Record-breaking muscle reveals its secret.

Someday athletes may carry toadfish and rattlesnake charms for good luck. Both animals have some of the fastest muscles of any vertebrate. The male toadfish (Opsanus tau) produces its mating call by contracting the muscles surrounding its gas-filled swim bladder 200 times per second. Swim bladders normally help fish maintain buoyancy, but the toadfish uses the organ primarily to whistle at passing females. Male and female western diamondback rattlesnakes (Crotalus atrox) also deserve a medal for muscle speed, as they can rattle their tails 90 times per second.

What makes these potential Olympians so fast? The contraction speed requires more than just a lot of energy, according to new findings. Although these sonic muscles work much like muscles of other animals, including humans, they perform a couple of key steps with impressive speed, say Lawrence C. Rome of the University of Pennsylvania in Philadelphia and the Marine Biological Laboratories in Woods Hole, Mass., and his colleagues.

Understanding these super achieving muscles provides clues to how different parts of all muscles work, even though the study doesn't have any immediate applications for humans, asserts Michael L. Fine of Virginia Commonwealth University in Richmond.

Most muscle cells contain a salt solution, called the myoplasm, and a storage compartment for calcium, called the sarcoplasmic reticulum. To make the muscle contract and relax, calcium goes from the sarcoplasmic reticulum to the myoplasm and back. To begin a contraction, a molecular bridge must form between two types of muscle filaments. Calcium entering the myoplasm enables those bridges to form, and its departure causes the filaments to separate, Rome explains.

The sonic muscles of toadfish and rattlesnakes pump calcium out of the myoplasm 50 times faster than the locomotor muscles do, Rome's group reports in the July 23 Proceedings of the National Academy of Sciences (PNAS). In addition, filaments in their sonic muscles release each other and get ready for the next contraction much more quickly than those in the locomotor muscles, Rome and his colleagues find. In fact, recent, unpublished work by the team suggests that sonic muscle filaments release 100 times faster.

Also, sonic muscles have fewer bridges connected at any one time than locomotor muscles do, the new study indicates. In their experiments, the researchers dissect small bundles of muscle fibers in a salt solution. To measure how fast the calcium moves, they inject a calcium-sensitive dye into the muscle cells. In the PNAS study, they measured bridge attachment and detachment rates by monitoring the maximum speed at which the muscles can shorten during contraction. Rome and his team have recently developed a more direct technique, which they've used in the unpublished studies, he says.