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Controlled Functional Electric Stimulation (FES) in the Rehabilitation of Spinal Cord Injured Persons and Stroke Patients

From Fachgebiet Regelungssysteme TU Berlin

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The overall theme of this project is to investigate the application of controlled functional electrical stimulation (FES) for the rehabilitation of spinal cord injured persons and stroke patients. It is well known that electrical nerve-stimulation can be used to generate contractions of paralysed muscles. In combination with appropriate sensor technology and feedback control, this can be exploited to elicit functional movements, such as walking and cycling, and hence to restore certain motor functions. Depending on the degree of disability, the goal may be temporary assistance, e.g., during re-learning of gait, or permanent replacement of lost motor functions (neuro-prosthesis). Beside these functional effects, FES has several secondary therapeutic benefits: it improves muscle size and strength, increases the range of joint motion and improves cardiopulmonary fitness by providing significant training effects. FES is therefore potentially more attractive for rehabilitation purposes than conventional methods such as passive bracing of the joints. Fig.1 explains the principle of controlled FES for a specific problem, the control of knee-joint angle by quadricep stimulation. The knee-joint angle is measured and fed back to the controller, which generates a suitable stimulation pattern to achieve tracking of a reference trajectory. Stimulation can either be applied directly to the peripheral motor nerves or, if the reflex arcs in the lower spinal cord are still intact, to the sensory nerves. The latter causes an indirect stimulation of motor nerves while ensuring the natural inhibition of antagonistic muscles.

Figure 1: Functional Electrical Stimulation (FES) for knee-joint angle control.

A general problem with FES is rapid muscle fatigue. External stimuli, which replace the missing commands from the central nervous system, tend to invert the recruitment order of muscle fibres: motorneurons with larger diameter are activated first as they have a lower threshold; they recruit the faster and more powerful (type 2 or white) fibres, which fatigue more quickly than the slower, but less powerful, type 1 or red muscle fibres. Electrical stimulation is realised by attaching surface electrodes to the skin, because the alternative, implanting electrodes, is much less convenient and carries a serious risk of infection.

The project is organised within four subprojects, addressing fundamental questions as well as aiming at transferring results into medical and therapeutical practice.

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