Non-contact muscle sound sensing by laser displacement meter and development of a wearable muscle sound recorder for daily life

In order to develop a new wearable muscle sound recorder (WMSR), non-contact muscle sound (MS) sensing by a laser displacement meter (LDM) was conducted. MS of biceps brachii was recorded by LDM at 20, 50 and 80% MVC under isometric contraction. The frequency range (transformed from displacement dimension into acceleration by multiplying the angular frequency squared) was from 5 to 50 Hz. The effect of sensor mass was evaluated by putting weights on the accelerometer to change the total mass. The capacitance accelerometer weighed 2 grams, and additional weights were changed from 0 to 280 grams. In 6 healthy subjects aged 22.5 on average, a sensor mass of more than 10 grams attenuated the rate that components over 20 Hz occupied in the total spectrum. Thus, a contact sensor mass for biceps brachii should be below 10 grams. The developed WMSR attached to the subject's waist consisted of not only a MS sensor, but also ECG, piezo-resistive 3-axis accelerometers and a vibrating gyroscope (W 6.4 cm, H 4.7 cm and L 14 cm). A 9 V-alkali battery could operate the WMSR for around 3.9 hours. The WMSR enabled the authors to measure MS outdoors. The S/N ratio was about 10 dB in performance evaluation, which indicated that the MS was recorded adequately. Furthermore, the S/N ratio was measured under some daily life situations; sitting, walking, muscle training, and getting on a train. Conclusions are follows; 1) compared with a non-contact sensor a contact sensor mass of more than 10 grams weakened frequency components higher than 20 Hz; 2) WMSR enabled the authors to measure MS outdoors, even when getting on a train.