Phonomyography (PMG) (also known as acoustic myography, sound myography, vibromyography, and surface mechanomyogram)[1][2] is a technique to measure the force of muscle contraction by recording the low frequency sounds created during muscular activity.
[1] Orizio states that the low-frequency response of the sensor is the most important feature, and should go as low as 1 Hz.
[3] Images of PMG waves are available in this creative commons-licensed document, "Mechanomyographic amplitude and frequency responses during dynamic muscle actions: a comprehensive review".
[3] Muscle sounds were first described in print by the Jesuit scientist Francesco Maria Grimaldi[4] in a posthumous publication of 1665, which influenced the work of the English physician William Hyde Wollaston[5] and the German scientist Paul Erman.
The past two centuries of repeated rediscovery and neglect of the phenomenon were summarised by Stokes and Blythe[7] in 2001.