Soft catch prepares power for frog leaps

Abstract

When it comes to a list of the top animal jumpers, frogs are right up there, along with other members of the superleague, such as grasshoppers and fleas. Yet, how frogs pull off their remarkable ballistic feat was unclear. Henry Astley from Brown University, USA, explains that insects wind up for a jump by locking their legs in place while their muscles pull on an elastic piece of exoskeleton that stores energy ready for release when the catch is unleashed to power the leg's thrust. However, Astley and Tom Roberts were puzzled how frogs could produce the same explosive feat in a soft body. ‘Jumping vertebrates lack a clear anatomical catch, yet face the same requirement to load the elastic structure prior to movement,’ says Astley. Intrigued, the duo began filming the leaping amphibians with X-rays to find out more about how frogs power jumps (p. 4372).Inserting minute metal markers that show up well in X-ray movies into the hind legs of three frogs (Rana pipiens) and filming the animals as they let fly, Astley and Roberts also recorded the forces exerted on the ground by the frogs' feet. Then the duo reconstructed the way that the frogs unfurled their legs in the last 150 ms before push off and calculating the amount of power produced by the amphibian's ankle extensor (plantaris) muscle. Amazingly, this muscle, which powers leaps, was capable of producing over four times as much power (1352 W kg–1) as a normally contracting muscle (322 W kg–1), confirming that the animals were storing elastic energy and using it to power take-off like some other frogs.Next, Astley and Roberts analysed the leg movement reconstructions, and realised that the frogs were storing elastic energy at the ankle by changing their posture to alter the leverage and forces acting around the joint. Describing this ‘dynamic catch’ mechanism, Astley explains that elastic energy can be stored in the plantaris muscle during the preparation phase of the jump when the leverage acting at the ankle is poor and the forces acting on the bent legs – such as the ground reaction force – resist movements at the ankle. However, in the later stages of preparation, the forces that had resisted the ankle's movement fall to a point where the ground reaction force and leverage becomes great enough to release the energy stored in the plantaris muscle, launching the frog into the air.Explaining that the leg forces and poor leverage that resist ankle movements in the early stage of a jump are analogous to the mechanical catch that allows jumping insects to store elastic energy, the duo suspects that other animals may also be able to take advantage of this dynamic catch mechanism to produce impressive leaps. However, they point out that there are situations where this mechanism will not work – such as leg kicking, when there are no ground reaction forces to hold the limb steady during preparation.