Applications of electrical stimulation for restoration of functional movements such as standing, gait, and grasp have always been hindered by the rapid fatigue of stimulated muscle. This paper describes an experimental investigation of stimulation with N-lets (a set of N closely spaced stimulation pulses) as a means of producing contractions with improved fatigue characteristics. Experiments were conducted on 27 able-bodied and four SCI human subjects using surface stimulation of the quadriceps muscle to produce isometric knee joint torque. Based upon evidence from the literature on muscle fatigue, parameters of the N-let trains for N = 1-6 were optimized to produce the most force per pulse. The results demonstrated that: 1) nonlinear summation of the twitch response occurs in human subjects with N-let surface stimulation; 2) for most subjects, doublet stimulation (N = 2) with a pulse interval of about 5 ms produced the maximum torque-time integral per pulse of the resulting twitch; and 3) on average, optimal N-let stimulation resulted in a 36% increase in isometric torque tracking when compared to traditional singlet stimulation. The results have immediate implications for alleviating the problem of premature fatigue during functional electrical stimulation.
Bibliographical noteFunding Information:
Manuscript received February 28, 1994; revised April 19, 1995. This work was supported by the Rehabilitation Research and Development Service of the Department of Veterans Affairs and by the Brit and Alex d’kbeloff Chair in Engineering Design. The work of Z. Z. Karu was supported by a fellowship grant from the Office of Naval Research. Z. Z. Karu is with the Department of Electrical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 USA. W. K. Durfee is with the Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455 USA. A. M. Barzilai is with the Department of Mechanical Engineering, Stanford University, Stanford, CA 94305 USA. IEEE Log Number 9412532.