Figure 3 shows the cross section of the MAV with activated and non-activated axons. If the membrane potential exceeded a certain threshold potential, activation of this particular axon was assumed. It can be seen that deep and small axons are less likely to be activated by FES compared to superficial and big axons. The diameter of the circles corresponds to the axon diameter. Red circles show activated axons, where the membrane potential surpassed the threshold potential. The parameters used for the FE model can be seen in Table 1.Ĭross section of the MAV. For the simulation a current of 20 mA with a duration of 150 µs was used. The electric potential caused by the electrical stimulation in the MAV was saved and exported to MATLAB to calculate the response of the motor axons. Three different active electrodes with the same area were considered in simulations and experiments: a longitudinal electrode (2 × 4 cm 2), a transversal electrode (4 x 2 cm 2) and a square electrode with an area of 8 cm 2(see Figure 2a). Thereby, the distal and bigger electrode was the indifferent and the electrode placed on top of the MAV was the active electrode. Two electrodes were placed on the surface of the skin. Three angles between the MAV and the longitudinal direction of the forearm were investigated (0°, 30° and 60°, see Figure 2b). It was assumed that there is one dominant axon orientation. The idea behind the MAV was to represent the way of the motor axons from the nerve entry point in the muscle to the motor endplates and was inspired by Gomez-Tames et al. The size of the MAV was estimated with regards to the results of Lieber et al. It was located 1.5 mm under the fat layer, had a height of 16 mm, a width of 24 mm and was 30 mm long. This cylinder will from now on be referred to as the motor axon volume (MAV). Besides the bones an elliptic cylinder was defined in which the motor axons, responsible for muscle activation, are located. Inside the muscle tissue the ulnar and radial were placed.
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This simple model of the human forearm consisted of a skin and a fat layer, surrounding the muscle tissue. In COMSOL Multiphysics a 3D finite element (FE) model was created (see Figure 1). 2 Methods 2.1 SimulationĪ two-step approach was used to simulate the outcome of FES. To validate these results, experiments with two subjects were performed to measure the force, generated by electrical stimulation of the finger flexor muscles.
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Therefore, a simulation study was performed to get insights on how axon activation is influenced by both axon alignment and electrode geometry. In this study, the goal was to determine the impact of electrode geometry on electrical stimulation.
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That is why it makes sense to spend time on simulation studies before expensive prototypes are developed. Modeling and simulation studies can be a great assistance in developing and improving systems for FES, especially for future multi-pad systems, since they are not commercially available right now and need to be custom made for research activities.
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They could help reducing fatigue by excitation of different motor axons within one muscle as well as easing the process of finding the best spot and electrode shape for stimulation. Multi-pad electrodes could be one way to solve several problems regarding FES. grasping, and can be used to help people suffering from spinal cord injury or stroke. Functional electrical stimulation (FES) is a way to restore lost movements, e.g.