Table 6 Use of MMG for studying endurance and fatigue

Study 1: analysis of torque and MMG fluctuations at various joint angles

[63]

Triaxal accelerometer-based MMG sensor (WAX3, Axivity, Newcastle upon Tyne, UK)

Erector spinae of the forearm and wrist flexors

5 male and 3 female subjects, age 22.9 ± 2 years, height 1.8 ± 0.1 m, weight 69.8 ± 14.9 kg

Pulses of 25 to 35 mA at pulse durations/intervals of 200/50 µs were delivered, and the muscle twitch contraction was measured. The endurance protocol consisted of 2 Hz, 4 Hz, and 6 Hz (3 min each) with 5 s of no stimulation between each stage

The EI values were reasonably reproducible, particularly those obtained with the 2- and 4-Hz stimulations

Remark: in young healthy individuals, the erector spinae muscle has a lower endurance index than the leg muscles

Future work: the experiment was conducted on young healthy subjects; thus, the application of the system for evaluating lower back pain in the healthy population should be investigated

Study 2: analysis of the effect of exercise on endurance

[62]

Accelerometer-based MMG sensor

GM

56-year-old female with muscle sclerosis

A 3-min, 2-, 4- and 6-Hz stimulation signal was applied, and the endurance index was determined as the percentage of the acceleration at the last stage of the stimulation frequency relative to the peak accelerometer records

The walking endurance and oxidative capacity were improved

Future works:

1. The effect of voluntary exercises on muscle plasticity and the role of muscle oxidative capacity on assisting individuals with MS should be evaluated

2. The use of a large sample size for examining the role of muscle plasticity to improve the walking function in people with MS should be investigated

Study 3: evaluation of rider time and virtual distance in SCI

[64]

ES: two self-adhesive surface electrodes (oval, 2″ × 4″); MMG: accelerometers (Entran EGAS, weight = 1 g, 2–150 Hz, gain of 25)

VM, RF and VL

12 SCI subjects, mean age 37 years, height 1.8 m, mass 80.6 kg

An electrical stimulation of 50 Hz for 500 µs was increased to a current of 140 mA to maintain a cadence of 32 RPM over 0.4 s and applied every 1.88 s

An improved electrically activated cycling alternated the activity of synergistic muscles in SCI, and different RMS values of MMG signals were obtained from all the muscles

Remark: the MMG amplitude decreased due to muscle fatigue and the virtual distance, and both the stimulation and co-activation protocols yielded the same mechanical output

Future work: the use of improved stimulation techniques to evaluate improvements during a longer training practice without an external rider should be evaluated

Study 4: analysis of endurance and feelings of fatigue in FRDA

[65]

MMG: tri-axial accelerometer (WAX-9; Axivity, UK); ES: electrodes (5.08 cm by 10.16 cm, Pro Advantage by NDC)

Forearm flexor

10 healthy subjects, 16 FRDA subjects

The forearm flexor muscles were stimulated with 3 min of ES at 2, 4, and 6 Hz with a 5-s rest between stages

A correlation was found among the mitochondrial capacity, disease severity and muscle-specific endurance

Remark: people affected by FRDA exhibit lower forearm muscle endurance than ABs

Future work: MMG and NIRS measurement methods are correlated with disease severity, and this correlation should be further investigated