Surface Electrical Stimulation of Skeletal Muscle After Spinal Cord Injury.

Hillegass EA, Dudley GA (Department of Physical Therapy, Georgia State University, Atlanta; Department of Exercise Science, University of Georgia, Athens, Ga), Spinal Cord. 1999;37:251-257

The use of functional electrical stimulation (FES) systems enables some individuals with spinal cord injuries to restore their standing and ambulation ability. This functional improvement requires the ability to develop sufficient lower-limb muscle torque using a surface electrode stimulation system. The purpose of this study was to examine the response of the quadriceps femoris muscle to surface electrical stimulation.

An experimental group of 8 people with complete spinal cord injuries of an average duration of 4 years and a control group of 8 adults without spinal cord injuries were recruited for this study. The cross-sectional area (CSA) of the quadriceps femoris musculature was measured in each subject using magnetic resonance imaging (MRI) techniques. Each subject then received 2 levels of electrical stimulation (ES) to the left quadriceps femoris muscle using a commercially available stimulator.

The first level (low-level ES) used an intensity that was sufficient to elicit full knee extension with a 2.3-kg weight attached to each subject's ankle. The second level (middle-level ES) used an intensity that elicited a contraction equal to 30% of maximal voluntary isometric contraction (MVIC) in the control group. A third level (high-level ES) was applied only to the control group, using an intensity that elicited 60% of MVIC in the control subjects. Each level of ES was applied using 3 sets of 10 one-second isometric contractions applied in a 50-Hz train of 400-[micro]s biphasic pulses with a 50-[micro]s phase delay. There was a 1-second rest between each contraction and a 90-second rest between sets. Immediately following each level of the electrical stimulation intervention, the cross-sectional area of the quadriceps femoris muscle that was stimulated was measured using MRI.

The results indicated that the average CSA of the quadriceps femoris muscle was significantly smaller in the experimental group (24 [cm.sup.2]) compared with the control group (75 [cm.sup.2]). Because the CSA was smaller, low-level ES caused a significantly larger proportion of the muscle to be stimulated in the experimental group (61%) compared with the control group (6%). Middle-level ES likewise stimulated a significantly larger proportion of the muscle in the experimental group compared with the control group (92% versus 20%, respectively).

The torque produced by the experimental group was also greater (30 N [multiplied by] m vs 22 N [multiplied by] m) than by the control group at the low level of electrical stimulation. At middle levels of electrical stimulation, the control group produced much greater amounts of torque (70 N [multiplied by] m vs 44 N [multiplied by] m). Increased fatigability was also noted in individuals with spinal cord injury, who had poor recovery of torque between sets.

The authors concluded that application of adequate amounts of ES causes a relatively large portion of the affected muscle to be stimulated in people with spinal cord injury. The torque produced by this stimulation, however, may be only modest compared to that produced in people without spinal cord injury because of the smaller size and increased fatigability of these muscles.

Robert Sandstrom, PhD, PT Creighton University Omaha, Neb