermore, constant modulation of stimulation parameters (like intensity) prevents both fatiguability and accomodation. If it happens, then simply increase the maximal stimulation intensity you need, and then modulate with the pedal.
Part 5. Indications, contraindications, precautions.
Indications:
- any neurological condition where the patient needs FES to accomplish a functional movement;
Contraindications:
- pregnancy;
- pacemaker or other cardiac implants;
- cancer (increased risk of metastasis;
- local presence of thrombophlebitis or phlebothrombosis (for risk of embolism)
- acute tubercolosis
- over carotid sinus
- cognitive impairments (unable to follow directions or provide feedback)
Precautions:
- seizure
- obesity
- absent or diminished sensation
- skin conditions (eczema, psoriasis, acne, dermatitis, infection, allergies to gel or tape)
- diabetes - fragile thin skin
- peripheral neuropathies or areas of denervation
- metal (internal or external)
- limitations (for range of motion or active excercise)
- spinal cord injury (dysreflexia)
To conclude, FES is a very powerful tool to recover functionality in neurorehabilitation, but is highly operator-dependent. Having the right, evidence-based neurorehabilitation perspective (motor learning-driven approach) and specific competences (anatomy, parameters, customized solutions) is fundamental to accomplish meaningful outcomes.
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