Both humans and animals have preferred walking speeds. This is partly influenced by the amount of energy required: they prefer to walk at the speed at which they use the lowest possible amount of energy. One of the ways to do this is to use something called resonance.
You already know how it works: when you’re on a swing, you can’t swing at any speed. If you want to do it right, you have to choose the right timing and swing to the beat of the swing. In other words: you have to resonate with it. And when you take a nice relaxing walk, your body parts resonate too. Walking a little slower doesn’t require less energy: you notice that it’s actually more difficult.
An animation of T. rex Trix walking according to the simulations of Van Bijlert et al 2021. The tail resonates, allowing a more realistic, slightly slower gait. Credits: Rick Stikkelorum, Arthur Ulmann and Pasha van Bijlert
It works for animals that walk on four feet and for two-legged animals like humans and ostriches. Pasha van Bijlert, a human movement science student at Vrije Universiteit Amsterdam (VU), applied the idea to an animal that walked differently from anything on earth today: Tyrannosaurus rex. These carnivorous dinosaurs not only had two legs, they also had a huge tail that helped them move around.
Like the bones in our neck, the tail bones are held together by ligaments. “You could compare it to a suspension bridge,” Van Bijlert explains. “A suspension bridge with a ton of muscle in it.” With each step, the tail swings up and down. This means that like the swing, it has a natural frequency at which it resonates.
To find out what this frequency is, Van Bijlert and his professors Anne Schulp (Naturalis / University of Utrecht) and Knoek van Soest (VU) built a 3D model of Trix, the Tyrannosaurus rex on display at the Dutch National Museum of Natural History, Naturalis. They added digital muscles to the famous skeleton, and on this muscle model they were able to perform biomechanical analyzes. From these, they derived the natural frequency and a preferred walking speed: 4.6 km / h (2.9 mph). So when Trix was out for a walk, she walked at about the same speed as you. If you had a T. rex pet, you wouldn’t have a problem walking it – at least in terms of speed.
Van Bijlert, Van Soest and Schulp published their results in the journal Royal Society Open Science today (April 21, 2021). “There were already studies on the walking speed of dinosaurs, but they mostly looked at the legs and ignored the tail – which makes dinos so unique,” says Van Bijlert. “They generally found much higher walking speeds. The one we calculated is lower, but it is similar to that of other animals. “
Reference: “Natural frequency method: estimation of the preferred walking speed of Tyrannosaurus rex based on natural tail frequency ”by Pasha A. van Bijlert, AJ ‘Knoek’ van Soest and Anne S. Schulp, April 21, 2021, Royal Society Open Science.
DOI: 10.1098 / rsos.201441