In 20 normal subjects, sounds or vibrational signals were recorded from the biceps brachii muscle during voluntary isometric contraction. Fourier analysis showed presence of predominantly low frequency components with little contributions beyond 60 or 70 Hz. Relatively high amplitude peaks occurring below 20 Hz were seen in the frequency spectrum with the most prominent of these peaks occurring at a mean frequency of 11.3 Hz. A majority of subjects had one or two additional peaks, each appearing on either side of the major peak with mean frequencies of 7.3 and 16.2 Hz. In general, subjects who could not sustain a very steady contraction had more peaks compared to those who were able to maintain a smoother contraction. Between 20 and 50 Hz, several well-defined but much smaller peaks were also seen. The frequencies of some of these small peaks can be expressed as exact harmonics of the previously described larger peaks in 85% of subjects.