BLADES OF THE NINJA

Kenji knew that he must be careful about how he entered
captured, Hon Castle. Even though he was disguised as a
merchant from a distant province, that would mean nothing
to Lord Katsugami’s brutal occupation troops. They would
treat and search him roughly, disgusted at even having to
touch a member of the lower classes. He would have to cower
before them and appear to be submissive. One wrong look,
just meeting the eyes of one of the Samurai, would be taken
as an infraction, reflecting great disrespect for the warrior
class. A deadly iai-jitsu draw and cut would follow, aimed at
the neck of such a vile, disrespectful merchant. On top of
everything else, most of the soldiers would be drinking,
celebrating their recent victory. After raping and pillaging
an entire province, they would need little excuse to spill the
blood of a defenseless merchant.

6 Specifics of Impact
Consider now the breaking process from the perspective of the target. When the
force of the strike is applied to the board or cinderblock, it accelerates in response to
that force. The key is that it does not accelerate uniformly—those areas where the
force is applied (the center of the target, if the strike is properly aimed) accelerate
much more than the outer regions of the target which are held in place by large
cinderblocks. This localized strain, the response to influence of stress imposed by the
strike, initiates the rupture. Strain is functionally the loss of height of the target that
occurs when the top surface is compressed and the bottom surface stretched. Because
of their molecular compositions, materials such as wood and cinderblocks withstand
compression better than stretching.

Use of Body Mass

Note that mass is a co-efficient in the formulae for force, momentum, and energy
transfer alike: all three are directly proportional to mass. Since a human being’s mass
for the time it takes to deliver a strike is constant—a karateka with a body mass of 70
kilograms before a strike will have a body mass of 70 kilograms after the strike—
mass is often and erroneously dismissed as a constant in the equations for force,
momentum, and impulse. What matters is not the karateka’s body mass, but how
much of that mass is involved in the strike. A body mass of 70 kilograms is beyond
the karateka’s immediate control; how many of those 70 kilograms contribute to the
strike is very much within the karateka’s control. It is therefore crucial not to use the
arm alone to extend the weapon and hope for sufficient force and energy to break the
target. The entire body should be used by snapping the hips and pushing with the legs
in the direction of the target. This explains why boxers are seldom knocked
unconscious by jabs, where little more than the mass of the arm contributes to the
punch, but are frequently knocked out by hook punches where the entire mass of the
body is thrown behind the punch. The same principle of using the entire body mass to
deliver a blow applies in breaking techniques as well.

Intuitively, this makes sense.

Since the hand cannot
move forward a distance greater than the length of the arm, it must have a velocity of
0 at full arm’s length extension. It follows that the hand must decelerate well before
the arm is fully extended. Advising beginners to attempt to hit an imaginary target
25% of their arms’ length on the far side of their targets would therefore be more
precise than the typical encouragement to aim for the floor, but the physical principle
is the same: maximum hand velocity is achieved when the point of focus of the strike
is well beyond the surface of the target.

Point of Focus

Karate black belts often advise white belts before their first attempt at breaking
not to try to break the board, but to break the floor under the board. This is to ensure
that the hand does not decelerate prior to contact with the target, a mistake that
beginners, fearful of injury and therefore mentally hesitant, often make. High velocity
of the hand is critical to successful breaking, and data taken from high-speed movies
of karateka show that maximum hand velocity is achieved when the arm reaches
approximately 75% of extension.

Consider a martial artist

capable of striking with 190 joules (J) of energy. A typical
human hand is about 6 inches long including the fingers and 4 inches across, which
means that a strike with the entire hand disperses those 190 J over 24 square inches,
about 7.92 J per square inch. If, however, the karateka strikes with only the fleshy part
of the palm, about 2 inches across and 1.5 inches long, the 190 J will be dispersed
over only 3 square inches. That strike will deliver about 63.3 J per square inch,
inflicting many times the amount of damage the whole hand could—the same amount
of energy dispersed over a smaller area delivers more energy per unit area.

Striking Surface

Any martial artist who has ever struck a board with improper hand technique can
attest to the physical pain associated with such impact. The human had is a complex
system of bones connected by tissue, and much can be said about the importance of
proper hand alignment in breaking. From the standpoint of physical science, however,
what is crucial about hand position upon impact is that all formulae for force,
momentum, and deformation energy are for a given unit of area. By minimizing the
amount of striking surface on the hand involved in collision with the board, a karateka
minimizes the area of the target to which force and energy are transferred and
therefore maximizes the amount of force and energy transferred per unit area.

As .E is proportional to the square of velocity, the more velocity the hand has, the

more energy will be transferred into the board. In the simplest possible terms, if the

board is infused with more energy than its structure can handle, it breaks. More

rigorously analyzed, energy transfer causes the board to dent. This process of

transferring energy is work (W). Work is force times distance (d): W=F· d. If the area

of the board that is struck dents a sufficient distance, it will break. Since the distance

it dents depends on the energy transferred to it and the amount of energy transferred

depends on the velocity of the karateka’s hand, a high-speed strike is most likely to

break the board.

energy transfer

enough relative to other parts of the board (which are generally held still by the

cinderblocks on which the boards are placed), breakage occurs.

This same phenomenon can be analyzed in terms of energy transfer and resulting

deformation damage. Given and object with mass m1 at rest (the board) and another

object of mass m2 (the karateka’s hand) moving at velocity v upon impact and

ignoring the negligible amount of energy lost as thermal energy (heat), the amount of

energy in the system lost to deformation damage (.E) is given by the following:

2

2 1 2

1 2

(1 )

2 ( )

m m e

E v

m m

· - . = · ·

+

where e is the coefficient of restitution, which measures how elastic the collision is. It

is a function of the hardness or softness of the colliding objects, which along with

velocity determines impulse. If hard objects collide (for a perfectly inelastic collision,

e=0), they will accelerate one another quickly, transferring a large amount of force in

a small amount of time while soft objects colliding (for a perfectly elastic collision,

e=1) transfer smaller amounts of energy to one another for longer periods of time.

Difference in how long momentum takes to transfer and therefore in force at a given

instant is why hitting a pillow with the fleshy part of the hand hurts much less than

hitting a brick with the knuckles.

Why, then, move should the karateka swing his or her hand with as much velocity
as possible? Because if the hand is moving quickly, it is likely to decelerate (strictly
speaking, accelerate in the direction opposite to its direction of travel) more quickly in
response to the force the board exerts on it upon collision, as per Newton’s third law.
If the amount of time involved in the transfer of momentum is therefore small, the
amount of force that will be transferred to the target all at once will be large. This
sudden transfer of a lot of force causes the part of the board that is struck and which
therefore experiences that force to accelerate. If that part of the board accelerates.

The first looks at the collision in terms of
force and momentum; the second looks at the collision in terms of energy.
Force (F) is acceleration (a) times mass (m): F = m• a. Momentum (p) is mass
times velocity (v): p = m• v. Since acceleration measures change in velocity over time
(t) (put another way, acceleration is the derivative of velocity with respect to time),
force is the derivative of momentum with respect to time. Equivalently, force times
time equals change in momentum, or impulse (p): p=F• t. This is significant
because momentum is a conserved quantity. It can be neither created nor destroyed,
but is passed from one object (the hand) to another (the board). The reason for this
conservation is Newton’s third law of motion, which states that if an object exerts a
force on another object for a given time, the second object exerts a force equal in
magnitude but opposite in direction (force being a vector quantity) on the first object
for the same amount of time so the second object gains exactly the amount of
momentum the first object loses. Momentum is thus transferred. With p a fixed
quantity, F and t are necessarily inversely proportional. One can deliver a given
amount of momentum by transferring a large force for a short time or by transferring
small amounts of force continuously for a longer time.

Force, Momentum, and Deformation Energy

That large objects moving at high speeds hit harder than smaller objects moving
more slowly goes without saying. In attempting to break a board, a karateka seeks to
hit the board as hard as possible. It therefore follows that the karateka should move
his or her weapon (for the purpose of this paper, the hand) as quickly as possible in
order to hit as hard as possible. But what makes for a “hard” strike? Two ways exist to
answer this question, both equally accurate

THE PHYSICS OF KARATE STRIKES

Introduction

In recent years, the ancient eastern art of Karate-Do (a Japanese word, literally

translated as “the way of the empty hand”) has become popular in the western world.

Karateka—practitioners of Karate—often break boards, cinderblocks, and other solid

materials in order to demonstrate the strength that their training develops. Much can

be said of the history and culture associated with the expansion of martial training, but

this essay—it is, after all, a physics paper—will examine the collision mechanics of a

hand strike to a solid target like a board.

“client,” and flowing from a grab position into an instant lock. Either way,

you want to control the situation -- and it’s best to control it without hitting,

if you can.

If there is enough interest, I’ll publish more tips for bodyguards. Good

practicing!

Keith Pascal is the author of Wrist Locks: From Protecting Yourself

to Becoming an Expert. If you need some great locks for Exercise #3, or

just want to perfect your counters and reversals, read more about this

book at:

http://www.kerwinbenson.com/?bodyguard

(You can also find FREE reports on martial arts and self defense.)

This book is available in both hardback and e-Book formats.

I’d up the ante a bit. I am not sure I’d hit at this point, because of legal
ramifications. But I would lock.
Even if you aren’t confident with a lot of different locks, a grab is one
of the easiest ways to snap a lock on an aggressor. Since they are grabbing
you, they aren’t hitting you, or your client ... yet.
Lock them down, before they do.
So, for Exercise #3, you need two practice partners to help out. As
one grabs the other, you practice removing the threat with a lock.
Practice the difference between brushing the hand away early, in
preparation for a lock, and actually picking the attacker’s hand(s) off your

But, as a professional, you can’t allow somene to put their hands on

your client, right? You have to react. And you have to react in a professional

manner. Like an expert.

So, what do you do?

I suggest using wrist locks or the beginning of a wrist lock. As Person

X moves in on your client, you can gently guide their hand away from the

body. You guide in preparation for a wrist lock ... you don’t put one on.

After all, they didn’t actually grab. So, you are just deflecting them a

bit ... it just so happens that your hand is in the right position, just in case.

If the person is actually grabbing your client, or one of your client’s

possessions, then I’d consider a real wrist or joint lock. Maybe even an

arm bar.

The Next Level: The Grab

So what’s left? If we are covering all lines a bit more efficiently, then what
do we practice now?
My suggestion is to take this whole scenario one step further. You
have just practiced moving through crowds. You have martial arts expertise
to deal with anyone attacking from the front or side. You have practiced
an exercise to remind you of a rear attack.
So, now practice what you’d do if someone reached in towards your
client. You aren’t sure if this is a violent or non-violent reach. So, you can’t
react with your destructive techniques, yet.

Of course,

you could choose to practice a back kick to the knee or
shin — with either distance. The point is, you need to cover with a
rear line of defense.
Get someone to approach from behind with a focus mit. Practice
kicking the mit -- your partner holds the target in the appropriate postions
for kicking.
If your client is behind you, get comfortable kicking around your client
without offending or injuring your protectee.
Pause For Reflection
Do you see what we’re doing? You are teaching yourself to move professionally
through crowds. You practice moving forward smoothly.
Then with just a shift of focus, you can almost guard your sides. You
just have to preview what will soon be passing on the sides. Anything else,
you are trusting to awareness ... for now (I have another e-report in mind
as a future project).
And in this last exercise, you covered the rear line of attack. You are
now, covering most lines of possible attack.
Note: Peripherally, we see below us easily. Still check the surface
as you move. On the other hand, most humans are very lazy about
looking up when entering a new situation. Keep both in mind.

On the other hand, I have never had to protect a true celebrity, other
than my wife (warm fuzzy for the compliment), so I don’t know about fan
control.
If a ton of folks were coming from the front, and I had to protect on my
own (no other bodyguards), I might be very inclined to shove my client
behind me for protection. See what I mean?
Back to the kick ...
Or to phrase it differently ... for this exercise, you need a back kick.
And not just any back kick.
If your client will be positioned behind you, you’ll need the extension
of a full back kick — called by different names in different styles, but
basically a donkey kick that reaches behind. I try to make contact with my
heel, with my toes pointed toward the ground.
Then I thrust backward with good leg extension.
This is a powerful kick.
On the other hand, if I instead position myself behind my client, then I
need a shorter kick. I don’t need to reach past my client to the attacker. I
only need to lift my heel, by bending at the knee, so it pops into my
attacker’s groin.

Exercise #2

Guarding the Rear
In Exercise #2, you will practice a kick. Not just any kick. You have a
choice of two ... and which you choose depends where you position your
client during your move through the crowd.
I have heard arguments for always keeping your client in front of you
jogged to the side —so you could walk with your hand lightly on the clients
back.
And I have also heard the argument of keeping your charge behind
you, so you can protect the oncoming threat.
Generally, I’d favor the former. I can guide from behind — and since I

know that I am more aware than anyone I am with, I can react to threats in
front of us, when need be.

You are putting a final touch on this exercise.

If you consider in front of you to be twelve o’clock on a clock (analog,
not digital), then I want you to constantly scan at 11:00 and 1:00. This
widens your perception (peripheral vision) just enough that you see
things/people in advance of them passing by your side.
For me, 2:00 and 10:00 are too wide. I can’t see both sides at the
same time (pardon the pun). With a narrower scope, I can see both sides
at the same time, without shifting my head.
By adding this small touch, I can see everyone before they pass. It
cuts down on the amount of attention I have to give to protecting the sides.
It doesn’t eliminate it — it just “almost” kills two birds with one stone.
And there you have it. Practice moving someone through the crowds
for 20 minutes a day for a month, and you will see marked improvements.
for some, it will take longer to become proficient. You really want to get
this down pat.