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Functional electrical stimulation and spinal cord stimulation: a comparison

By Tommy Sutor, MS, CSCS

Former Program Director at Push to Walk NJ, currently pursuing a Ph.D. in movement science exploring the coordination between walking and breathing.

In October, a scientific article written by researchers from the University of Louisville and Frazier Rehab Institute presented with some pretty incredible results; a man who had been paralyzed from a spinal cord injury for over 4 years had regained the ability to voluntarily move his legs and stand. These results came after another study performed at UCLA in 2015, which reported similar results, of the return of voluntary movement at least two years after a spinal cord injury. Importantly, all the participants in these studies had been diagnosed with “complete” spinal cord injury, meaning they had no ability to move or feel any part of their body below their injury. In both studies, the movements were not perfect and were not always useful to the study participants in everyday life. Nonetheless, they served as important proof-of-concept studies, showing that return of movement after a spinal cord injury is at least possible.

How is this possible? 

Both studies involved long periods of physical rehabilitation and exercises. What set both of these studies apart is they employed methods of electrically stimulating the spinal cord of the research participants, giving the participants more “bang for their buck” from the exercises they were doing. In the Louisville study, an epidural stimulator was used, which is a device that is actually surgically implanted inside the spine to stimulate nerves in the spinal cord. In the UCLA study, a transcutaneous stimulator was used; transcutaneous means “over the skin”. Thus, this stimulator used adhesive pads on the lower back and the abdomen, combined with a special electrical waveform that could stimulate nerves in the spinal cord from outside the body.

In both studies, the electrical stimulation to the spinal cord was the crucial ingredient needed to enable voluntary movement after it had been lost due to spinal cord injury. In the words of Dr. Reggie Edgerton, who has been involved in many transcutaneous and epidural stimulation studies, the electrical stimulation excites the spinal cord, and acts as a “hearing aid”, allowing nerves in the spinal cord to “hear” the intention to move that comes from the brain.

As studies like this one have been released, many people have wondered, “Can riding an FES bike do the same thing?” This is a reasonable question – after all, an FES bike uses electrical stimulation to excite nerves in your body to make your muscles contract. However, many people with complete spinal cord injuries who diligently use their FES bike know that, in most cases, the use of an FES bike alone is not enough to return the ability to voluntarily move one’s legs. But why is that true, when both FES cycling and spinal cord stimulation involve electrical stimulation?

Comparison of FES and spinal cord stimulation

To understand this better, let’s explore the two main ways movement occurs: reflexes and/or the voluntary intent to move. In a reflex, like when a doctor hits below your kneecap with her reflex hammer and your leg kicks out, the reflex hammer hitting below your kneecap activates sensory nerves (the green circles and lines in the drawings below) in your leg. These sensory nerves send “activation” signals to other nerves in your spinal cord (represented by the purple box), including motor nerves (the red stars in the drawings). These motor nerves send long projections out from your spinal cord, called axons (the red lines that go to the muscle), to your leg muscles, and your leg muscles contract, briefly kicking your leg out in front of you. Similarly, when a person voluntarily intends to move, this intent originates in the brain; nerves in the brain send signals down the spinal cord (represented by the blue arrow and lines) to other nerves, including motor nerves, which then send signals down their axons to a muscle to make it contract.

How Does FES Work?

FES bikes use a specific type of electrical waveform to target just the axons that project out from motor nerves. By placing FES pads over a specific muscle, the electrical waveform will activate those motor nerve axons directly (as seen by the lightning bolts in drawing “A”). The intention of the FES bike is to skip over the sensory nerves and nerves from the brain and provide enough electrical stimulation to a motor nerve axon to bring it above a certain threshold to induce a muscle contraction, whether the user is thinking about moving or not.

How Does Spinal Cord Stimulation Work?

Spinal cord stimulation, whether epidural or transcutaneous, works quite differently. Because of the waveform and the location where it’s applied, spinal cord stimulation actually activates all the nerves in and around the spinal cord, including both sensory and motor nerves. An even bigger difference between spinal cord stimulation and FES, however, is that with spinal cord stimulation, the intention is to provide enough electrical stimulation to be just below the threshold that will induce a muscle contraction (represented by the smaller lightning bolts in drawing B, now near all the nerves in the spinal cord). Why is that? The idea is, the spinal cord stimulation plus the voluntary intent to move/reflex activation together cause the motor nerve axon to fire and the muscle to contract. As was mentioned earlier, the intention of spinal cord stimulation is to enable motor nerves to “hear” activation signals from sensory nerves or brain nerves.

Despite promising results, spinal cord stimulation studies are still in their infancy in humans, and it will take much more safety data and many years of development before spinal cord stimulation is used regularly for spinal cord injury rehabilitation. Because of this, almost all rehabilitation professionals agree that people with a spinal cord injury should do everything they can to keep their bodies in optimal condition to be ready for better medical treatments in the future. For example, spinal cord stimulation may benefit people more if they already have strong muscles and cardiovascular systems when they begin using it. Thus, people who are already using FES bikes to keep their muscles and cardiovascular system in shape should continue doing so, with the added knowledge that you are preparing your body for even better things to come. If you don’t currently use an FES bike but want to start, the MyoCycle is a low-cost, high-quality option that I highly recommend.