Getting around can be tough; but thanks to specialised muscle-tendon units in their limbs, most creatures cope well, no matter how rough or hilly the terrain. Many limb muscles are long and involved in moving joints, while some muscles are so short it seems unlikely that they can contribute to joint movements. Instead, these short muscles are thought to act as dampers,cancelling out unwanted vibrations with the help of the long elastic energy-storing tendons that they attach to. But is that all there is to these small muscles, or can they contribute to the increased demands when an animal changes gear; for example when speeding up or climbing hills? Although some animals can use these muscles for extra power it was not clear whether super-efficient wallabies rely on these muscles for this purpose as they hop around the outback. In an international collaboration that spanned the Pacific Ocean, Andrew Biewener and his colleagues tested whether tammar wallabies use two short lower leg muscle groups either to contribute power in the animal's legs, or for economical force generation and vibration damping(p. 211).

Biewener hopped off to Adelaide University in Australia to train four wallabies to bounce on treadmills, either on the flat or up a moderate incline. According to Biewener, the wallabies were extremely cooperative,resting quietly between experiments in pouch-like bags. Once the animals were happy on the treadmill, the team fitted them with instruments to record contraction forces and length changes in the lateral gastrocnemius and the plantaris muscles, as well as measuring the muscles' electrical activities. But there was no difference in the muscle lengths or the amount of work done as the animals hopped along the flat or uphill. So the muscles weren't contributing to the increased work of hopping uphill, but were sticking to their main job of minimising work and the cost of force production by elastic energy-savings in their tendons.

Biewener suspects that there is a `division of labour' between the muscles in the upper and lower portions of the leg. He suggests the upper muscles are responsible for the increasing work output as the animal ascends a hill or accelerates, while the shorter lower leg muscles are responsible solely for energy-savings. Although wallabies appear not to use their short muscles to power themselves uphill, other creatures, such as running turkeys, may be able to adjust their short muscle-long tendon units to contribute power over tougher terrains. Biewener is keen to put more species through their paces to find out whether the tammar wallaby is unique in this respect.

But the mystery of the wallaby's efficient hop is not entirely solved. Biewener explains that the secret of their low-energy bounce must depend on the muscle-tendon unit's design, but other animals have also evolved similar highly specialised designs that do not result in the same energy savings. So,it's still unclear exactly how the wallabies can operate so efficiently, but Biewener intends to continue his wallaby studies with the hope of understanding how different creatures have adapted their legs to bounce, hop and run across varying terrains.

Biewener, A. A., McGowan, C., Card, G. M. and Baudinette, R. V. (
). Dynamics of leg muscle function in tammar wallabies (M. eugenii) during level versus incline hopping.
J. Exp. Biol.