Frogs' incredible jumping ability comes from their tendons, not their muscles

Frogs really should have fantastically strong leg muscles to jump as far as they do. And yet their leg muscles are only a fraction as powerful as they would need to be...so where does this leaping power really come from?

That question has long baffled scientists, but now Brown researcher Henry Astley thinks he's found the answer. The big breakthrough came from advances in observation techniques, as he and his colleagues were able to film frogs jumping at an incredible 500 frames per second. What's more, they filmed the jumps with X-ray cameras, allowing them to see inside the animals' legs at every moment of the jump.

The secret, it turns out, is in the tendons. When the frogs prepare to jump, their tendons stretch out as far as they can. The leg muscles shorten at this point, transferring energy into the tendons. The frog then blasts off as the tendon recoils like a spring. This elastic structure is the key to the frog's ability to jump long distance. Though its muscles still have a vital role - after all, a quarter of the frog's entire mass is in its legs just for this purpose - these jumps would be physically impossible without its springy tendons.

Here's a more complete description of the jumping process, taken from the researchers' observations of the northern leopard frog:

As the frog readies itself to leap, its calf muscle shortens. After about 100 milliseconds, the calf muscle stops moving, and the energy has been fully loaded into the stretched tendon. At the moment the frog jumps, the tendon, which wraps around the ankle bone, releases its energy, much like a catapult or archer's bow, causing a very rapid extension of the ankle joint that propels the frog forward. The entire jump — from preparation to leap — lasts about a fifth of a second, the experiments showed. Other frog species jump much faster.

Frogs aren't the only leapers whose jumps far outstrip their apparent capabilities. The researchers are confident that similar relationships between tendons, muscles, and joints may well explain how other jumping creatures from grasshoppers to guinea fowl manage their fantastic leaps. Researcher Thomas Roberts observes:

"Frogs are interesting in their own right, but we are also confident that this study gives us insight into how muscles and tendons work together in animal movement. Other studies have presented evidence for an elastic mechanism, but Henry [Astley]'s gives us the first glimpse of how it actually works."

Via Biology Letters. Image via.