Abstract
Clinical applications of Functional Electrical Stimulation (FES) provide both functional and therapeutic benefits. To enhance the functionality of FES systems and to improve the control of the activated muscles through open-loop or feedback controllers, solutions to gather information about the status of the system in real time and to easily detect the intention of the subject have to be optimized. This chapter summarizes the state of art of sensors used in motor neuroprostheses. These sensors can be classified in two categories: sensors of biological signals, such as electromyogram, electroencephalogram, electroneurogram, eye tracking, and voice control, and sensors of non-biological signals, such as sensors of force/pressure (e.g. force sensitive resistors and strain gauges) and sensors of movement (e.g. accelerometers, electrogoniometers, inertial measurement units, and motion capture systems). Definitions, advantages and disadvantages, and some example of applications are reported for each sensor. Finally, guidelines to compare sensors for the design of motor neuroprostheses are drawn.
TopBackground
NMES refers to the electrical stimulation of an intact lower motor neuron to activate paralyzed or paretic muscles. Clinical applications of NMES provide either functional or therapeutic benefits. Moe and Post (Moe & Post, 1962) introduced the term functional electrical stimulation (FES) to describe the use of NMES to activate paralyzed muscles in precise sequence and magnitude so as to directly accomplish functional tasks. FES was started with the simple and ingenious idea of Liberson et al. (1961) of lifting the drop-foot of a hemiplegic patient with a portable electronic stimulator (Liberson, 1961).
Restoration of motor functions based on FES has been widely studied since the first developments by Vodovnik and Grobelnik (Vodovnik & Grobelnik, 1977). Clinical application provides both therapeutic and functional benefits by retraining atrophied muscles. Once trained, the muscles can be used again to generate functional movements. NMES is also used for therapeutic purposes. NMES may lead to a specific effect that enhances function but does not directly provide function. One therapeutic effect is motor relearning, which is defined as “the recovery of previously learned motor skills that have been lost following localized damage to the central nervous system” (Lee & van Donkelaar, 1995). Indeed, in addition to the well-known peripheral effects on muscles themselves, FES is considered to have some central therapeutic effects. Some hemiplegic patients treated with FES for foot-drop correction during walking have shown a relearning effect that outlasts the period of stimulation. This was firstly observed by Liberson and colleagues (Liberson, 1961) and it is currently known in literature as “carryover effect” (Ambrosini, 2012; Ambrosini, 2011; Burridge, 2001). However, we know from literature (Burridge, 2001; Merletti, 1979) and from clinical practice that it is not possible to infer which patient will get the carryover effect from a peripheral evaluation. This further supports the hypothesis that FES induces some plasticity mechanisms in the reorganization of the central nervous system that allows maintaining recovery of motor control, whose mechanisms of action are still under investigation, although some possibilities have been hypothesized (Bergquist, 2011; Gandolla, 2014; Rushton, 2003).