Although the tracing drawn by a lever attached to a muscle in tetanus is straight, and does not show any variation in the tension of the tetanized muscle, some variations in tension must occur, since a low humming sound is produced during contraction. A muscle tone, like the purring of a cat, can be heard by applying the ear firmly over any large muscle (biceps) while in tetanus, by throwing the muscles attached to the orbit and Eustachian tube into powerful action, or by spasm of the muscles in mastication.
The number of vibrations which has been estimated to occur in the voluntary contraction of human skeletal muscles does not produce an audible note; hence it has been supposed that the note we hear has been an overtone. When a muscle is thrown into tetanus by a current interrupted by a tuning fork, a tone is produced which corresponds with that of the fork causing the interruption in the current by definite vibrations, which regulate the number of stimulations the muscle receives. If, on the other hand, a contraction of the muscle be brought about by stimulating the spinal cord, with the same rate of breaking the current, the normal muscle tone is produced, as if the contraction were the result of a nerve impulse coming from the brain.
There is no satisfactory proof, however, that the variation in tension of the continuous contraction of voluntary muscle is strictly rhythmical. The sensation of a sound like the muscle tone is produced by any nearly periodic vibrations of less rate than 25 per second. The pitch of the muscle tone varies with the tension of the membrana tympani. Hence, it has been suggested that it corresponds with the resonant tone proper to the membrane of the drum; which may be evoked by any trembling movements of the muscle fibres due to slight variations in the force or distribution of the impulses transmitted by the motor nerves.