A Piezoresistive Sensor to Measure Muscle Contraction and Mechanomyography

Abstract

Measurement of muscle contraction is mainly achieved through electromyography (EMG) and is an area of interest for many biomedical applications, including prosthesis control and human machine interface. However, EMG has some drawbacks, and there are also alternative methods for measuring muscle activity, such as by monitoring the mechanical variations that occur during contraction. In this study, a new, simple, non-invasive sensor based on a force-sensitive resistor (FSR) which is able to measure muscle contraction is presented. The sensor, applied on the skin through a rigid dome, senses the mechanical force exerted by the underlying contracting muscles. Although FSR creep causes output drift, it was found that appropriate FSR conditioning reduces the drift by fixing the voltage across the FSR and provides voltage output proportional to force. In addition to the larger contraction signal, the sensor was able to detect the mechanomyogram (MMG), i.e., the little vibrations which occur during muscle contraction. The frequency response of the FSR sensor was found to be large enough to correctly measure the MMG. Simultaneous recordings from flexor carpi ulnaris showed a high correlation (Pearson's r > 0.9) between the FSR output and the EMG linear envelope. Preliminary validation tests on healthy subjects showed the ability of the FSR sensor, used instead of the EMG, to proportionally control a hand prosthesis, achieving comparable performances.

Keywords: electromyography; force sensitive resistor; human machine interface; mechanomyography; muscle contraction; prosthesis control.

Figure 1

(a) Exploded view of the mechanical components of the force-sensitive resistor (FSR)-based muscle sensor. (b) A picture illustrating the FSR sensor and the rigid dome mounted above.

Figure 2

Conditioning circuit for the FSR muscle sensor.

Figure 3

Experimental set-up to measure the frequency response of the FSR sensor.

Figure 4

Electromyography (EMG) electrodes and FSR sensor placed on patient’s muscle.

Figure 5

FSR static calibration: scatter plot of the experimental data (o) and regression line.

Figure 6

FSR drifts at different constant loads (400, 800, 1200, and 1600 grams).

Figure 7

FSR dynamic response: amplitude (upper panel) and phase (lower panel) frequency response.

Figure 8

Simultaneous recordings from flexor carpi ulnaris when performing three grasp actions at increasing strength: (a) Raw EMG signal; (b) EMG linear envelope; (c) FSR force signal (raw output); (d) mechanomyogram (MMG) extracted from FSR.