Muscle contraction generates discrete sound bursts

Isolated frog sartorius muscles were stimulated to shorten under lightly loaded conditions. A piezoelectric transducer was placed alongside the muscle to record sounds generated during contraction. Shortening was accompanied by the generation of a series of discrete sound bursts. The bursts were found to be moderately repeatable among successive contractions; 44% repeated from contraction to contraction. The duration of each sound burst was on the order of 400 mus, and the temperature dependence of the interval between successive bursts had a Q10 of approximately 2. Sound intensity was variable: average acoustic power ranged from 0.05–0.4 mW/g, or approximately 1% of the heat generated during contraction. The generation of discrete bursts of sound during contraction, rather than continuous sound, implies that contractile behavior may be discontinuous.