Table 5 Studies of fatigue and muscle physiology

Study 11: relationship between temporal and spectral MMG features during fatiguing electrical muscle excitation

[58]

MMG: freescale MMA7260QMEM tri-axial accelerometer, sensitivity = 800 mV/G at 1.5 g

RF and VL

10 HV subjects, age 28.30 ± 6.58 years

10 SCIV subjects, age 32.06 ± 9.68 years

A single pulse at 1 kHz based on a 15% duty cycle was delivered with a rest of 2 to 5 min, and the maximum electrical stimulation was determined based on the voltage required to vary the knee angle from approximately 90° to 40°

Both the HV and SCI analyses yielded correlation coefficients of − 12 up to − 0.82

Remark: the negative correlation between \({\mathrm{MMG}}_{\mathrm{RMS}}\) and \({\mathrm{MMG}}_{\mathrm{MF}}\) justify their divergence due to fatigue and motor neuron adaptation

Future work: strategies for differentiating the timing among muscle fibers events during the NMES-based recovery process should be investigated

Study 12: analysis of electrical and mechanical behaviors of stimulated pre-fatigued muscles

[59]

EMG: silver bars (diameter = 1 mm, length = 10 mm, inter-electrode = 10 mm); MMG: accelerometer (model ADXL202JE, Analogue Devices, Norwood, MA, USA); force: load cell (model #SM-200 N, Interface, UK; operation range = 0 and 200 N)

GM

11 healthy male subjects, age 21 ± 2 years, body mass 75 ± 4 kg, stature 1.79 ± 0.06 m

A set of three 50-Hz, 10–100-mA, 307-µs pulses lasting 5 s with a 1-min rest between contractions was delivered before and after the fatiguing protocol (35 Hz for 120 s) and stretching maneuvres (elongation up to 45 s with 15-s rest periods)

EMG, MMG and force features decreased and recovered after a 420-s rest period. A stretching protocol reduced the MMG and force signals

Remark: passive stretching remains questionable during a cooldown routine

Future work: studies on passive muscle tendon units should help verify the force reduction after stretching

Study 13: evaluation of the features of human muscle and mechanomyography from interpolated twitch methods

[60]

MMG: uniaxial accelerometer (9-mm square, thickness = 4.5 mm, mass = 0.75 g, sensitivity = 500 mV/g where g = 9.8 m/s²; MP110-10–101, MediSens INS, Japan)

GM

12 male subjects, age 27 ± 2 years, height 0.5 ± 5.2 cm, weight 68.5 ± 9.7 kg

The plantar flexion force was measured at 20, 40, 60, 80 and 100% followed by a supramaximal 1-ms stimulus to the twitch resting torque. The superimposed twitch amplitude, MMG amplitude and ultrasonic images at each force level were recorded

The superimposed MMG amplitude and the extent of fascicle shortening with increasing intensities showed similar patterns

Remark: superimposed MMG might strongly mirror changes in the muscle architecture rather than the twitch amplitude

Study 14: analysis of passive stretching on the electromechanical properties of muscles

[61]

EMG: silver bar electrodes (diameter = 1 mm, length = 10 mm, inter-electrode distance = 10 mm); MMG: one accelerometer (ADXL202JE, Analogue Devices, Norwood, MA, USA); force: load cell (model SM-200 N, Interface, UK; operation range = 0 and 200 N)

GM

12 healthy male subjects, age (means ± standard errors) 23 ± 1 years, body mass 76 ± 5 kg, stature 1.79 ± 0.005 m

Six electrical stimulations with a rest period of 5 s between stimulations. The force signal was induced by two impulses of 100 Hz for 307 µs during a 1-s period. The stretching signals during five maneuvres lasting 45 s with a 15-s rest were obtained

Acute passive stretching altered the mechanical but not the electrical properties

Finding: attention should be paid to the use of MMG to examine stretch-induced changes in the mechanical properties of skeletal muscles