EVALUATION OF PATTERNED STIMULATION FOR USE IN SURFACE FUNCTIONAL ELECTRICAL-STIMULATION SYSTEMS

The rapid onset of muscle fatigue is one of the major limiting factors when using electrical stimulation to restore functional movement in i ndividuals with a neurological deficit. It has been proposed that musc le efficiency can be improved by using variable frequency pulse trains (VFT), consisting of repeated doublets, rather than a constant freque ncy train (CFT) of stimulating impulses. The aim of this study was to test this hypothesis. Experiments were conducted on fifteen non-impair ed and one spinal cord injured subjects. It was shown that a VFT deliv ering the same electrical energy with a CFT produces a relative increa se in muscle output, attaining a maximum for VFTs composed of doublets with an inter-pulse interval of 5 ms. It was also shown that this eff ect is highly dependent on the experimental conditions and especially the intensity of applied stimulation and this effect diminishes at hig h intensity levels. We suggest that any improvement in muscle output o bserved when using a VFT can be attributed to changes in motor unit re cruitment and is not an intrinsic property of the muscle. Our results show that there is practically no benefit in using VFTs for alleviatin g the problem of fatigue in functional electrical stimulation systems. (C) 1998 IPEM. Published by Elsevier Science Ltd. All rights reserved .