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Why medium-sized animals are better built for speed

From the cheetah to the antelope to the yellowfin tuna and the needletail swift – these speedy creatures may look very different but they each share common traits.

Not only are they among the fastest animals on the planet, but for the groups they represent, they are all intermediately sized.

The reason for this has puzzled biologists for generations. But a team of researchers – including Dr Taylor Dick from UQ’s School of School of Biomedical Sciences – has taken a giant step towards solving the mystery, using cutting-edge musculoskeletal simulations to better understand how size and weight affects speed and energy output.

Key points:

• The research team created predictive musculoskeletal simulations based on human locomotion that replicate real-world physical constraints and biomechanical processes.
• Results showed there are fundamental biomechanical constraints that limit the speed of both very small and very large animals.
• Understanding how body size affects speed and energy could help conservationists predict how animals might respond to changes in their environment, palaeontologists understand why many giant animals have gone extinct, and biomechanists develop training strategies to optimize speed or efficiency in athletes.

Dr Dick said understanding why intermediate-sized animals were the fastest has been a major question in evolutionary biology, biomechanics, and ecology.

“Small animals, like mice and shrews, often seem quick in short bursts but can’t sustain high speeds for long,” she said.

“And while intuition may suggest that larger animals should be faster due to their longer strides, the reality is far more complex.

“Several theories have been proposed – including the role of metabolic constraints, muscle power, and the biomechanics of limb movement – however, the diversity of animal shapes, gaits, and environments has made it difficult to pinpoint the exact factors responsible for this pattern.

“This innovative approach has helped clarify the mechanisms behind the unusual scaling of speed and revealed generalised rules that can predict how animals of various sizes move.”

Pushing the models to their limits

Dr Dick’s research explores the mechanisms of neuromuscular function, using both experimental and modelling approaches to understand how the anatomy and biomechanics of the musculoskeletal system adapt to challenges such as size, age, and disease.

In this latest study, she joined Associate Professor Christofer Clemente, from the University of the Sunshine Coast, and Associate Professor Friedl De Groote, from KU Leuven in Belgium, to create predictive musculoskeletal simulations based on human locomotion that mimic the way muscles, bones, and tendons work together to produce movement.

This allowed the models to replicate real-world physical constraints and biomechanical processes.

Using OpenSim – a freely available, virtual model of the human body with bones, muscles and tendons – the models were scaled across a wide range of body masses, from the size of a mouse (under 100 grams) to the size of an elephant (up to 2,000 kilograms).

“While the models were based on human anatomy, they were generalised to mimic basic principles of locomotion found in many animals, such as the use of legs for support and propulsion,” Dr Dick said.

“The simulations allowed us to control for factors like muscle force and limb structure, which vary widely across species, and focus on the general principles that govern movement across different body sizes.”

The research produced 3 key findings that mirrored real-world observations of animals:

1. Intermediate sizes are the fastest

Only models weighing between 100 grams and 900 kilograms were capable of moving, while the 1000-kilogram and 2000-kilogram models could not move at all. This suggests an upper limit on human body size.

Among the models that did move, the simulations showed that the fastest speeds were achieved by models of intermediate size. The fastest model weighed around 47 kilograms – similar to the average weight of a cheetah.

“To move faster, animals need to push off the ground harder. But it appeared the larger models were limited by their muscles.

“At the other end of the spectrum, the miniature models have relatively stronger muscles but are just too light.”

To try to produce more force on the ground, the smaller models crouch their limbs, much like mice or cats, which allows them to stay on the ground longer and produce more force.

2. Transition from crouched to upright postures

The research team observed a transition from crouched postures in smaller models to more upright postures in larger ones.

Dr Dick said this change in posture is commonly seen across animals as they change in size.

“Smaller animals, like rodents, tend to move with a crouched posture, while larger animals, like horses and elephants, adopt more upright postures,” she said.

“This transition is, in part, linked to the need for larger animals to support more weight with their limbs.”

3. Decreased cost of transport

The simulations also showed that as body size increased, the energy required to move a certain distance decreased.

“This aligns with real-world data, where larger animals tend to be more energy-efficient movers compared to smaller animals,” Dr Dick said.

“It also highlights the importance of energetics in determining movement strategies and speeds in animals of different sizes.”

The evolution of animal movement

Dr Dick said the findings have important implications for understanding animal locomotion and evolution.

“We have created a framework that can predict how animals of different sizes will move, with potential applications in fields ranging from ecology and conservation to robotics and clinical biomechanics,” Dr Dick said.

“Understanding how body size affects speed and energy could even help conservationists predict how animals might respond to changes in their environment, such as habitat loss or climate change.

But what does this tell us about human evolution?

“Throughout history, the size of hominins has varied from the small (about 30 kilograms) Australopithecus afarensis to the larger (about 80 kilograms) Homo erectus, so body mass has tended to increase – and presumably so too has our running speed,” Dr Dick said.

” The average body mass of modern adult humans is 62 kilograms – a bit heavier than the 47-kilogram body mass that our models predicted was the fastest.

“Intriguingly, many of the worlds fastest long distance runners, such as Eliud Kipchoge, weigh about 50 kilograms.”

Dr Dick said the research team can also take their simulations and develop models that represent an elite sprinter training for the Brisbane 2032 Olympics and Paralympics.

“This can help us understand how minor changes to musculoskeletal form, or how strengthening quadriceps rather than hamstrings, for example, can help an athlete improve their performance while using less energy,” she said.

“Although, based on our research, we now know humans today are about as fast as we will get – without large changes to our muscular form.”