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Inside JEB |
ELASTIC POWERS SPRINGY STEP
Everyone knows that when it comes to leaping, no one beats the flea's striking athletic performance. How the humble bloodsucker launched itself into the record books was a complete mystery until the 1960s, when Henry Bennet-Clark discovered the insect's secret weapon; a catch mechanism to store elastic energy in the leg before catapulting the insect forward. But insects aren't the only super-acceleration athletes; Tom Roberts is amazed by the way that frogs launch themselves from a standing start. But how do the frogs enhance their performance? Richard Marsh knew that as frogs took off, their legs produced far more power than possible from a single muscular contraction. Had they opted for the flea's catch-and-catapult mechanism or had they come up with an alternative approach? Marsh and Roberts teamed up, putting bullfrogs through their paces to deconstruct the frog's take-off technique, finding that as they prepare to leap, the frogs store enormous amounts of elastic energy in their leg's springy tendons, ready to propel them off the ground (p. 2567).
But getting to grips with the frog's complex anatomy was tricky, so Roberts decided on a simplified strategy; he designed a computational cyber frog. Roberts represented the leg as a single muscle with a springy tendon. By attaching this to a lever and gear system that connected to a frog-sized weight, Roberts could drive the muscle's contraction and watch the effect as the lever and gear `launched' the frog's weight through cyber space. By running the simulation with and without the elastic tendon, he discovered that his cyber frog could only produce enough power to leap like a flesh-and-blood amphibian when he included the springy tendon.
Unfortunately, the live bullfrogs were less cooperative, `they weren't enthusiastic jumpers' remembers Roberts. But after resorting to startle tactics he got them to leap while he recorded length changes in the animal's leg muscles as they took off. At first, Roberts was puzzled by the muscle's unexpected behaviour. Before take-off, the muscle's shortening velocity was high, even though the animal was static. As the animal began moving, the shortening velocity decreased, and only began increasing again during the final stages as the frog pushed off. But Roberts had seen this strange muscular contraction pattern before; in the cyber frog simulations. He realised that the muscle was working hardest before the frog left the ground to store energy in the leg's elastic tendons, ready for release in a single elastic contraction as the frog bounded forward.
But why didn't the muscle launch the frog from the ground during the fast initial contraction? Roberts suspects that the force exerted on the ground simply isn't enough to get the frog airborne. But as the contraction continues and more energy is stored in the tendon, the force slowly increases until the frog begins moving. At that point, the muscle's leverage increases, giving the frog a forward push, until the final moments when the muscle contracts fast again, to give the frog it's final shove off the ground.
Roberts points out that although this doesn't rule out the chance that the frogs use an anatomical catch mechanism to get the most from their muscles, for bullfrogs elastic energy certainly puts a spring in their step.
References
Roberts, T. J. and Marsh, R. L. (2003). Probing
the limits to muscle-powered accelerations: lessons from jumping bullfrogs.
J. Exp. Biol. 206,2567
-2580.
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