The study, conducted at the University of the Sunshine Coast (UniSC) in Australia and now published in eLife, explains a long-standing puzzle in locomotion biomechanics about why the energetic cost of hopping in kangaroos remains stable as they move faster.
Kangaroos, wallabies and other macropods use a pentapedal gait involving forelimbs, hindlimbs and tail at low speeds, but switch to a bipedal hopping gait at higher speeds, and their energetics do not follow the patterns predicted by many terrestrial locomotion models.
The work tests the refined cost of generating force hypothesis, which predicts that animals should spend more energy as they run faster and reduce ground contact time, a pattern that macropods do not follow.
"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.
Thornton and colleagues built a 3D musculoskeletal model of a kangaroo using 3D motion capture and force plate measurements that recorded the forces exerted on the ground during hopping in red and grey kangaroos.
Using this model, the team examined how body mass and hopping speed affected hindlimb posture, effective mechanical advantage of the joints and associated tendon stress in the ankle extensors, as well as mechanical work at the ankle.
They proposed that the hindlimb would become more crouched at faster speeds, mainly through changes at the ankle and metatarsophalangeal joints, and that posture-driven changes in joint moment arms would increase tendon stress and elastic energy storage, allowing energy savings at higher speeds without extra muscle work.
Analyses confirmed that hindlimb posture changed with both body mass and speed, and partially supported the first hypothesis by showing that the limb was more crouched at higher speeds due largely to altered angles at the ankle and metatarsophalangeal joints.
Joint-level energetics showed that most work and power per hop in the hindlimb occurred at the ankle, and that as posture became more crouched with speed, ankle effective mechanical advantage 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 at UniSC, and Honorary Senior Fellow at the University of Queensland, Australia.
The authors note that they could not determine whether effective mechanical advantage at the hip and knee, which are harder to measure with surface motion capture, changed with speed, even though these proximal joints contribute less work than the ankle while housing most of the kangaroo's skeletal muscle.
They conclude that further research is needed to map how posture and muscles across the entire body contribute to kangaroo energetics, including possible changes in effective mechanical advantage at proximal joints.
"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."
Research Report:Postural adaptations may contribute to the unique locomotor energetics seen in hopping kangaroos
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