Ask Ms. Jessica Issue 31 – Long legs and walking speed

(The content for Ask Ms. Jessica comes from real questions from real people sent to AskMsJessica@gmail.com)

Hello Ms. Jessica,

Recently I saw a mutant with legs that were easily six feet long if not longer. The thing is, she walks at a pretty normal speed. I always figured she should be slower because she is bigger, heavier, has so much more mass and volume. How do big mutants move different from smaller mutants?

Well, it depends on what you mean by faster? Speed is a function of stride distance and stride rate. Stride distance is the amount of ground you cover in one step while stride rate is the amount of steps you can make in any given period of time. The same holds true no matter how many legs you have or how long they are.

Let’s look at some hypothetical numbers.

Let’s say you have a normal human with a stride length of three feet and a stride rate of one stride per second. This person then moves at three feet per second.

Now let’s look at your long legged mutant friend. Let’s say her long legs allow her to have a stride length of nine feet. 

If she was moving at the same ground speed of a normal human, i.e. three feet per second, she would end up having a much lower stride rate. A human would need to take three strides to move nine feet while our long legged mutant would only have to take one. This would make it appear as if she is moving “slower” than the human, since she is taking fewer steps, but she is still moving at the same speed.

However, there is another interpretation of fast. Instead of setting their ground speed to be equal in our calculations, we can set their stride rate to be equal, in which case their bodies are moving at approximately the same speed relative to their body size. If this is the case, our long legged mutant friend would actually be covering much more distance than our human friend, three times as much, which means by ground speed calculations only they are three times as fast!

We all assume that bigger means slower because of the movies, but that’s not necessarily true. After all, elephants are some of the biggest land mammals there are but they can move up to speeds of 40km/h (25 mph). While it is true that more energy is needed to move more body mass, usually bigger creatures have bigger muscles and more effective metabolisms to compensate. 

That’s why you always see mutants with leg mutations in the mutalympics. Long legs means faster ground speed. More legs equals less force and work needed to achieve the same ground speed. There are a ton of other mutations that give mutant athletes an advantage in races and other track and field events. 

In short, bigger mutants are not fated to be slow, but it’s all a matter of how you look at it. If their body moves as fast as a humans, then they move faster across the ground. If they match a human’s ground speed, then their body moves slower.

Really, I think all you saw was the mutant matching the human’s speed to be polite. She probably could have pulled out ahead but that would be rude, don’t you think? We all match the speed of others that we are walking with.

Dr. Jessica Park

AskMsJessica@gmail.com