A giant leap for frog-kind.

By Dave Armstrong - 19 Dec 2014 9:34:0 GMT
A giant leap for frog-kind.

Ready for lift off. The leopard frog can make giant leaps, but how does its body magnify the energy of its leg muscles exactly? Rana pipiens image; Credit: © Shutterstock

The frog is a marvel of engineering. We always suspected that unique shape had hidden assets. Henry C Astley and TJ Roberts from Austin’s Brown University in the US have looked at The mechanics of elastic loading and recoil in anuran jumping in the Journal of Experimental Biology and concluded that the soft body of the amphibian has a dynamic catch mechanism similar to a jumping insect. The grasshopper and flea have hard chitinised limbs to lock together as a storage device, unlike the soft-bodied vertebrate.

This mechanism lies in the elastic energy that can be stored in the powerful legs when they brace against the ground in preparation for those world-beating leaps. Kicking in water for example would not be able to exert the same force that is achieved in elastic energy. Henry put it succinctly with Jumping vertebrates lack a clear anatomical catch, yet face the same requirement to load the elastic structure prior to movement

The investigation to discover the mechanism began with X-ray filming of that power leap. Using minute metal markers to help the X-rays detect every change, Rana pipiens (the leopard frog) displayed an unfurling of the limbs just 150msecs before lift off. With further records of the forces exerted on the ground by the feet, power could be calculated. The plantaris extensor muscle of the ankle produced maximum power at 1352W per kg. This is quadruple the amount of power that a normal muscle can produce, which means elastic energy is being stored and used to power the leap.

The leg movement reconstruction indicated how the ankle was the site where frogs adjusted their posture to alter leverage and forces acting around the joint. Leverage is poor as the frog prepares its leap and there is no movement at the ankle because the ground has great resistance. Then the dynamic catch mechanism releases the stored energy to launch the truly dynamic frog! Simple Huh!

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