Effect of electrical stimulation pattern on the force responses of paralyzed human quadriceps muscles

Most studies examining the effect of electrical stimulation pattern on the force response of muscle have been done in able-bodied persons. The purpose of this study was to examine the electrically elicited force responses of the paralyzed quadriceps femoris muscles of persons with spinal cord injuries (SCI) to see whether stimulation patterns that increase the force response in non-paralyzed muscle will do so in paralyzed muscle. Thirteen subjects ranging in age from 11 to 24 years old with motor-complete SCI were studied. Isometric muscle performance was tested using 6-pulse constant-frequency trains (CFTs), variable-frequency trains (VFTs), and doublet-frequency trains (DFFs) delivered at mean frequencies of 10, 20, 33, 50, and 100 Hz. In the non-fatigued and fatigued condition, the VFT and DFT peak forces were greater than the CFT peak forces at 10 Hz. In addition, in the fatigued condition the 20-Hz VFT peak forces were greater than the CFT peak forces, and there was a trend for the DFT peak forces to be greater than the CFT peak forces. In the non-fatigued condition, the 33-Hz and 50-Hz DFT force-time integrals were greater than both the CFT and VFT force-time integrals. In the fatigued condition, there was no significant effect of train-type on the force-time integrals. These results differ from those previously reported from studies using able-bodied persons and indicate that findings from studies of the electrically elicited force responses of the muscles of able-bodied persons do not apply to the paralyzed muscles of persons with SCI.