Most animals expend more energy as their speed increases. This is because faster movements typically require muscles to generate force more rapidly, leading to higher metabolic costs. However, kangaroos and their relatives, known as macropods, defy this rule. Decades of treadmill studies have consistently shown that red kangaroos and tammar wallabies can hop at higher speeds with only a minimal increase in oxygen demand, a phenomenon that has long baffled biomechanics researchers.
Previous investigations explored the role of their ankle extensor muscle-tendon units, which function like biological springs, storing and releasing elastic energy. While crucial, these alone couldn’t fully explain why large macropods maintain such low energy costs compared to other quadrupeds of similar size. Attempts to find answers in stride timing or breathing coordination also proved inconclusive, failing to differentiate between small and large macropods.
Now, a groundbreaking study by an international team of researchers from Australia, the UK, and the US appears to have uncovered the missing piece of the puzzle: posture. Their findings suggest that the specific combination of joint angles a kangaroo adopts when its foot is on the ground actively modulates the leverage at the ankle. This allows for a greater return of elastic energy as the kangaroo moves faster. If this model holds true, it implies that kangaroos can meet increased mechanical demands without their muscles having to work harder, effectively decoupling speed from metabolic expenditure.
To arrive at this conclusion, the researchers meticulously recorded 3D motion and ground forces from 16 red and eastern grey kangaroos hopping at speeds ranging from 2 to 4.5 meters per second on specialized force plates. They then constructed a detailed musculoskeletal computer model to analyze joint kinematics, rotations, effective mechanical advantage, ankle work, and Achilles tendon stress.
Their analysis revealed that as kangaroos increased their hopping speed, they bent their legs more upon slowing down, with the ankle flexing upward and the toes pressing down harder. This action caused the Achilles tendon to be stretched more intensely, much like pulling a thicker rubber band. Concurrently, ground forces and the twisting force at the ankle also rose. This particular leg geometry enabled the tendon to store a greater amount of energy during the initial landing phase and then efficiently release it as the animal pushed off the ground.
Crucially, the study found that while both the energy ‘absorption’ during landing and the ‘push-off’ during propulsion increased at higher speeds, they remarkably balanced each other out over each hop. As a result, the total work performed by the ankle per hop remained largely constant. This mechanism meant the tendon was undertaking more of the mechanical work, thereby eliminating the need for muscles to consume significant extra energy.
However, the study also identified a potential trade-off: because kangaroos heavily rely on their tendons, there might be a limited safety margin before structural failure could occur. This suggests that the unique biomechanics of their hop could impose limits on how large these animals can become and how sharply they can maneuver.
The researchers recommend that future studies investigate a wider range of body sizes and thoroughly assess tendon stress at high speeds. This might require alternative experimental or modeling approaches, as kangaroos in enclosed environments tend to be reluctant to hop faster over force plates. Furthermore, they emphasize the importance of understanding how posture and the entire body’s musculature contribute to the overall energetics of kangaroo locomotion.