for clarity of explanation, they will be treated separately.
a. Physical Principles
Needless to say, the human body follows the physical laws of nature. For example, it is attracted to the ground with a force which is directly proportional to its weight; this is the gravity force. When you move, you generate a momentum which is the product (Mx V) of your weight (M) and velocity (V). If you hit the ground or a wall with a force F, it creates a reaction force R which is directly proportional to F and in the opposite direction. In general, for any action there is a corresponding reaction of proportional intensity. Also, the impact generated by a mass (or weight) M moving with a velocity (or speed) V is proportional to the square of the speed (kinetic energy = 1/2M x V2). Finally, the last important force brought into play in karatedo is the muscular force resulting from the contraction and/ or extension of some of the 400 muscles of the body. Certain muscles are very powerful but move slowly while others, less powerful, move much faster. One of the characteristics of muscular force is that it is cumulative. In other words, the effects can be added, two identical muscles producing twice as much force as one.
How can one apply these physical principles in karatedo?
Use of body dynamics
First, good balance is needed. Because of the law of reaction, a punch creates reaction in the opposite direction. Therefore, it would be completely ineffective if the body were to move back on impact. Thus gravity force is used to nullify the effect of the reaction force. The stability of the body is determined by three major factors: the weight, the base, and the position of the center of gravity. The weight is a constant factor: the base is determined by the position of the feet on the ground; and the center of gravity is located approximately one inch below the navel. As the surface of the base is increased, the mobility of the body decreases, the best compromise being when the feet are shoulder width apart. It can also be easily demonstrated that maximum stability is achieved when the center of gravity is located vertically over the center of the base. In this position, the weight is equally spread on both legs. The knees must be bent to insure a correct dispersion of weight on each foot and to be able to use the spring of the legs to move faster. The body should be kept erect so that its center of gravity continues to fall over the center of the base.
From this basic position of stability, the motion of the body can now be fully utilized. There are three kinds of motion possible: linear motion, which is a translation of the center of gravity; circular motion, which is a rotation around the center of gravity; and pendular motion, which is a swing of the center of gravity in a vertical plane. As the center of gravity is located between the hips, and as the hips and abdominal region account for about one-third of the weight of the body, it is clear that a considerable momentum can be generated by either moving the hips forward, rotating them, or swinging them. Thus, every action in karatedo must start from the hips. Another reason for this is that muscles of the waist and abdominal region are very powerful but because of their configuration can only be slowly brought into action and must therefore be contracted at the earliest stage of the movement.
A linear momentum is generated when the body is propelled forward by the rear leg. At first the hips are moved backward to shift the weight of the body on the rear leg. The spring of the leg (muscular action) and the reaction force created by the ground are then transmitted to the hips resulting in a forward motion. The center of gravity describes, in fact, a slightly curved trajectory because of the vertical component of the impulse force.
A circular momentum is generated when the body rotates around a vertical axis going through the center of gravity. The spring of the rear leg, the reaction force, and the force of the abdominal region are used to create a rotation of the hips, shoulders, and head in horizontal planes. The circular momentum is increased when the body is first twisted in the opposite direction, in a winding up motion, and by the antagonistic action of the hands (push-pull type of motion). These principles are used primarily in circular techniques.
A pendular momentum is generated when the hips swing backward and forward in a vertical plane going through the center of gravity. The pendular motion of the hips is used mainly in direct kicking techniques.
Concentration of force
The importance of the hips and abdominal region cannot be overemphasized. Because muscular force is of a cumulative nature, as many muscles as possible must be used in the motion (obviously these muscles have to be convergent and not antagonistic). Because of the reaction law, the long and flexible muscles of the extremities have to rest on the stronger and more stable muscles of the body to produce an effective result. Of all the profound muscles, the waist muscles are the strongest and therefore they must be contracted first to support the next closest muscles which in turn support adjacent long muscles and so on to the extremities of the body used for punching or kicking. The movement has to be executed fast to generate a high momentum. For the arm or foot to move quickly, muscles have to be strongly contracted. But if the muscular tension is kept throughout the motion, it will slow it down. Therefore the initial contraction of a muscle should be followed by a decontraction while the fist travels. At the time of impact, the muscles have to be tightened again to straighten the joints of the arm, insuring the maximum transmission of force. Otherwise, the reaction force will cancel out the force of the elbow. These periods of intense contraction and decontraction have to be well coordinated and it is only through intensive practice that one can develop a feeling for the right time at which the contraction or decontraction of the muscles should take place.
The relation between the force and the momentum is given by the equation: Momentum=M x V=FxT, T being the time during which the force F is applied to produce the momentum M x V. In the execution of a punch, M would be the mass of the fist; if M and V are constant, it can be seen from this equation that the force will increase when T decreases .
Thus the shorter the time, the greater the force generated. As a result, the force should be concentrated in as little time as possible. Therefore, speed being directly proportional to force, it is conceivable that a person of smaller physical structure (who is generally capable of greater speed than his counterpart of a larger structure) is able to create enough force to match and possibly supercede the force of a much larger man. Thus, it is said of a karate practitioner that size has no bearing on capability in the fighting art.
In modern karatedo, not only are the hips used in linear, circular, or pendular motions, but also the shoulders and head. This horizontal counterbalancing of the head and shoulders, added to (A) the horizontal pendular motions, (B) horizontal and vertical motions of the hips, and (C) circular motions of the feet, increases the momentum of the back counterbalance motion of the elbow. This supports and enhances the executions of the hand technique. This support counterbalance theory is particularly of importance in the Kenkokan system, where the motion is not stopped at the time of impact, but, in fact, followed through. With this, the acceleration of the striking mass is increased and thus creates a greater force M X A, A being the acceleration). Other reasons for using follow-through motions will be given in the next paragraph.
The reaction force
The reaction force is that which results from the application of a force to a specific point and directly opposes the initial impact. If you kick on the ground with your foot, using your muscular force and gravity, it creates an upward force in your foot and body which can be reutilized in another direction. It is this reaction force which is used by high jumpers when they stamp on the ground before jumping. It is used in karatedo in much the same way. The ground is kicked just before punching or kicking and the reaction force is transmitted through the body to the fist or the foot, adding a momentum proportional to the force of the kick on the ground [when there is no loss of momentum. (If the ground is soft it may' absorb part of the energy generated by the kick.)]
While the reaction force can be used positively, it can also have detrimental effects. When hitting a target, a reaction force results, which may lessen the force
