Acoustic myographic activity increases linearly up to maximal voluntary isometric force in the human quadriceps muscle.

Sounds produced during voluntary isometric contractions of the quadriceps muscle were recorded by acoustic myography (AMG) in seven healthy adults. With the subject seated, surface AMG and electromyography (EMG) were recorded over rectus femoris (RF) during isometric contractions, at different levels of maximum voluntary force. The AMG and EMG signals were amplified and integrated (IAMG and IEMG). The relationships between force and IAMG (r = 0.98 +/- 0.01, mean +/- 1 SD) and force and IEMG (r = 0.99 +/- 0) were linear in all subjects. The results for EMG confirm previous reports but those for AMG differ from the relationships reported for other muscles. Physiological and technical explanations are proposed for these differences and the necessity for further validation of the AMG technique is stressed.