kralj’s approach was extended by graupe et al. into a digital fes system that employs the power of digital signal processing to result in the parastep fes system, based on us patents 5,014,705 (1991), 5,016,636 (1991), 5,070,873 (1991), 5,081,989 (1992), 5,092,329 (1992) and related foreign patents. the parastep system became the first fes system for standing and walking to receive the us fda approval (fda, pma p900038, 1994) and become commercially available.