Table 3 Overview of the use of ES and MMG in sensor validation

[38]

Ag–PDMS composite

BB

Not reported

Single and array electrodes composed of Ag/Ag–PDMS and PDMS/OHP substrates were used, and MMG signals were recorded for testing purposes

The responses were similar to those obtained with a single commercial electrode, with an average peak of 7000–10,000 mV

Remark: the Ag–PDMS can be bent and stretched, which was a limitation of the Ag–OHP paper electrode

Study 2: evaluation of a lever indentor and a moving magnet galvanometer for MMG recording

[39]

ES: galvanically isolated stimulator (MYOSTIM); MMG: scanner galvanometer; Force: ankle dynamometer (RAFOLT and GALLASCH, 1996)

Calf muscles

6 healthy subjects

9 stimulation pulses were delivered to achieve a contraction of 70 N, whereas the indentor was adjusted to 0.1, 0.5 and 5 N. Under isometric contraction of the calf muscles, an ankle dynamometer was used to record the surface response from the gastrocnemius muscle

The amplitudes of MMG-RMS showed direct increases during contraction

Remark: a unity cross-correlation coefficient confirmed the validity of using a galvanometer for recording MMG signals

Future work: further studies should verify indirect stimulation

Study 3: analysis of the accuracy of an accelerometer for tensiomyography

[40]

TMG: optical encoder (4 µm, 0.25 mm⁻¹; a spherical tip of 12 mm⁻²); MMG: displacement accelerometer

BB

Not reported

A single-twitch stimulus consisting of a 1-ms, 20-mA square pulse was delivered to the BB using two self-adhesive electrodes. The double integration of the acceleration records were compared with the optical encoder records

The MEM accelerometer efficiently detects short-term small muscle displacement

Remark: the difference in \({D}_{m}\) recorded from an accelerometer and a displacement sensor and the time parameter must not differ by more than 0.05 mm and 0.5 ms, respectively

Study 4: characterization of muscles and subcutaneous tissues

[41]

ES: Ag–AgCl; DMMG: (LK-G80, Keyence, Osaka); AMM: (MP-110-10-101, MediSens, Saitama)

TA

6 healthy males, age 22–25 years

A monopolar rectangle pulse with a 500 µs in width and an inter-pulse interval of 600 ms was applied

Good identification of the longitudinal and transversal mechanics of the muscle, subcutaneous tissue and skin was achieved

Remark: the natural frequency of an acceleration sensor fluctuates more than that of a displacement sensor, but the latter is limited to longitudinal muscle mechanics

Future work: the effect of the mass of subcutaneous tissue on the natural frequency should be investigated

Study 5: analysis of the reliability of MMG and a laser-displacement sensor

[42]

MMG: laser-displacement sensor (LDS; class 2 laser; model LG10A65PU, Banner Engineering Australia) and contact-displacement sensor (CDS; Positek P101 Stand Alone Linear Position Sensor)

RF

16 female and 14 male subjects, age (means ± SDs) 22 ± 2.7 years, height 1.70 ± 0.09 m, body mass 68.9 ± 11.0 kg

Stimulation with a voltage of 400 V, a pulse duration of 200 µs, and a current amplitude of 40—280 mA with 10-mA increments was applied until the muscle reached full displacement; five successive single twitches were delivered at maximum intensity

Both sensors showed good test–retest reliability over the four testing sessions

Remark: the two sensors are not interchangeable: the CDS appears to be more sensitive to muscle belly displacement, whereas the LDS shows increased sensitivity

Future work: the allocation between the stimulation interval and ½ Tr should be well monitored in order to ensure an efficient recovery time for all MMG parameters

Study 6: evaluation of MC sensors

[43]

ES: 5–9 cm (RehaTrode, Hasomed GmbH, Magdeburg, Germany); MMG: (TMG-BMC Ltd., Ljubljana, Slovenia)

RF

9 SCI subjects, age 41.6 ± 14.5 years

A stimulation with 35 Hz, 200 µs and an amplitude of 70—110 mA was followed by MC recording to predict the torque

The MC sensor torque and the dynamometer knee torque were linearly correlated

Remark: MC sensor measurements are reliable and can be used as an alternative for fatigue estimation

Future work: an experiment using a high number of subjects with SCI and different protocols should be performed to validate the use of an MC sensor for real-time data transmission