Researchers have taken a leap in understanding how kangaroos can increase their hopping speeds without incurring an associated energetic cost.
Their study, first published in eLife as a Reviewed Preprint and appearing this week as the final Version of Record, shows that changes in kangaroo posture at high speeds increases their tendon stress and energy storage and return. This increased energy storage/return counteracts the higher muscular force required at speed, explaining why the animals’ energetic cost remains unchanged.
eLife’s editors say the research, from the University of the Sunshine Coast (UniSC), Australia, provides convincing evidence to help address this long-standing question in locomotion biomechanics, and sets the stage for further studies to investigate how kangaroo hopping speeds relate to metabolic cost more definitively.
Kangaroos, wallabies and other macropods are unique in both their form and locomotor (movement) style. At slow speeds they use a pentapedal gait, where their forelimbs, hindlimbs and tail all contact the ground, while at faster speeds they use their distinctive hopping gait. This uniqueness also extends into the energetics behind these movements.
“The ‘cost of generating force’ hypothesis, refined in a previous study, implies that as animals move faster and decrease their ground contact time, their energy cost should increase – but macropods defy this trend,” explains first author Lauren Thornton, who was a PhD student in the Biomechanics and Biorobotics lab, School of Science, Technology and Engineering, UniSC, at the time the study was carried out. “The underlying mechanisms that explain how macropods are able to uncouple their hopping speed and energy cost are still unclear, so we set out to address this by investigating their hindlimb motion, or kinematics, and the forces that impact this motion – the kinetics – as they hop around at various speeds.”
Thornton and colleagues created a 3D musculoskeletal model of a kangaroo based on 3D motion capture and force plate data – concerning the force exerted on the ground during hopping – to analyse the movements of red and grey kangaroos.
An illustration of the research team's kangaroo model. Image credit: Thornton et al., 2025
(CC BY 4.0)
Using this model, they evaluated how the animals’ body mass and speed influence three aspects of their form and movement during hopping: their hindlimb posture; effective mechanical advantage (EMA) – a measure for understanding the efficiency of movement – and its associated tendon stress in the ankle extensors; and their ankle work.
An image of a kangaroo from the study with reflective markers used for the team's 3D motion capture system. Image credit: Craig McGowan
(CC BY 4.0)
The team hypothesised two results ahead of their analyses. The first was that the kangaroo hindlimb would be more crouched at faster speeds, primarily due to the distal hindlimb joints (ankle and metatarsophalangeal) and independent of changes with their body mass. The second was that changes in moment arms resulting from this posture change would contribute to the increase in tendon stress with speed. This may lead in turn to the animals’ energetic savings by increasing the amount of positive and negative work done by the ankle, without the need for additional muscle work.
Their analyses revealed that kangaroo hindlimb posture varied with both body mass and speed. In partial support of their first hypothesis, the hindlimb was more crouched as they moved at faster speeds, mainly due to the ankle and metatarsophalangeal joints.
Analysing the kangaroo joint-level energetics showed that the majority of the animals’ work and power per hop in the hindlimb was performed by the ankle joint. As the hindlimb became more crouched at faster speeds, the EMA of the ankle decreased.
“We found that as kangaroos hop faster, they crouch more, mainly by changing their ankle and metatarsophalangeal joint angles. This alters the geometry of the hindlimb, in particular, the moment arms to the Achilles tendon force and the ground reaction force, which decreases ankle EMA. Achilles tendon stress increases as a result, and therefore so does the amount of elastic energy it can store and return per hop,” Thornton says. “We found that this helps kangaroos maintain the same amount of net work at the ankle, and the same amount of muscle work, regardless of speed.”
“Our findings suggest that kangaroos’ posture-controlled increases in energy absorption at the ankle provide energetic efficiency during hopping – although potentially constraining the maximum size achievable by larger species,” adds senior author Christofer Clemente, Associate Professor in Animal Ecophysiology, and Group leader in the Biomechanics and Biorobotics lab, UniSC, and Honorary Senior Fellow at the University of Queensland, Australia.
The authors note that their study has a few limitations. For example, they were unable to determine whether the EMA of proximal hindlimb joints, which are more difficult to track via surface motion capture markers, remained constant with speed. Although the hip and knee contribute substantially less work than the ankle joint, the majority of kangaroo skeletal muscle is located around these joints. Further research is therefore needed to understand how posture and muscles throughout the whole body contribute to kangaroo energetics.
“The contributions of changing posture can’t be overlooked,” Thornton says. “Changes in EMA have an effect on tendon stress comparable to that of the increase in peak ground reaction force that naturally occurs at faster speeds.”
“We’ve pieced together more of the puzzle of locomoter energetics in kangaroos, highlighting how EMA may be more dynamic than previously assumed,” Clemente adds. “Our work also shows how musculoskeletal modelling and simulation approaches can provide insights into direct links between form and function, which are often challenging to determine from experiments alone. It will be interesting to see future studies expand on this work to build a bigger picture of kangaroo kinematics.”
Accompanying multimedia for this study are available to download here.
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