1 Center of Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria.
2 Ludwig Boltzmann Institute for Electrical Stimulation and Physical Rehabilitation, Wilhelminenspital, Vienna, Austria. winfried.mayr@meduniwien.ac.at.
3 Otto Bock Healthcare Products GmbH, Vienna, Austria. winfried.mayr@meduniwien.ac.at.