Transmission of acoustic or vibratory signals from a contracting muscle to relatively distant tissues

In 20 normal subjects, acoustic or vibratory signals generated during sustained voluntary isometric contraction of the biceps brachii muscle were recorded directly over the muscle belly. The signals were also recorded simultaneously at a distance from the contracting biceps muscle over the posterior upper arm near the triceps muscle. Frequency amplitude spectra (FAS) of the recordings were determined. The amount of transmission of the most dominant vibratory signal from the biceps muscle to the posterior arm was estimated from the amplitude ratio of similar frequency peaks in the FAS obtained from recordings in these two separate regions. The mean amplitude ratio was 0.46. This finding suggests that when several groups of muscles are contracting simultaneously, there may be significant transmissions of acoustic signals from one muscle to the other. These transmissions could potentially interfere with the selective recording of sounds produced by a specific muscle.