Not too big, not too small: Why it’s the ideal size for modern people to speed up.

The speed and size pattern of running animals (blue) shows that intermediate-sized species (such as cheetahs) are usually the fastest. Resizing the computer-generated human model (right) from a mouse to a horse (orange dots) shows the same pattern, revealing the underlying biomechanical reasons. Credit: Nature Communications (2024). DOI: 10.1038/s41467-024-52924-z
The fastest animal on land is the cheetah, which can reach a top speed of 104 kilometers per hour. The fastest underwater animals are yellowfin tuna and wahoo, which can reach speeds of 75 km/h and 77 km/h, respectively. In the air, the title of fastest horizontal flight (excluding diving) goes to the white-throated swift at over 112 km/h.
What do these swift creatures have in common? They’re neither particularly large nor particularly small for a group of animals. In fact, they are all medium in size.
The reason for this is a bit of a mystery. As the mass of an animal increases, some biological characteristics also change. For example, leg length generally increases steadily. But the largest land animals like elephants aren’t the fastest, so long legs are clearly not the answer.
However, my colleagues and I have taken an important step toward solving this mystery. Using a scalable virtual model of the human body, we were able to study the movements of limbs and muscles, discover what limits their speed, and gain important insights into the evolution of the human body over thousands of years. The findings will be published in the journal Nature Communications.
From a human the size of a mouse to a giant
Scientists have been building OpenSim since the early 2000s. OpenSim is a freely available virtual model of the human body, complete with all bones, muscles, and tendons.
This model is used in a variety of scientific studies to help understand human movement, explore exercise science, and model the effects of surgery on soft tissues.
In 2019, a group of Belgian researchers took this a step further and used OpenSim to build a physically-based simulation. Rather than telling the model how to move, they asked it to move forward at a constant speed. The model then determined which muscle combinations to activate to allow the person to walk or run at a specified speed.
But what if we took this a step further and scaled the model down to the size of a mouse? Or what if we scaled the model up to the size of an elephant? Then we could see which models run at what speeds. I did.
This is exactly what my team did. Based on a standard human body model (75 kg), we created a miniaturized model down to 100 grams. We also challenged ourselves to make the model larger, up to 2,000kg, and run as fast as possible.
Understand mass properly
When I did this, some interesting things happened.
First, the 2,000kg model could not move. Even the 1,000kg model couldn’t do it. In fact, the largest model that can move is 900kg, suggesting an upper limit for human form. If you exceed this size, you will need to reshape it to move it.
We also found that the fastest model is neither the largest nor the smallest. Instead, it weighed about 47 kg, about the same as an average cheetah. The important thing is that you can look under the hood and understand why it happened.
The curve that describes the shape of the maximum travel speed due to mass is the same shape as the curve that describes the maximum ground force due to mass. This is not surprising. To move faster, you need to push the ground harder.
So why weren’t the larger models able to push harder off the ground? The larger models seemed to be limited by their muscles.
The ability of a muscle to produce force is determined by its cross-sectional area. And as the animal increases in size, muscle mass increases faster than cross-sectional area.
This means that large animals have relatively weak muscles. As the muscles begin to “max out” beyond their maximum speed, the model must slow down.
At the other end of the spectrum, miniature models have relatively strong muscles but have problems with gravity. It’s too light. You try to push against the ground and exert a lot of force, but this only causes your body to leave the ground faster.
To try to generate more force on the ground, they bend their limbs, similar to mice and cats. This allows you to stay on the ground longer and generate more force, similar to when you stand up and jump. But this takes time. And the longer it takes you to develop your strength, the slower your stride will be, and you still won’t be able to run as fast.
Therefore, the trade-off between ground force and stride frequency begins and does not end until the weight reaches a good intermediate size.
as soon as possible
What does all this tell us about human evolution?
Throughout history, modern humans and extinct humans (groups known as “hominins”) have grown in size from Australopithecus afarensis, which existed about 3.5 million years ago, to about 30 kg. We know that we have become very different from the 80 kg Homo erectus that existed before us. 2 million years ago.
In general, weight tends to increase, and so does running speed. Homo naledi, which existed about 300,000 years ago and weighed about 37 kg, and Homo floresiensis, which existed about 50,000 years ago and weighed about 27 kg, had to sacrifice some speed due to their small size. I’m sure it wasn’t there.
The average weight of a modern adult is around 62kg, which is a bit heavier than the peak weight of 47kg found in modeling, but still close to the ideal size.
Interestingly, many of the fastest distance runners, like Eliud Kipchoge, weigh around 50kg.
So, based on our new research, we find that today’s humans have roughly the same speed as they will gain in the future, without major changes in muscle morphology.
Further information: Christofer J. Clemente et al. Predictive musculoskeletal simulations reveal mechanical links between speed, posture, and energy in extant mammals, Nature Communications (2024). DOI: 10.1038/s41467-024-52924-z
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