Modeling transcutaneous electrical stimulation

Abstract

Transcutaneous electrical stimulation (TES) is a technique to artificially activate motor nerves and muscles by applying electrical current pulses to pairs of electrodes placed on the skin surface. TES can be used for sports, fitness, rehabilitation, and restoration of lost motor functions, e.g. in subjects with brain or spinal cord lesions. In this thesis a versatile TES simulation framework (TES model) that calculates nerve activation using a finite element model combined with non-linear nerve models was developed. The aim of this model was to investigate new TES technologies and to better understand specific effects in TES. Nerve activation in the TES model was calculated either in nerve bundles or by using activation volumes which describe whole regions where nerves are activated. Computer graphics methods were applied to the activation volumes in order to introduce measures to estimate the activation depth and the selectivity. The TES model and its parametrization were verified using experimental measurements that were undertaken on human volunteers. Three types of measurements were performed and compared with the TES model: • Voltage measurements using surface electrodes that measured the potential on the skin and needle electrodes in order to measure intramuscular voltages. • Motor thresholds on the volunteers were measured using an accelerometer that was placed on the stimulation electrode. • Finger forces were measured during TES using a grasp assessment system. A sensitivity study was performed in order to quantify the influence of different model parameters on nerve activation (e.g. capacitive effects, electrode-skin contact resistance, electrode material resistance, or nerve depth). This revealed the parameters with the largest influence in TES and allowed us to conclude which model simplifications can be made. One of the goals of TES systems is to selectively activate motor nerves in order to accomplish a specific motor function. However, TES