A recent study has uncovered how kangaroos can increase their hopping speed while maintaining a relatively low energy expenditure. This research, conducted by a team from Australia, the UK, and the US, was published in the journal eLife.
Traditionally, animals tend to consume more energy as they accelerate. This is attributed to the need for muscles to generate force more rapidly during shorter ground contact times. However, kangaroos and their relatives, known as macropods, challenge this norm. Earlier studies have indicated that species like the red kangaroo and tammar wallaby can hop at greater speeds with only a minimal rise in oxygen consumption, leaving researchers puzzled.
Previous explanations for this phenomenon pointed to the unique properties of the kangaroos” ankle extensor muscle-tendon units, which function similarly to springs by storing and returning elastic energy. Nevertheless, these factors alone could not fully clarify why large macropods do not experience the same energy costs as other quadrupeds.
The new investigation zeroed in on the kangaroo”s posture, specifically the angles formed by its joints when its foot strikes the ground. The researchers discovered that posture plays a critical role in adjusting leverage at the ankle, enabling kangaroos to return more elastic energy as their speed increases.
To conduct the study, the team recorded three-dimensional motion and ground forces from 16 red and eastern grey kangaroos hopping at speeds ranging from 2 to 4.5 meters per second. They developed a scaled musculoskeletal model on a computer to analyze joint kinematics, rotations, mechanical advantages, ankle work, and stress on the Achilles tendon.
As the kangaroos hopped faster, the researchers noted that they bent their legs more during slower phases, with the ankle flexing upward and the toes pressing down more forcefully. This action resulted in the Achilles tendon being stretched more, similar to how a thicker rubber band behaves. Additionally, ground forces and the torque at the ankle increased, creating a geometry that allowed the tendon to store additional energy upon landing and release it during takeoff.
Crucially, while both the energy absorption upon landing and the push-off increased with speed, the overall work done at the ankle remained relatively constant. This indicates that the tendon was responsible for a greater share of the work, permitting the muscles to avoid significant extra energy expenditure.
However, the study also highlighted potential risks, noting that the intense use of their tendons leaves little margin for safety, which could restrict the maximum size of kangaroos and their agility in turning. The authors concluded that future research should examine a wider range of body sizes, investigate tendon stress at higher speeds, and explore how overall body posture and muscle function contribute to the energy dynamics of kangaroos.
