For researchers intrigued by the less conventional ways animals use to get around, wallabies and kangaroos are ideal study subjects. `They are just like pogo-sticks' says Craig McGowan of the University of Colorado, Boulder. When a wallaby is using its specialised design to bounce around on a flat surface, its ankle extensor muscle tendon acts like a spring, storing up energy for each hop. But when a wallaby is hopping up a hill, the ankle extensor still stores the same amount of energy and doesn't work any harder against gravity, so McGowan and his colleagues concentrated on the activity of the two largest muscles in the leg – a knee extensor called the vastus lateralis and a hip extensor called the biceps femoris – to see if they give wallabies the extra `oomph' they need to bounce up hills (p. 1255).
To find out how much the wallabies' muscles were shortening and lengthening, and therefore how hard they were working as they hopped along on level ground or up a slope, the team used a method called sonomicrometry. The team surgically implanted a pair of crystals, about 15 mm apart, into the vastus and biceps muscles. One crystal emits a high frequency sound wave to the second crystal, which transmits this information via a very fine wire to a receiver outside the animal. The time it took for the sound to travel between the crystals told the team how long the muscle was and whether it was lengthening or shortening. `It's a very nice method as you're directly measuring length change', says McGowan. They also inserted tiny wires into the muscles to measure their electrical activity during hopping, telling them when the muscles were contracting.
Once the wallabies had recovered from their operations, they went to work on level or inclined treadmills. Comparing the muscles' electrical activity between level and incline hopping, the team found that the vastus and biceps muscles were active for the same amount of time during both level and incline hopping. However, when they examined changes in the muscles' lengths, they found that both muscles were working harder as the wallabies bounced uphill, but in different ways.
The biceps muscle shortened more during uphill hopping, showing that it was contracting and producing more force to help move the wallaby against gravity. The vastus muscle behaved differently: during level hopping, it lengthened more than it shortened, acting like a shock absorber. Hopping uphill, the muscle lengthened less, meaning that it could absorb less energy. Instead, it acted as a knee stabiliser to counteract some of the very high forces being produced by other muscles at the hip and the knee.
Interested to know how much work the biceps and vastus were contributing, the team put their data, plus data from wallabies hopping on force plates, into a mathematical model which estimates how much force each muscle contributes. Their calculations showed that both the muscles were working harder to help the wallabies hop up the slope, but weren't working as hard as their size suggested they would. McGowan suspects that other muscles in the hips also work harder to help wallabies hop up hills.
- © The Company of Biologists Limited 2007