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Research Shows Frogs Really Have Springs For Legs

It's the stuff of proverbs: Don't judge a frog's leap by its muscles. Or something like that. New research shows that frogs' prodigious jumping ability isn't so much a factor of how beefy their leg muscles are. Instead, frogs' physiology allows them to...

by Derek Mead
Nov 16 2011, 4:01pm

It’s the stuff of proverbs: Don’t judge a frog’s leap by its muscles. Or something like that. New research shows that frogs’ prodigious jumping ability isn’t so much a factor of how beefy their leg muscles are. Instead, frogs’ physiology allows them to load their tendons like a spring, helping them explode into the air.

Henry Astley and Thomas Roberts, both of Brown University, recently published a paper on frog physiology in Biology Letters. They showed that, before jumping, a frog’s leg muscles contract to stretch its tendons and load them with energy. So while the leg muscles are still the source of that energy, the tendons are what give the frogs’ leaps a boost.

Astley and Roberts worked with the northern leopard frog (Rana pipiens). They inserted metal beads into the shin, ankle and leg of a quartet of frogs. Using 3D X-ray video tech, Astley and Roberts were able to watch frogs’ jumps in slow motion, with the metal beads showing up on the film in a similar fashion to those goofy white balls used for CGI motion capture.

One of Astley’s frogs leaping while taped with X-ray video.

As a frog initiates a jump, its calf muscle contracts, loading the tendon. It takes about 100 milliseconds for the calf to fully stretch the tendon, after which the frog leaps. The tendon, whichs wraps around the ankle bone, then releases its energy in a quick burst.

Although the mechanics differ from humans—our jumping is caused by our muscles rapidly contracting—the idea that some animals, like horses, use elastic tendons as springs is a tenet of biomechanics. But Astley is one of the first researchers to really demonstrate the inner workings of how frogs leap. Interesting as it is on its own, the research should help scientists working with other top leapers like fleas, grasshoppers and bush babies.

Lead photo of Henry Astley: Mike Cohea/Brown University.