Varying pulses help stimulate 'paralyzed muscles'
Vol. 17, No. 26April 2, 1998

Research results

Varying pulses help stimulate 'paralyzed muscles'

When Superman actor Christopher Reeve broke several upper cervical vertebrae in a Virginia steeplechase accident in 1995, he was paralyzed from the neck down, and the prognosis was poor. Such devastating mishaps are not uncommon.

Potential solutions have ranged from possible spinal cord regeneration to use of bionics-with little real success so far. But in the future, scientists such as Stuart A. Binder-Macleod, physical therapy, say they hope that such tragedies can be reversed, at least in part, by artificial stimulation of the skeletal muscles.

In the April issue of the Journal of Neurophysiology, Binder-Macleod, with doctoral candidate Samuel C.K. Lee and undergraduates April D. Fritz and Lorin J. Kucharski, explores the relationship between the induction of small electrical shocks and the strength of progressively fatigued leg muscle reactions in seven men and five women who volunteered for a laboratory study. The paper, titled New Look at Force-Frequency Relationship of Human Skeletal Muscle: Effects of Fatigue, will add to the medical literature that might someday help people such as Reeve improve function.

"Electrical (muscle) stimulation is used for many things by physical therapists. My research, in general, tries to understand the relationship between stimulation rate, the force the muscle produces and the fatigue it induces," Binder-Macleod said.

In a typical situation, "we use a 20- or 30- or 40-pulse-per-second electrical stimulation to activate a muscle," he explained. "But when the central nervous system turns on a muscle it uses an irregular frequency of pulses during activation. What we've tried to do is look at the best pattern rather than the best frequency...comparing forces produced over a large range of different frequencies. These are really very brief pulses of current. A microsecond is only a millionth of a second, and each pulse might only last 600 microseconds, just enough to activate the muscle, with a 'pulse train' of maybe 10 pulses."

Binder-Macleod, who has been at UD 11 years, is an associate professor and associate chair of the physical therapy department. He earned a bachelor's degree in physical therapy from the State University of New York in Buffalo in 1974; a master's degree in medical science from Emory University in Atlanta in 1979; and a doctorate in physiology from the Medical College of Virginia in Richmond in 1987.

Why use electrical stimulation on muscles? Binder-Macleod cited several reasons. He points to the now common and relatively simple heart pacemaker, which sends minuscule "shocks" to the heart to maintain its rhythm. "And, we use electrical stimulation to make muscles in people with central nervous system problems functional, for instance in someone who's had a stroke and whose foot drags. You can put on an ankle brace, or you can stimulate it so the foot lifts up when they walk. One problem is that the muscles get tired more easily when you activate them artificially," he said. "But, there are certain functions of the central nervous system that we don't understand," he added.

To help people such as Reeve, "the primary focus of most of the research is to try to get them to stand and walk by using electrical stimulation," Binder-Macleod explained. But success has been limited, he said: "To date, it has still not enjoyed widespread public acceptance. There are important control problems... trying to get the muscles to coordinate naturally. We're a very long way away from getting someone like Reeve to stand up and walk."

In conducting the study, the volunteers were seated on a chair-like device, a dynamometer, with a seat belt. The equipment is designed to measure muscle force, both induced naturally by the subject or by electrical stimulation of the muscles in the area of the knee, he said. To measure reaction of the muscles, the subjects would "try to straighten out their knee as hard as they could," using the quadriceps femoris muscle on the front of the thigh. "Then they just relaxed, and we used electrical pulses to activate them to about 20 percent of maximum force," according to Binder-Macleod.

"We used a range of frequencies going from 8 pulses per second to 100 pulses per second, and we used variable frequency trains, with some (sets of pulses) closer together than others, and we'd look at the force produced in response to the stimulus," Binder-Macleod said.

"The basic results were very exciting," Binder-Macleod explained. "With variable frequency trains we were able to produce more force than with any constant frequency trains. And it's the first study that shows this. No one has ever done this before."

The study suggests that using electrical stimulation with varying intervals, rather than regular intervals, "may be useful during clinical applications of electrical stimulation," according to his research.

The experiments described in the article are part of a federally funded, five-year project through a $500,000 grant from the National Institutes of Health.

-Phil Milford