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Functional Neuromuscular Stimulation (FNS) in Walking and Pedaling
Felix E Zajac, PhD; Lisa Schutte, PhD; Scott Tashman, PhD; Gary Yamaguchi, PhD; Inder Perkash, MD
Objective - The paralyzed muscles of patients with spinal-cord injuries can be activated by electrical stimulation (FNS, functional neuromuscular stimulation). However, for the patient to use FNS to perform useful tasks, such as walking or pedaling an ergometer for cardiopulmonary exercise, stimulation patterns for many muscles must be carefully coordinated. In this study we tried to determine the most effective stimulation patterns for walking and pedaling using mathematical models and computer simulations.
Approach - The model for pedaling took into account the activation and contractile properties of three muscle groups in each leg (gluteals, hamstrings, and quadriceps) and the dynamical properties of the ERGYS ergometer and controller. Feedback about crank position and cadence were used to adjust the amplitude and timing of stimulation to each muscle group. The strength and fatigue rate of each muscle group were chosen to best reproduce the measured cadences and power output of a spinal-cord-injured patient.
The walking simulations employed a three-dimensional dynamical model of both legs and the torso. Improper muscle stimulation patterns would cause tripping due to premature ground contact or knee hyperextension due to prolonged swing (Figure 1). The pattern optimized to match the kinematics of normal walking agreed with the EMG activity recorded from able-bodied individuals during normal walking.
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Figure 1. Stick figure simulation sequences of a normal step (d), stance leg collapse (a), tripping because of premature ground contact (b), and knee hyperextension of the end of swing due to a long step (c). |
Simulations were also performed of a patient with crutches and a walking orthosis (a brace that allowed him to swing his legs by twisting his trunk)(Figure 2). The simulations revealed that high arm and trunk muscular effort is normally required to accelerate the swing leg. However, this high trunk and arm effort can be substantially reduced if the swing leg begins its flexion differently, and it may be possible to achieve this by electrically stimulating the hip muscles in the preceding double support.
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Figure 2. Walking orthosis and four-segment model of an ambulating SCI subject.
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3. Different seating configurations studied by computer simulation.
Conclusions - In the pedaling study, simulated cadences, power outputs, and oxygen consumption agreed with measurements taken from spinal-cord-injured patients. The model was used to assess how seat configuration and the intermuscle stimulation pattern affect the rate of metabolic energy use and the ability of the patient to pedal steadily.
The walking study showed that undisturbed level gait at normal speeds is possible, though difficult, using FNS of seven muscle groups in each leg. The walking orthosis study indicated that FNS of hip muscles to augment orthotic double support can produce efficient ambulation.
Republished from the 1994 Rehabilitation R&D Center Progress Report. For
current information about this project, contact Felix E Zajac.
