Leaping frogs maximize muscle potential.

In Mark Twain's tale about the notorious jumping frog of Calaveras County, frog owner Jim Smiley tries to win some money by betting that his pet, Dan'l Webster, can outjump all others.

Now, biologists have trained highspeed cameras on frogs wearing electrodes and simultaneously monitored the animals' muscle movement and activity. By catching the frogs' motion on film and also studying iso1ated frogmuscle fibers, they have learned more about why frogs like Dan'l are such great leapers.

For each bound, these animals rev up their muscles and put every fiber to maximum use, Lawrence C. Rome and Gordon J. Lutz of the University of Pennsylvania in Philadelphia report in the January 21 Science.

"It seems that [frogs] are designed to give all that they can give," says Lutz. To do this, "many things had to evolve in a coordinated, concerted way," he adds.

With this setup, the researchers saw that muscles wait about 26 mi!!iseconds after they receive a signal to contract. During that time, they set in motion all the chemical reactions that prepare the force-generating units inside the muscle fiber for shortening. If these units, called sarcomeres, shorten too little or too much -- or too slowly or too quickly - then a muscle fails to achieve its maximum potential.

But these amphibians shorten their sarcomeres by just the right amount at just the right velocity for an explosive takeoff, says Lutz. What's more, the frogs' muscles attach to joints in just the right places. This placement ensures that the muscles shorten just the right amount to create a given change in the angle of the joint.

The biomechanics of leaping frogs contrast sharply with those involved in the cyclical movements of swimming fish (SN: 7/17/93, p.39), says Rome. Frogs give their all with each jump, whereas fish have to back off with each swing of the tail in order to keep opposing muscles from fighting each other.