Is posture how kangaroos can speed up without burning more energy?

Most animals spend more energy as they move faster. Shorter periods of contact between the foot and the ground require muscles to generate force more quickly, driving up the metabolic cost.

Kangaroos and their relatives (macropods) are exceptions to this rule, however. Classic treadmill studies have shown that red kangaroos and tammar wallabies can hop faster with only a slight increase in oxygen demand, puzzling biomechanics researchers.

Past studies have pointed to their ankle extensor muscle-tendon units, which can store and return elastic energy like springs, but that alone couldn’t explain why large macropods don’t incur energy costs that other similar quadrupeds do. Attempts to crack the mystery using stride timing or breathing coordination fell short as well as didn’t distinguish small from large macropods.

A new study by researchers from Australia, the UK, and the US, reported in eLife, may have finally found the answer. The researchers focused on posture, i.e. the combination of joint angles a kangaroo adopts when its foot is on the ground. They found that posture actively modulated the leverage at the ankle and could raise the amount of elastic energy returned as the kangaroo moves faster. If this model is validated, it would mean kangaroos can meet higher mechanical demands without their muscles performing extra work, thus delinking speed from metabolic cost.

The researchers recorded 3D motion and ground forces from 16 red and eastern grey kangaroos hopping at 2-4.5 m/s on force plates. Then they built a scaled musculoskeletal model on a computer to model the joint kinematics, joint rotations, effective mechanical advantage, ankle work, and Achilles tendon stress.

As they hopped faster, the team found that kangaroos bent their legs more when they slowed down, with the ankle bending upward more and the toes pressing down harder. The Achilles tendon was pulled harder, like stretching a thicker rubber band. Ground forces and the twisting force at the ankle also rose. This geometry allowed the tendon to store more energy when the foot first landed, and return it as the animal pushed off.

Importantly, even though both the ‘soaking up’ part upon landing and the ‘push off’ part got bigger at higher speeds, they balance out over each hop. As a result, the ankle’s total work per hop stayed roughly constant. Instead, the tendon was doing more of the job, and the muscles didn’t have to burn much extra energy.

On the flip side, because kangaroos run their tendons hard, the study warned that there wouldn’t be much of a safety margin before something could fail. This in turn meant the biomechanics of the kangaroos’ hop could limit how big they could get and how sharply they can turn.

The researchers also wrote in their paper that “future work should examine a broader range of body sizes”, could assess “tendon stress at fast speeds” — perhaps “with a different experimental or modelling approach … as kangaroos in enclosures seem unwilling to hop faster over force plates — and also “understand how posture and muscles throughout the whole body contribute to kangaroo energetics”.