Simulating Wheelchair Posture

A three-dimensional biomechanical model predicts how effectively functional electrical stimulation (FES) stabilizes seated postures

Implanting electrodes into paralyzed torso muscles can help individuals with spinal cord injury balance in their seats. So say researchers at Case Western Reserve University, who have built a three-dimensional biomechanical model that predicts how effectively functional electrical stimulation (FES) stabilizes seated postures.


In 2003, the late actor Christopher Reeve received implanted electrodes for FES to help him breathe, and various other types of FES are under investigation for help in bowel and bladder control, coughing, walking, and standing. However, relatively little attention has been paid the subtle muscle movements of torso stabiliztion required for balanced, steady sitting, says Ari Wilkenfeld, MD, PhD, first author of the study that appeared in the March/April issue of the Journal of Rehabilitation Research & Development.


For those with spinal cord injury, hooking one arm over an armrest for stability is a common strategy to maintain balance when reaching. Courtesy of Cleveland FES CenterA stable seated position means being able to reach with one or both hands and not fall over, Wilkenfeld says. A healthy posture also prevents skeletal deformities, pressure wounds, and too much pressure on internal organs.


The Cleveland group’s model of the human torso simulates how three muscle groups work in synergy to rotate the spine and bend it forward and sideways. Knowing from previous research that a paralyzed muscle stimulated by FES produces, at most, about 50 percent of the force of a non-paralyzed muscle, Wilkenfeld, along with investigators Ronald Triolo, PhD, and Musa Audu, PhD, at the Cleveland FES Center, used the model to calculate the largest range of stable movement that a paralyzed torso could attain under ideal FES.


They found that with the help of FES, paralyzed individuals can hold the weight of one or two bricks at arm’s length, bend forward enough to extend their reach by almost a foot, and bend to the side a bit more.


In addition to creating the model, the Cleveland researchers compared its predictions to the actual sitting of a test volunteer with one pair of implanted spine electrodes. They found that one pair is not ideal because it does not fully activate even one of the sets of muscles. Yet they found that the model describes seated postures well.


“It is a promising start,” says Jason Gillette, PhD, an assistant professor who specializes in biomechanics and motor control at Iowa State University. He suggests testing more individuals and expanding the tests to include active reaching, not just still postures.


Additionally, says Wilkenfeld, they’ll need a more sophisticated system of FES implanted electrodes to get the kind of results predicted by the model. Yet, now that they have a model that shows two-handed reach and the stable sitting postures theoretically possible, they can work on the practical details for attaining them.


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