How electrical stimulation can change propulsion
With 24 channels, you can image how many options there are for stimulating muscles using the implanted neural prosthesis. True, the system was originally designed to allow people with paralysis to stand, but there is more to this system than that.
In fact, exploring what the system can do is part of the discovery process. The engineers of the implanted standing system designed it to do just that, stand. As people began using the system, questions were raised as to whether or not they could program it to do something else. With electrodes implanted in the lower back, could we design a stimulation system for seated balance. Using input from the system users, computational modeling and advances in implanted technology, the evolution of stimulation system for seated balance was born.
What is seated balance? Think about sitting on a Pilates ball — it is all about seated balance. Muscles in your core keep you sitting upright allowing you to reach and have stability. For those of us whose trunk muscles are paralyzed, you can image how we need to compensate. For instance, with no core muscles, it is not possible to do a two-handed reach to pick up the Amazon box off your front porch. Instead, I hang onto something stable with one hand and use the second hand to try to grab the box. If the box is too big, I give up and push it in the house with my feet and empty it on the floor. Does that work? Sure. Is it ideal? No way.
Exploring the use of trunk stability for people with paralysis started with the use of two electrodes in the erector spinae (the lower back) when a research participant worked with the researchers so he could have stability while he was skeet shooting. But only two electrodes could not provide the desired trunk control. Once the implanted technology advanced from the original 8-channels to options for 12 or 16, then a dynamic seated balance system became an option. Then the gluteus maximus (butt muscles) were added, followed by the quadratus lumborum (more back muscles) and the adductor magnus. With these muscles working together, someone with complete trunk paralysis like me has seated balance. Using these muscle groups, the efficiency of wheelchair use increases as well.
This leads to the subject of propulsion. To maneuver a manual wheelchair, it takes upper body strength and trunk control. To propel up a ramp takes some level of trunk muscles. But efficient propulsion equates to an efficient use of kinetic energy. A rigid structure can retain the kinetic energy more so than one that is not rigid. Think of the difference between a folding and non-folding (rigid) wheelchair. A folding wheelchair has many moving parts and those hinges in the structure that lose energy while propelling. This is one reason why self-propelled manual wheelchair users tend to prefer rigid chairs. They are simply more effective.
So, what if we made the body rigid too? Another point of energy loss during propulsion is through uncontrolled fluidity of the body in the chair. By using coordinated stimulation of the trunk muscles the body becomes more stable and there is less energy loss while maneuvering around.
During the long weeks of waiting for the incisions to heal, I was not able to swim or use my handcycle which are two ways that I get cardio exercise. The only other option is to go for a roll; just get out and propel the wheelchair for a few miles. The research team programmed a stimulation pattern labeled as wheelchair propulsion. The program stimulates the trunk muscles at various levels to make the body more rigid in the wheelchair. Does it make me wheel any faster? No, but it is does allow for propulsion with less effort. Part of the reason is that it lowers the loss of kinetic energy.
If I’m going out to get some cardio exercise and wheel around the neighborhood or a municipal park, why not use the stimulation pattern? Keep in mind that over the last eight months, I have not used some of the muscles in this pattern and it was evident. Before the original IST-16 failed, I had really strong adductor magnus muscles (post ads for short). These are the inner thigh muscles that technically adduct the hip and for me, bring my knees together. Mine were so strong that I could hold a piece of paper between my knees.
But after eight months of disuse, it was obvious the muscles were weak. The wheelchair propulsion program stimulates the back, butt and inner thigh muscles. While using the wheelchair propulsion program for a simple two-mile stroll, within less than 1/2 mile, the post ads were barely contracting. This was because the muscles are weak and fatigued quickly, not that the stimulation to the muscles was lowered. The advantage of having strong muscles from the daily use of stimulation over years had been lost in just a few months. Now is the hard climb back to regain that strength.
Note: The statements and views posted here are of my own and do not reflect those of Case Western Reserve University, the Department of Veteran Affairs, Metro Health Medical Center or the National Institutes of Health along with their representives involved with this program. If you are interested in this clinical trial, please visit https://clinicaltrials.gov and search NCT00623389 or NCT01923662.
