SYME, DA*; JOSEPHSON, RK: How to Build Fast Muscles I. Synchronous Muscle.

Skeletal muscles throughout the animal kingdom are turned on by neuron action potentials, resulting in muscle fiber depolarization that triggers the release of intracellular calcium. Calcium activates contractile filaments and allows interaction between myosin cross bridges and actin filaments. Relaxation is brought about by sequestration of the calcium and detachment of the cross bridges. Muscles in which there is congruence between each contraction and fiber depolarization are termed synchronous. Some synchronous muscles operate at high frequencies, in one extreme case up to 500 Hz. The capacity for high frequency contraction in synchronous muscles is achieved by hypertrophy of the cellular systems involved in calcium release and uptake, allowing more rapid calcium cycling, and by increases in association and/or dissociation rates of the myosin cross bridges in some instances. Increase in the volume of the calcium control systems within a fiber reduces the space available for contractile material, and increases in dissociation rates appear to limit the number of attached cross bridges. In addition, there are substantial metabolic costs associated with the high rates of calcium exchange and cross bridge cycling. Thus high-frequency synchronous muscles tend to be weak and are expected to be less efficient than those that operate at lower frequencies. Often they are not well suited for powering sustained locomotion, but they are capable sound producers. The trade-offs between frequency, force and efficiency will be demonstrated using, as examples, high-frequency muscles from fish, snakes and insects.