11th Annual Conference of the International FES Society September 2006 – Zao, Japan
Gradual Muscle force potentiation during constant electrical stimulation -Dependence on stimulation current and pulse-duration Ji-Won Kim1, Min-Young Kang1, Gwang-Moon Eom1 1
Biomedical Engineering, Konkuk University, Choongju, Choongbuk, 380-701, Korea
[email protected] the muscle length [2]. In this work, we investigated the dependence of muscle force potentiation on both the pulse-amplitude (current) and the pulse-duration with different ramp-up time.
Abstract The purpose of this work is to investigate the fundamental properties of the gradual muscle force potentiation for the prediction of muscle force and body movement due to electrical stimulation. We investigated the dependence of force potentiation on both the pulse-amplitude and the pulse-duration with different ramp-up time. The experimental results showed that the force increment ratio (FIR) during constant electrical stimulation decreased with pulseamplitude and also with pulse-duration. The FIR was greater with short ramp-up time in both the pulse-amplitude and pulse-width modulation. The feasible mechanism might be that the myosin light chain phosphorylation induces the force potentiation and it occurs only in the fast type muscle fibers which are recruited first. These observations indicate that muscle potentiation must be understood well for the accurate control of muscle force.
1. INTRODUCTION Functional Electrical Stimulation (FES) has been used for neuro-rehabilitation of spinal cord injury (SCI) or cerebral apoplexy patients. Currently, open-loop control schemes based on trial and error are used in FES, since Liberson et al. purposed the use of electrical stimulation for the drop-foot patients [1]. It is very difficult for the musculoskeletal system model to provide accurate prediction of FES-induced muscle force and body movement, due to the time-variant and nonlinear properties of musculoskeletal system. One of the important properties which are not well understood is the very slow increase of muscle force during constant electrical stimulation named as the gradual muscle force potentiation [2]. The purpose of this study is to investigate the fundamental properties of the gradual muscle force potentiation. It was reported that the muscle force potentiation depends on stimulation frequency, stimulation history, and
DA c onversion 20Hz pulse signal
Stimulator and isolator
RTLinux system
4.096kHz Sampling
Subjec t
Force sensor (CPU strain gage)
Figure 1 Experimental setup
2. METHODS Figure 1 shows the block diagram of the experimental system setup. Nine healthy subjects (23-27 years old) participated in this experiment. The subjects were seated on Biodex system (Biodex Medical Systems, Inc.) with their trunk and thigh fastened. The realtime Linux System generated the constant current stimulation pulse by using an isolator (NEC Sanei, 5384). Vastus Lateralis muscle is stimulated via surface electrodes (ValuTrode, Axelgaard co.) attached on the motor points. Knee extension force was measured using a bidirectional force sensor (CPU gauge, Aikhoh Eng.) under isometric condition of 30° knee joint flexion. The stimulation frequency was fixed to 20Hz and the constant-current stimulation was used. Before the experiment, the motor point and the maximum tolerable current amplitude (Imax) with 250us pulse-width was manually determined. Ramp-up stimulation was applied prior to the main (constant) stimulation during tramp to prevent any reflexive muscle contraction, as shown in Figure 2. The time at the beginning of constant stimulation was set to be zero.
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11th Annual Conference of the International FES Society September 2006 – Zao, Japan
We investigated the effects of stimulation pulse-amplitude (20, 40, 60 and 80% of Imax) with fixed 250µs pulse-width, and the effect of pulse-duration (250, 500, 750 and 1000µs) with fixed 20% of Imax on the gradual force potentiation. All the conditions of pulseamplitude were designed to match those of pulse-duration in terms of the electric charge per pulse. The ramp-up time (tramp) was fixed at 5s in 6 subjects and was fixed at 2s in the other 3 subjects. The main stimulation time (tconstant) was set to be 25s. The muscles were rested long enough (more than 6 hours, depending on the stimulation condition) before the experiment, to minimize stimulation-induced muscle fatigue and to exclude the residual effect of the previous stimulation.
tramp=5s, and from that at 80% of Imax in case of tramp=2s (p
