A Finite Element Model of the Electrically Stimulated Human Thigh: Changes due to Denervation and Training *Yvonne Stickler, *†Johannes Martinek, ‡Christian Hofer, and *Frank Rattay *Institute for Analysis and Scientific Computing,Vienna University of Technology; †Center of Biomedical Engineering and Physics, Medical University of Vienna; and ‡Ludwig Boltzmann Institute of Electrical Stimulation and Physical Rehabilitation, Institute of Physical Medicine and Rehabilitation, Wilhelminenspital, Vienna, Austria Abstract: The complete denervation of muscles leads to changes in the muscle fibers as well as in the surrounding tissue. Concerning excitability the most important changes are reductions in fiber diameter, in muscle cross-sectional area, and in electrical conductivity of the muscle tissue. These changes can be partially reversed by intensive elec- trical stimulation. Evaluation of a 3D finite element axial symmetric model of the human thigh shows that the train- ing leads to a reduction in threshold values between 17 and 51 percent, depending on the position of the fiber in the thigh. Single parameter variation clarifies the influence of each of the different factors. The electrode position was found to be most effective with the electrodes as far apart from each other as possible. Due to (i) comparatively higher changes in potentials at the distal electrode; and (ii) variations in sodium channel dynamics, lowest threshold values can be reached with a hyperpolarizing first phase of the biphasic impulse at the distal electrode. The tissue of the denervated muscle is known to be highly inhomogeneous. Simulations demonstrate that the related irregularities in the field can actually initiate fiber activation. 3D finite element simulations show the overall positive effects of FES on muscle tissue, especially an improved excitability of the muscle fibers. Furthermore the method gives an insight into the relations between potential distribution, electrode position, geometric effects, and muscle fiber activation that cannot be obtained by measurements. Key Words: Denervated muscle— Muscle fiber—Functional electrical stimulation—Finite element model—Activating function—Threshold. INTRODUCTION The complete denervation of muscles leads to changes in muscle fibers as well as in the surround- ing tissue. Long disuse or immobilization causes fiber atrophy and a reduction of total muscle cross- sectional area (1–3). After 1–2 years of denervation, the muscle area is reduced to 40% of the status before injury (4). Furthermore, the structure of the muscle fibers partially dissolves and the muscle gets enriched with fat and loose or fibrous connec- tive tissue, which leads to a reduction in electrical conductivity of the muscle tissue and to poor elec- trical excitability (1–3). Intensive electrical stimulation with large surface electrodes can partially reverse these changes (5–8). In the early stages of training very long impulses are needed to actually produce single twitches. After the excitability has improved due to the training, a tonus can be achieved with a train of shorter impulses and finally force training starts (4). During this training process, the mean fiber diam- eter of the trained muscles grows, the structure of the muscle tissue improves, and the total cross-sectional area of the muscle increases (4,5). Because of the simultaneous reduction in fat tissue, single fiber diameters may increase comparatively more than the corresponding cross-sectional area (4). After months of training patients can regain the ability to stand up (9,10). The following computer simulations should help to analyze the relations between geometry, muscle status, electrode placement and stimulus parameters. doi:10.1111/j.1525-1594.2008.00612.x Received May 2008. Address correspondence and reprint requests to Dipl.-Ing. Yvonne Stickler, Institute for Analysis and Scientific Computing, Vienna University of Technology, Wiedner Hauptstrasse 8-10 A-1040 Vienna, Austria. E-mail: yvonne.stickler@tuwien.ac.at Presented in part at the 9th Vienna International Workshop on Functional Electrical Stimulation held on September 19–22, 2007 in Krems, Austria. Artificial Organs 32(8):620–624, Wiley Periodicals, Inc. © 2008, Copyright the Authors Journal compilation © 2008, International Center for Artificial Organs and Transplantation and Wiley Periodicals, Inc. 620