Question

The odd hopping gait of a kangaroo has a very practical purpose: It lets the kangaroo cover great distances with minimal energy input. How is this efficiency possible?

Answer 1

In physics, elastic energy refers to the energy released when a spring elongates. When a spring is compressed it stores energy that can be used later, at this point the spring contains elastic potential energy. Releasing the spring, or elongating it, releases the elastic energy, allowing the spring to move. Releasing the spring does not require any work, as the elastic energy utilized is the same as the elastic potential energy that is stored when the spring is compressed. In this example, the work done onto the spring happens when someone compresses it. Alternatively, if someone pulled apart a spring and held it, the spring would contain elastic energy until it was released. When releasing the spring, it snaps back together, utilizing the elastic energy that was stored previously. Kangaroo's hind legs work like springs, compressing and elongating, storing and releasing elastic energy in the process. In the kangaroo example, however, the kangaroo does not need to do work in order to elongate the springs that are their hind legs - gravity takes care of that.

Kangaroos have long hind legs that make them easy to spot in the wild. When taking a closer look at the muscular-skeletal make-up of these hind legs, one will notice unique characteristics of both the tendons and muscles. A kangaroo has extremely long tendons in its back legs that undergo drastic length changes when the kangaroo is hoping. Acting like springs, the tendons stretch under the weight of the kangaroo, and, while elongated, contain elastic energy. The muscles in a kangaroo's legs are impressively strong and stiff, allowing them to handle the stretching of the tendons. One study conducted on the animals revealed that their tendons can store up to ten times as much energy as their muscles. All of this stored energy is released when the kangaroo pushes up and the tendon contracts again. While the muscles in a kangaroos legs still work to help them hop, so much of the energy they use comes from the tendons. Unlike muscles, tendons do not fatigue and they do not require oxygen to work.