A New Piezoelectric Sensor for Forcemyography Application

Forcemyography (FMG) is a technique that provides information on muscle activity by measuring the variation of muscle cross-section during contraction. FMG is a viable alternative to Electromyography (EMG) for monitoring muscle activity in applications such as human machine interfaces. Typically, FMG makes use of Force Sensitive Resistor (FSR) sensors. A new piezoelectric sensor based on common and inexpensive lead zirconate titanate (PZT) disk is presented. To evaluate its operation, signals from the new sensor, an FSR sensor and EMG electrodes were simultaneously acquired on the biceps brachii of 4 healthy subjects while performing about 50 isometric contractions of different intensity and duration. To estimate the level of muscle activity, the EMG signal was processed by extracting its linear envelope (EMG-LE). Each muscle contraction was then segmented. The similarity between the three signals was assessed via their normalized cross-correlation index (NCCI). The NCCI computed between the FMG signals and the EMG-LE and averaged across all the contractions scored 0.89 (SD: 0.042) for the PZT sensor and 0.88 (SD: 0.050) for the FSR sensor. Instead, the average NCCI computed between the two FMG signals scored 0.97 (SD: 0.020). The delay between the contraction onsets in the FMG signals and those in the EMG-LE was about 20 ms on average. The results confirm the good performance obtained by the new sensor in monitoring muscular activity, an excellent degree of equivalence between the signal provided by the PZT and the FSR sensors, and the good correspondence with the information provided by the EMG-LE.