Standing-up exerciser based on functional electrical stimulation and body weight relief M. Ferrarin 1 E. E. Pavan 1,2 R. Spadone 1 R. Cardini 3 C. Frigo 1,2 1 Centro di Bioingegneria, Fondazione Don Carlo Gnocchi ONLUS IRCCS, Politecnico di Milano, Italy 2 Dipartimento di Bioingegneria, Politecnico di Milano, Milano, Italy 3 Unita ` Operativa Rieducazione Funzionale, Fondazione Don Carlo Gnocchi ONLUS IRCCS, Milano, Italy Abstract—The goal of the present work was to develop and test an innovative system for the training of paraplegic patients when they are standing up. The system consisted of a computer-controlled stimulator, surface electrodes for quadricep muscle stimula- tion, two knee angle sensors, a digital proportional-integrative-derivative (PID) controller and a mechanical device to support, partially, the body weight (weight reliever (WR)). A biomechanical model of the combined WR and patient was developed to find an optimum reference trajectory for the PID controller. The system was tested on three paraplegic patients and was shown to be reliable and safe. One patient completed a 30-session training period. Initially he was able to stand up only with 62% body weight relief, whereas, after the training period, he performed a series of 30 standing-up=sitting-down cycles with 45% body weight relief. The closed-loop controller was able to keep the patient standing upright with minimum stimulation current, to compensate automatically for muscle fatigue and to smooth the sitting- down movement. The limitations of the controller in connection with a highly non- linear system are considered. Keywords—Functional electrical stimulation (FES), Closed-loop control, PID control, Standing up, Paraplegia, Body weight relief Med. Biol. Eng. Comput., 2002, 40, 282–289 1 Introduction STANDING UP from a chair is a fundamental motor act of daily living and is normally performed many times every day. It is a prerequisite for upright posture and gait, and without such an action many types of patient would not stand or walk, even if they are potentially able to do so. On the other hand, it has been pointed out that rising from a sitting position is the most mechanically demanding functional task undertaken routinely during daily activity (RILEY et al., 1991). Actually, this motor task calls for the production of mechan- ical work to increase the potential energy of the body. Therefore patients with musculo-skeletal pathologies of the lower-limbs (KERR et al., 1994) and even elderly people without specific pathologies (HUGHES et al., 1994) can find standing up consid- erably difficult. People presenting paralysis of the lower-limb muscles (e.g. due to spinal cord injury (SCI)) are not able to stand up at all without external assistance (BAJD et al., 1982). The possibility of standing for such patients has both func- tional and therapeutic benefits related to bone loading, joint extension, cardio-circulatory stimulation and pressure-sore prevention, as demonstrated by HANGARTNER et al. (1994) and SOLOMONOW et al. (1997). In fact, as far back as the 1970s, research groups began proposing functional electrical stimulation (FES) to restore standing in paraplegics (BRINDLEY et al., 1979; BAJD et al., 1981; PETROFSKY et al., 1984a;EWINS et al., 1988; YARKONY et al., 1990). Interest in FES-supported standing, including standing up and sitting down, has increased in recent years (VELTINK and DONALDSON, 1998), and several control strategies have been proposed. Some, like the CHRELMS (DONALDSON and YU, 1996) andPDMR (RIENER andFUHR, 1998) strategies, have tried to combine and co-ordinate two concurrent controllers: the central nervous system, which controls the intact neuromuscular system above the spinal cord lesion, and an artificial control system, devoted to the electrical activation of below-lesion paralysed muscles. However, all the systems available for FES-induced standing up in paraplegics still require upper-limb involvement. The simplest and most adopted method, proposed by BAJD et al. (1982), considers an open-loop stimulation of the quadriceps muscles of both legs, activated by hand switches. To use such systems autonomously, the patients must follow a training protocol to strengthen the knee extensor muscles and to learn how to synchronise the artificial activation of paralysed muscles with the voluntary movements of the upper limbs and the trunk. In any case, increasing muscle strength and resistance to fatigue through a specific muscle-conditioning programme is of crucial importance before any functional application of FES in para- plegic subjects (GLASER, 1994; SOLOMONOW et al., 1997). The most widely used quadriceps muscle-conditioning tech- nique is the electrical stimulation of the muscle while the patient Correspondence should be addressed to Dr M. Ferrarin; e-mail: ferramau@mail.cbi.polimi.it Paper received 19 November 2001 and in final form 18 February 2002 MBEC online number: 20023676 # IFMBE: 2002 282 Medical & Biological Engineering & Computing 2002, Vol. 40