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 individuals with a neurological deficit. It has been proposed that muscle efficiency can be improved by using variable frequency pulse trains (VFT), consisting of repeated doublets, rather than a constant frequency train (CFT) of stimulating impulses. The aim of this study was to test this hypothesis. Experiments were conducted on fifteen non-impaired and one spinal cord injured subjects. It was shown that a VFT delivering the same electrical energy with a CFT produces a relative increase 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 effect is highly dependent on the experimental conditions and especially the intensity of applied stimulation and this effect diminishes at high intensity levels. We suggest that any improvement in muscle output observed when using a VFT can be attributed to changes in motor unit recruitment and is not an intrinsic property of the muscle. Our results show that there is practically no benefit in using VFTs for alleviating the problem of fatigue in functional electrical stimulation systems.