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Table 8 Overview of the use of ES and MMG on the assessment of muscle stiffness

From: Assessment of muscle activity using electrical stimulation and mechanomyography: a systematic review

Authors Sensor and electrode type Electrode site Dataset Methodology Results and discussion
Study 1: analysis of muscle stiffness at various workloads, cadences, and power levels
[75] MMG: capacitor microphone (MXE-4758; Primo, Tokyo, Japan); ES: 10-mm diameter Ag–AgCl (Vitrode J-150; Nihon Kohden) VL 8 able subjects, age 21–24 years The subjects pedaled an electrically braked ergometer set to 47 W. Stimulation pulses were delivered at 30° with 3-, 2- and 1.5-s inter-pulse intervals and a constant power of 40, 60, and 80, respectively With the knee angle set to 80°, the muscle stiffness was in direct proportion to the workload and power
Remark: the muscle stiffness progressively increased with increases in the pedaling rate
Future work: the effect of non-active and active muscles on the viscous coefficient and the muscle response after the application of various phases per cycle should be investigated
Study 2: estimation of the muscle stiffness from evoked MMG
[71] ES: Ag–AgCl electrodes (F-150S; Nihon Kohden); DMMG: capacitor microphone (MX-E4758; Primo, Tokyo, Japan; weight = 0.78 kg, sensitivity = − 43 ± 3 dB at 1 kHz, gain = 0.5 to 4000 Hz); AMMG: (MP-110–10-101, MediSens, Saitama, Japan) TA 7 healthy male subjects, aged 22–24 years A monopolar pulse with a 500-µs width and an inter-pulse interval of 1 s was applied in each trial and repeated five times No significance difference was found between the natural frequencies from the velocity and displacement obtained by MMG
Remark: the velocity component measured with a differential circuit reached a steady state value within a short time, and motion is thus recommended
Future work: the transverse muscle stiffness should be further estimated
Study 3: analysis of muscle strength
[74] MMGs: capacitor microphone (MX-E4758; Primo, Tokyo, Japan); ES: Ag–AgCl surface electrodes (F-150S, Nihon Kohen) GM 8 healthy male subjects, age 22–24 years The subject was asked to pedal at speed of 3 km/h with a gait cycle of 1.8, 1.5, 1.2, and 1.1 s, and a monopolar rectangular pulse width of 500 µs and an amplitude of 20-mA was applied A progressive increase in muscle stiffness was detected with increases in the pedaling rate
Remark: the stable viscous coefficient was attributed to the friction effect from active and non-active fibers
Future work: the cause of the stable viscous coefficients should be investigated
Study 4: analysis of the stiffness of the gastrocnemius muscle and the waking speed
[73] ES: Ag–AgCl (F-150S, Nihon Kohden); MMG: capacitor microphone (MX-E4758; Primo, Tokyo, Japan) GM 8 healthy male subjects, age 22–24 years Stimulation for 500 µs with a 20-mA amplitude was delivered to the GM while the subject walked on a treadmill at 2, 3,4, 5 km/h with a gait cycle of 1.8, 1.5, 1.2, and 1.1 s The stiffness was indicated by an increase in the natural frequencies f1 and f2, which increased with increases in the gait speed
Remark: a constant natural frequency f3 is likely caused by subcutaneous tissue
Future work: the natural frequency corresponding to the soleus muscle should be further studied, and the natural frequency of GM should be clarified
Study 5: analysis of the effect of static stretching (SS) on joint angle stiffness
[72] EMG: silver bars (model ELSCH004, diameter = 1 mm, length = 10 mm, inter-electrode distance = 10 mm, OT Bioelettronica, Turin, Italy); MMG: accelerometer (model ADXL202JE, Analogue Devices, Norwood, MA, USA); force: load cell (model SM-2000 N, Interface, UK; operation 0–2000 N) GM and GL 19 healthy male subjects, age (mean ± SD) 24 ± 3 years, body mass 76.4 ± 8.9 kg, stature 1.78 ± 0.09 m MMG, EMG and force signals were recorded before and after SS. The joint stiffness and the force from plantar flexors were measured after a supramaximal + 10% tetanic stimulation (3 s, 50 Hz, 10–100 mA, and 304 µs) applied at 0°, 10° and 20° The SS and joint stiffness increased with increases in the joint angle
Remark: the increase in dorsiflexion with \({\mathrm{R}-\mathrm{Delay}}_{\mathrm{TOT}}\) might be due to joint stiffness