Girl With A Muscle Swimsuit

A non-invasive elastography technique that measures muscle noise is undergoing experimentation to provide a way of monitoring neuromuscular disease. The sound produced by a muscle comes from the shortening of actomyosin filaments along the axis of the muscle. During contraction, the muscle shortens along its longitudinal axis and expands across the transverse axis, producing vibrations at the surface.
Evolution
Muscles have evolved at least twice, once in cnidaria and once in bilateria; they seem to have evolved from contractile cells in sponge-grade organisms.