The Piezo-resistive MC Sensor is a Fast and Accurate Sensor for the Measurement of Mechanical Muscle Activity

Abstract

A piezo-resistive muscle contraction (MC) sensor was used to assess the contractile properties of seven human skeletal muscles (vastus medialis, rectus femoris, vastus lateralis, gastrocnemius medialis, biceps femoris, erector spinae) during electrically stimulated isometric contraction. The sensor was affixed to the skin directly above the muscle centre. The length of the adjustable sensor tip (3, 4.5 and 6 mm) determined the depth of the tip in the tissue and thus the initial pressure on the skin, fatty and muscle tissue. The depth of the tip increased the signal amplitude and slightly sped up the time course of the signal by shortening the delay time. The MC sensor readings were compared to tensiomyographic (TMG) measurements. The signals obtained by MC only partially matched the TMG measurements, largely due to the faster response time of the MC sensor.

Keywords: electrically stimulated skeletal muscle contraction; muscle contraction sensor; tensiomyography.

Figure 1

Muscle contraction (MC) sensor design and placement. (a) Four piezoresistors, connected in Wheatstone bridge, were attached at the root of the tonguelet. Piezoresistors contact pads and cable interface were connected with microelectronic golden wires and covered with epoxy casting compound for protection. (b) The sensor was attached to the skin through the supporting part using double-sided adhesive patches. (c) The depth of the sensor tip was set to 3, 4.5 and 6 mm. The sensor compressed the skin and subcutaneous tissue, exerting pressure on the measured skeletal muscle.

Figure 2

Typical MC sensor recording. Parameters extracted from measured response: Td—delay time, Tc—contraction time, Ts—sustain time, Tr—relaxation time, Dm—maximal force. Zero time corresponds to the start of electrical stimulus.

Figure 3

Response of vastus medialis to twitch stimulation measured with tensiomyographic (TMG) (a) and MC sensor at three tip depths: 3 mm (b), 4.5 mm (c) and 6 mm (d). Solid line, average values (n = 20, left and right muscles of 10 subjects were measured); envelope, SD. Average signal from both sensors has been normalised to maximum (e). (f) shows first 100 ms of the normalised signals from panel e. In some recordings, a small peak appears 4 to 5 ms after the start of the stimulus. The short duration of the peak (less than 0.5 ms) indicates non-physiological origin.

Figure 4

Comparison of four different temporal parameters extracted from responses of muscle belly to electrical stimulus, measured with TMG and MC. (a) Delay time, Td; (b) contraction time, Tc; (c) sustain time, Ts (d) relaxation time, Tr. Measurements with TMG are compared to those with MC sensor at three different depths (3, 4.5 and 6 mm). Asterisks indicate the level of statistical significance (* P < 0.05, ** P < 0.01, *** P < 0.001). Mean values and standard deviations are shown.

Figure 5

Maximal slopes of signals. Measurements with TMG are compared to those with MC sensor at three different depths (3, 4.5 and 6 mm) and their maximal slopes are averaged. Mean values and standard deviations are shown.