Electrodeless FSR linear envelope signal for muscle contraction measurement

From the evaluation of electrical activity of muscles to the development of myoelectric prosthetic control/manmachine interfaces, the electromyography (EMG) signal has always been the first choice for both clinicians and engineers. However, due to the many drawbacks of EMG (e.g. skin preparation, electromagnetic interferences, high sample rate, etc.), researchers have strived to find suitable alternatives. We propose as a valid alternative, the dry-contact, low-cost sensor based on a force sensitive resistor (FSR). This sensor applied to the skin through a hard, circular base senses the muscle contraction mechanically and this signal can be actually employed to directly replace the EMG linear envelope (EMGLE) that is typically used as a control signal in prosthetics applications. To reduce the output drift (resistance) caused by FSR edges and to maintain the FSR sensitivity over a wide input force range, its signal conditioning is implemented with a reference voltage strategy (voltage output proportional to force). In this paper, we focus on the validation experiments aimed at finding the best FSR position(s) to replace a single EMG lead. Simultaneous recording of EMG and FSR, using up to three FSRs placed directly over the EMG electrodes, in the middle of the targeted muscle was performed on a small sample of two volunteer subjects. Our results show a high correlation (up to 0.92) between FSR output and EMG linear envelope.