Non-contact assessment of muscle contraction: Laser Doppler Myography

Electromyography (EMG) is the gold-standard technique used for the evaluation of muscle activity and contraction. The EMG signal supports analysis of a number of important parameters including amplitude and duration, engagement of motor units, and functional characteristics associated with factors such a force production and fatigue. Recently, a novel measurement method (Laser Doppler Myography, LDM) for the non-contact assessment of muscle activity has been proposed to measure the vibro-mechanical behavior of the muscles that conventionally is referred to as the mechanomyogram (MMG). The fact that contracting skeletal muscles produce vibrations and sounds has been known for more than three centuries. The aim of this study is to report on the LDM technique and to evaluate its capacity to measure without contact some characteristics properties of skeletal muscle contractions. This is accomplished with the very high vibration sensitivity inherent in the Laser Doppler Vibrometry method (in comparison to commonly used devices such as microphones, piezo electric pressure sensors, and accelerometers). Data measured by LDM are compared with signals measured using standard surface EMG (sEMG) which requires the use of skin electrodes. sEMG and LDM signals are simultaneously acquired and processed. The LDM and sEMG signals are compared with respect to the critical features of muscle activation timing, signal amplitude and force production. LDM appears to be a reliable and promising technique that allows measurement without the need for contact with the patient skin. LDM has additional potential advantages in terms of sensor properties, insofar as there are no significant issues relating to bandwidth or sensor resonance, and no mass loading is applied to the skin.