Kangaroos Can Hop Faster Without Burning More Energy Thanks to Smart Posture Changes
Kangaroos have long fascinated scientists for one simple reason: they can hop faster and faster without paying the usual energy penalty that most animals face when they move at high speeds. A new study published in eLife finally offers a clear biomechanical explanation for how they pull off this impressive feat. The answer lies not in stronger muscles or exotic physiology, but in something surprisingly subtle — how kangaroos adjust their posture while hopping.
The research shows that at higher speeds, kangaroos slightly change the way they hold and move their hindlimbs. These posture changes increase stress on key tendons, especially the Achilles tendon, allowing more elastic energy to be stored and released with every hop. This stored energy effectively offsets the extra muscular force required at faster speeds, keeping the overall energetic cost nearly constant.
This finding helps resolve a long-standing puzzle in locomotion biomechanics and provides strong experimental evidence for why kangaroos defy the usual rules of animal movement.
Understanding the energy mystery of kangaroo hopping
In most animals, moving faster means using more energy. As speed increases, ground contact time decreases, muscles must generate force more quickly, and metabolic cost rises. This relationship is central to the widely accepted “cost of generating force” hypothesis, which predicts a clear increase in energy use with speed.
Kangaroos, wallabies, and other macropods are a major exception. For decades, researchers have observed that kangaroos can increase hopping speed with little to no increase in metabolic cost. This unusual pattern has been measured repeatedly, yet the mechanical explanation behind it remained incomplete.
The new study set out to directly investigate the kinematics (movement patterns) and kinetics (forces involved) of kangaroo hopping across different speeds and body sizes. By doing so, the researchers aimed to identify how posture, limb geometry, and tendon mechanics interact to keep energy expenditure in check.
Who conducted the study and how it was done
The research was led by Lauren Thornton, who conducted the work as a PhD student in the Biomechanics and Biorobotics Laboratory at the University of the Sunshine Coast (UniSC) in Australia. The senior author was Christofer Clemente, an Associate Professor in Animal Ecophysiology at UniSC and an Honorary Senior Fellow at the University of Queensland.
The team studied red and gray kangaroos, collecting detailed motion and force data while the animals hopped at a range of speeds. To do this, they used:
- 3D motion capture systems with reflective markers placed on the kangaroos’ bodies
- Force plates to measure ground reaction forces during each hop
- A 3D musculoskeletal model of the kangaroo hindlimb to simulate muscle forces, joint mechanics, and tendon loading
This modeling approach allowed the researchers to analyze movements and forces that are extremely difficult to measure directly in living animals.
What the researchers wanted to test
Before analyzing the data, the team proposed two main hypotheses.
First, they predicted that as hopping speed increased, kangaroos would adopt a more crouched hindlimb posture, mainly through changes in the ankle and metatarsophalangeal (toe) joints. They also expected this posture change to be largely independent of body mass.
Second, they hypothesized that these posture changes would alter the moment arms around the ankle joint, increasing tendon stress in the ankle extensors. This increased stress would allow more elastic energy to be stored and returned by the tendons, reducing the need for additional muscle work.
Both hypotheses were grounded in biomechanical theory, but required detailed modeling to confirm.
How kangaroo posture actually changes with speed
The results showed that hindlimb posture does change with speed, and body mass also plays a role. As kangaroos hop faster, their hindlimbs become more crouched, primarily due to adjustments at the ankle and metatarsophalangeal joints.
These changes in joint angles alter the overall geometry of the hindlimb. One key outcome is a decrease in effective mechanical advantage (EMA) at the ankle joint. EMA is a measure of how efficiently muscle force is transmitted to produce movement. A lower EMA means muscles must produce more force for the same external load.
At first glance, this seems inefficient. However, this decrease in EMA turns out to be the key to kangaroos’ energy-saving strategy.
Why the ankle and Achilles tendon matter so much
The study found that the ankle joint dominates hindlimb energetics during hopping. Most of the work and power per hop are generated at the ankle, far more than at the hip or knee.
As the ankle EMA decreases at higher speeds, Achilles tendon stress increases. This increased stress allows the tendon to stretch more and store greater amounts of elastic energy. When the tendon recoils, that energy is returned to the system, contributing to propulsion without requiring extra muscle work.
In simple terms, kangaroos let their tendons do more of the work as they hop faster.
Crucially, the researchers found that despite changes in posture and force, the net work at the ankle and the amount of muscle work remain essentially constant across speeds. The increased elastic energy storage and return perfectly counterbalances the higher muscular forces required at faster hopping speeds.
Why energy cost stays flat even at high speeds
This tendon-driven mechanism explains how kangaroos uncouple hopping speed from metabolic cost. As speed increases:
- Ground reaction forces increase
- Ankle EMA decreases
- Achilles tendon stress increases
- Elastic energy storage and return per hop increases
Together, these changes allow kangaroos to hop faster without increasing net muscle work. The muscles still do roughly the same amount of work per hop, even though speed and force demands rise.
The researchers also showed that changes in posture have an effect on tendon stress that is comparable in magnitude to the effect of increasing ground reaction forces. This highlights posture as an active and important control mechanism, not just a passive consequence of speed.
Limitations and unanswered questions
While the findings are compelling, the authors acknowledge several limitations. Surface motion capture makes it difficult to accurately track proximal joints like the hip and knee. Although these joints contribute less mechanical work than the ankle, they contain the majority of kangaroo skeletal muscle.
This means future research is needed to determine whether the EMA of the hip and knee remains constant with speed, or whether additional posture-related mechanisms are at play throughout the body.
The authors also note that posture-controlled increases in tendon stress may place constraints on maximum body size, potentially helping explain why extremely large hopping macropods are rare.
Why this study matters beyond kangaroos
This research does more than solve a kangaroo-specific mystery. It challenges the assumption that effective mechanical advantage is static and shows that animals can dynamically tune limb geometry to manage energy costs.
The study also demonstrates the power of musculoskeletal modeling and simulation, which allow scientists to explore links between anatomy, posture, and energy use that are difficult or impossible to isolate experimentally.
These insights could influence fields ranging from evolutionary biology to bio-inspired robotics, where efficient legged locomotion remains a major engineering challenge.
Research paper:
https://elifesciences.org/articles/96437
