Functional Electrical Stimulation
Info on FES from ActiveLinx
How does FES Actually work?
FES stands for Functional electrical stimulation which can be applied through using adhesive electrodes or conductive garments, such as shorts with electrodes built in. With currently available technology, adhesive electrodes are best for conductivity with garments following up ease of use, although the muscle contraction is less efficient.
Stimulation is done through peripheral nerves in the body. These would usually be triggered by your brain, which is how the able bodied walk around. But for involuntary contractions, a computer can supply electricity to the peripheral nerves, circumventing the brain. There are ways to trigger muscle directly with a longer pulse width which we’ll talk about later.
Benefits of FES
Functional Electrical Stimulation has many benefits to those with paralysed limbs but also for abled bodied too, depending on the setup. It has been well known for Athletes to build additional muscle using all sorts of Electrical Muscle Stimulating devices.
For those with Spinal Cord Injury, or other Neurological problems the reliance on FES is high. It can support healthy bone density, muscle growth, improved circulation and also prevent pressure sores from muscle tone as well as increased blood flow. For those suffering from disability at a young age, keeping limbs functional supports bone growth and can prevent deformities due to in-activity. There is on-going FES Research into exactly how NMES and also FES Cycling can help build Skeletal muscle.
The Science Behind FES
Using a low powered biphasic current with a Pulse Width of up to 2500 Microseconds (1/1000000th of a Second), sensory and motor neurons are triggered. This happens when a negative charged electrode placed on the bulk of the muscle, and a positive electrode on the end of the muscle. The electrical current (Ions) can travel back and forth between each electrode causing the muscle to contract.
FES parameters should be set so at maximum stimulation levels, overfeed does not occur. Usually for a 1 hour cycle ride it’s good use 30 – 35hz with a 400 – 450us (microsecond) pulse width with a maximum of 130 – 150 mA (milliamps) for a two 3 x 4 inch electrodes. Different training programs can be setup for various cycling regimes, which you would need if purchasing an FES Bike.
For those with Spinal Cord Injury, the easiest way is to use an FES Bike, such as the Berkelbike which has up to 8 channels. Each channel has 2 leads, a negative electrode lead and positive lead. Traditionally the glutes, quads and hamstrings are triggered using a channel each with optional Abdominal, Errector Spine or Dual Channel Quad setup like in the FES Cycling video below.
By placing the adhesive electrodes further apart it’s also possible to penetrate more deeply causing a wider range of nerves to be activated since the pulse width increases too. However, by doing this your FES parameters would need to be adjusted accordingly as overfeed could cause unwanted muscles to contract at the wrong timings. Sometimes putting them closer together is better to prevent overfeed but for pinpoint accuracy it’s wise to use a motor point pen. If you want, you can check out this guide for electrode positioning for every single muscle group to get an idea.
How to know if you have Denervated Muscles
If you have a lower Spinal Cord Injury at lumbar or sacral vertebrae then it is possible you have damaged your reflex arch. By tapping the patellar tendon with a small hammer, ths can evoke a knee jerk response. If nothing happens, chances are you have denervated muscles which will not respond to FES, EMS, NMES or TENS. The good news is there is still a solution to maintain muscle mass with the Den2X stimulator using more powerful electrical waveforms to trigger skeletal muscle.
Biphasic Current for Denervated Muscles
Muscle Denervation means for low level SCI injuries that the reflex arch has been damaged, disabling or reducing peripheral nerve function in the legs. Atrophy from Spinal Cord Injury can occur within 2 weeks with up to a 50% loss in muscle mass over 2 – 3 months. For lower level injuries, it’s important to get started as soon as possible as FES Cycling can’t be used to trigger the peripheral nerves in the legs. Fortunately, part of the Skeletal muscle itself does not require nerves to evoke a contraction. However the Current is more powerful because the Pulse Width for use on larger muscle groups is between 60 – 75 milliseconds as opposed to being in microseconds. Instead a 2hz frequency is recommended for a Twitch Contraction. This helps activate large atrophied muscles efficiently and is suggested for use of 5 – 6 times per week for progressive results.
The Dens2x Stimulator is specifically designed to support those with paralysis suffering from muscle denervation and feeling the effects of not being non responsive to FES.
Electrodes for Carbon Electrodes
Large Carbon rubber electrodes using a wet sponge contact using an elasticated wrap are the standard choice for stimulation using a wide pulse width. This ensures current is evenly distributed and is very important to prevent hot spots by there being good conductivity to the skin.
For safety reasons it is recommended to ensure the electrodes are moistened and bound tight by the elastic straps.
In-adequetely prepared electrodes could result in skin burn or discomfort, especially when using high powered stimulation. Always follow the programs allocated by your physician and double check the display is on the correct stimulation program for you. This is not a toy and it does use electricity which if used improperly, can have serious consequences. For example caution should especially be taken with unused leads because high powered current could give you a shock and skin burns. Same goes for improperly connected leads, so always do your safety checks before beginning a session.
Exponential Current using Triangular Waves
There is also a setup for smaller muscle groups in the arms for example that uses triangular shaped pulses. Using this technique is great for ensuring neighbouring muscles that are not denervated are not activated. This setup uses a 100ms – 250ms pulse width depending on the severity of the denervation.
In Oct 2016 we were at the ‘World’s First Bionic Olympics’ called the Cybathlon. It took place in Zurich where teams from around the world gathered to compete in a variety of different events which merged man and machine for a Paralympic spin off. In the FES Cycling category the UK Team came second place, winning a Silver Medal.
CEO Paul Moore had been on a Press tour with Imperial College and a lot of effort went into logistically preparing for the event.
A quarter of my working feat has been working on the Cybathlon, especially during the last 6 months. I’m chuffed, I am – excellent! – Paul Moore