"The frog's back legs are what do most of the work during jumping and landing. A frog's front legs are his shock absorbers when he lands a jump.Aquatic and semi-aquatic frogs live most of their lives in the water or near it. Swimming is an essential skill and leaping is mostly done on level surfaces or for dives. Because the frog's habitat relies on these kinds of movement, the back legs have developed to be much larger than the front legs. Some frogs live in environments where the front legs are just as important as the back legs and are about equal in size. Tree frogs use their front legs heavily. If you watch a tree frog leaping through branches, you can see him reach out to his target with his front legs and feet to take hold of a surface, then draw his back legs onto it. In the case of tree frogs the front and back legs split the work of locomotion almost equally." (animals.pawnation.com).
