All the Best Bits: Physics in Martial Arts: Ukemi ("Accepting Falls")

After over 30 years of studying Judo with some Karate, Tae-Kwan-Do, and Aikido thrown in from time to time, I have finally come to conclude that I have never seen a decent technical description of the physical principles surrounding the very specialized and effective techniques that martial arts have evolved over the centuries to protect people from injury when falling. Many practitioners are quite expert in the practice, and can teach the techniques quite effectively, but traditionally do not speak in any real detail on the scientific aspects. Frankly, I have found that this is a good thing, as the technical descriptions, even to MIT undergrads, have little impact on student improvement. But the study of different refinements and variations across different martial arts could be instructive. So I guess I'll write it myself and invite comment.

In the interest of reaching the broadest possible audience, I will restrict this discourse to fundamental physical principles without any real mathematics, though possible later posts might go into specific numerical examples. In order to understand this discussion, readers should be, or make themselves, familiar with the basic physics surrounding velocity, acceleration, linear and angular momentum (and conservation thereof), work, energy (kinetic and potential), impulse, pressure, levers and leverage, springs, and collisions.

The very name Ukemi is often mistranslated as falling. A more correct translation takes into account the etymology of the root word Uke, or receiver. In Judo ( The Art or Way of Gentleness ) techniques are generally executed by Tori or the Giver and taken by the receiver, Uke. In that sense, a person who receives a technique must accept, and protect themselves from any attack or incident. The fundamental precept of Judo is: Maximum efficiency with minimum effort, for the mutual welfare and benefit of all. So the techniques surrounding the receipt of any technique is never to meet force with a directly opposing force, but rather to go with the incoming force, accept its energy to move in concert with it, and then deflect it in a more useful direction without harm to either party. By extension, Ukemi extends these same principles to protect a person physically from even brutal impacts with the ground. The basic goal, of course, is to be able to fall without injury, even under extreme conditions otherwise out of a person's control.

The efficacy of these techniques is truly awesome. They are, by far, the most useful and often-used aspects derived from my many years of martial arts training, and have helped me survive some potentially serious injuries. One particular incident comes to mind from when I was about 12 or 13 years old, with around 6 years of Judo training behind me at that point, when I was pushed off of a 3-meter diving board head first onto the concrete pool deck. You know, one of those "saved your life" kind of jokes gone bad. Net result was a slight road rash on one shoulder, but the look on the guy's face when I just dove of the board and rolled to my feet was priceless. Other examples in sports, and on Ithaca, NY ice are just too numerous to count. I just have absolutely no fear of falling anymore, and that is a very empowering thing.

The basic principals that can be used to describe the minute refinements that give Judo Ukemi its incredible protective power turn out to cover a very wide range of mechanisms.

The area of the body impacting the ground in the instant when sensitive body parts strike is maximized in order to minimize the absolute force felt by any one part of the body.
The duration of impact is maximized, so that the momentary impulse or force felt at any one instant is minimized.
Tensioned, but not stiff or locked, muscles are used as springs, to absorb kinetic energy and dissipate it harmlessly, giving, but not breaking.
The extension of arms and legs, and even bending of the waist are actively controlled so as to change the angular moments of inertia and thereby vary the rate of rotation of the body and guarantee a landing on a soft part of the body, just as a cat gyrates to land on its feet.
The limbs are used to do work, and thereby affect relative rotational and linear velocities so that the body lands in a safe position.
The Body is bent into wheel-like shapes in order to convert linear momentum in a fall, to controlled angular momentum of the body rolling along the ground in a wheel-shape, until the flexible muscles dissipate the energy of the fall with little or no actual impact.
The limbs and joint angles are actively positioned so that no hard bones or joints (skull, knee, elbow, ankle bone, wrist bones) can possibly impact the ground directly, and so that no part of the body can be driven to strike another part of the body (as in one leg crossed over the other to slam together).
The head is tucked, chin towards chest, and inside the circle of arms to protect it from any impact.
The breath is exhaled smoothly and continuously (traditionally in a Kiai or shout, translated more literally as an expression of energy or self ) so that upon impact there is no sharp increase in blood pressure from the closed bag of water (your body) hitting the ground. (which in specially hard falls, can cause a person to black-out.)
Fingers and toes are held together so that in a unit they are stronger than individual fingers that can catch on things and break.
Any contact with the ground, or an attacker that exists at the beginning of a fall or throw, even if Uke is very off-balance, is used as something to push against in order to alter the body's trajectory towards a safer landing position.