Muscle shortening/lengthening velocity is an important component of muscle response to motor neuron input. What structural features help determine lengthening velocity is thus important to understanding how nervous systems generate behavior. Using data from muscles with similar electrical properties and a wide range of sarcomere and muscle lengths, I verify here Huxley and Neidergerke’s hypothesis that shortening (and lengthening) velocity depends on sarcomere number. Using a fluorescent marker for F-actin, muscles from the lobster (Panulirus interruptus) stomatogastric system, one of the best studied motor pattern generating model systems in neurobiology, were stained. Prior work has shown that the widely differing dynamics of these muscles play a crucial role in determining how these muscles respond to neural input. Therefore, this work not only verifies that sarcomere number is a fundamental determinant of muscle shortening velocity, but also the important role that shortening/lengthening velocity can play in determining motor output.