Locomotion

To talk about the human body as a unit, it is customary and simple to make use of the center of mass. Despite the fact that the center can change, it is still usefull to understand and model the body as a whole. We will view movement of the center of mass as locomotion. In order for the center of mass to move relative to external objects, it must be acted on by a force. What are the common forces which will act upon the body to make it move? The most common ones are: the forces between the feet and the ground, the force of gravity and the force of an opponent's push or strike. Without one of these (or some other interfering force such as wind or other odities which we will discount for now), the center cannot displace itself.

Acceleration Limits for the Center of Mass

Now let's ask some questions about limits. How fast can a person move forward or backward? How fast can a person rise or fall?

The Gravity Limit

Without external assistance (i.e. from an opponent), a person cannot fall faster than gravity, but a person can rise faster than gravity because the rising force they can create with their legs can be larger than the force of gravity. This is equivalent to realizing that a strong person may be able to lift more than their own body weight, but they can never pull down with more than their body weight even if they hang from the object they are pulling!

The Friction Limit

Now what about moving forward and backward? Under the power of one's feet, we are theoretically limited by the force of friction between our feet and the earth. This force is relative to the coefficient of friction which in theory cannot be greater than one. I believe there are materials which do actually have higher coefficients, certainly a sticky cohesive material such as glue does. Ignoring greater than 1 coefecients, we can deduce that moving forward or backward also cannot be faster than gravity since at best with a coefficient of 1, we cannot create a lateral acceleration force (of our center) greater than the force of gravity. This limit is not inherently human, it actually applies to all horizontal locomotion. Even race cars have to be concerned with this limit.

Breaking the Friction Limit

So how can we accelerate even faster? One way to "cheat" friction is to increase the force being applied to the friction surface without increasing the mass that you need to accelerate. But, how do we do that? Well there are several arrangements that could achieve this. A really low ceiling could be used to push down with, increasing our friction force. If we expand our view and incorporate our opponent, we can use their weight to help accelerate us. If you pull on your opponent while running forward, you will be able to take advantage of the friction his feet create with the ground to accelerate yourself faster than gravity!