title: Acoustic myography - The signal from contracting skeletal muscles
Creators: name: Yao, Yan-Dong
Publisher: name: University of Glasgow
Year: 1996
Subjects: subject: Biophysics, Acoustics.

description: Muscular contraction is associated with low frequency transverse mechanical waves called acoustic myograph (AMG). It can be recorded by microphones and accelerometer. The origin of AMG is thought to be due to the lateral movement of the muscle as a whole. The signal is implicated in many reports as having great potential as force indicator during muscular contraction and could be use clinically in distinguishing healthy and disease, and evaluate force in muscles normally with little clinical access. The aim of this series of investigation was to assess the feasibility of using AMG as a force indicator, especially in situations where direct force measurement is not practical and in FES applications as fatigue indicator. Attempts were made to clarify AMG signal characteristics and assess the possible parameters which could be deployed to describe the AMG signal. Two series of experiments were carried out: one on voluntary contractions of human quadriceps and the other on stimulated contractions of rabbit anterior tibialis. Strain gauges were used to measure force and accelerometers were used to record AMG signal from the skin surface of the thigh on human and from the muscle surface of rabbit anterior tibialis. The AMG signal was recorded between 0.5Hz and wideband frequency and sampled at 512Hz. AMG signal amplitude was calculated by both the rectify-integrated and room-mean-square methods. Frequency content of AMG signal was analyzed by Fast-Fourier-Transform method. The studies carried out on human quadriceps were to: 1) Locate the possible optimal recording site for AMG. 2) Investigate the relationship between AMG and force production during isometric constant force and varying force contractions. 3) Study the AMG-force relationship during sustained and intermittent contractions till fatigue. The study performed on rabbit anterior tibialis were to: 1) Investigate the AMG signal characteristic with changing muscle length both under twitch and tetanic contractions. 2) Examine the AMG signal change with stimulation frequencies both at fixed and varied muscle lengths. 3) Assess the possible association between force and AMG in stimulated contraction till fatigue by both continuous and intermittent stimulation. 4) Study the influence of fatigue on AMG-force relationship under different stimulation frequencies. Results shown that there is no single optimal position for AMG recording on human quadriceps. But high AMG signal intensity were recorded at the mid and proximal-lateral region. The data presented in this report collaborated a linear relationship between AMG and force level in isometric contractions of human quadriceps muscle. This close link between AMG signal intensity and force production is also present in fatigue induced by sustained muscle contractions but not by intermittent contractions. The data obtained from rabbit tibialis anterior muscle showed less systemic relationships between AMG and force production. AMG signal intensity did not display a close relationship with force under condition of changing muscle length, stimulation frequency and fatigue. There were no significant change in AMG signal median frequency during voluntary contractions and AMG signal recorded during stimulated contractions were dominated by stimulation frequency. The good correlation between AMG and force in voluntary contractions, shows promise as an indicator of voluntary force from isometric contractions and fatigue by sustained contractions. The possible clinical use of AMG could be in the area of assessing force output from muscles with limited access, such as paraspinal muscle and facial muscles. It could also be used in conjunction with EMG to assess the state of the muscle function in health and disease and muscle mechanic in training. The use of AMG as force indicator in stimulated contractions, such as FES applications, requires further investigation. It did not appear to relate strongly to force output under conditions investigated. Alternative properties of AMG signal, such as power content in specific frequency range, should be investigated further for FES applications. The AMG median frequency certainly is not a good force indicator under all conditions tested in the experiment.