come back to this idea of gravitational potential energy again and again in our chapter on footwork. By positioning your upper body in a certain way, you create more gravitational potential energy for yourself with accompanying footwork. In many cases you offset your weight just enough to help you move in a particular direction with more speed and less effort, all compliments of earth's gravitational pull.
Bruce Lee spent a lot of time in developing the JKD stance. Of course, he was incorporating strategic factors (e.g. narrow and closed stance, stability, etc.), but the stance also was designed to maximize potential energy, specifically for throwing the straight lead. As we'll soon discuss, the correct on-guard position, is positioning of your body to throw the most effective punch—for example, hip position for uncoiling of the body during rotation, foot position to maximize the push off and, thus, gravitational potential energy, and a slight lean forward to cheat inertia. All are examples of maximizing potential energy.
STRAIN ENERGY: THE SLINGSHOT EFFECT
The other type of potential energy important to understanding JKD is called strain energy. This is potential energy generated by the deformation of an object. Think of a slingshot or a rubber band. The further you stretch you it, the further you deform it, and the more capacity it has to do work. In discussions of the martial arts, we sometimes interchange the term strain energy with leverage. Strain energy is dependent on the degree of the object's deformation and the stiffness of the object, which may also be referred to as the spring constant of the material.
Strain energy is mathematically represented by the equation:
Where:
From the equation, we see that the greater the deformation, the greater the potential energy. In the Tao, Bruce Lee describes strain energy as it relates to throwing a ball:
"The arm is kept sofar behind that the chest musclespulling against it are tensed and stretched. The final wrist snap is postponed until the last instant before release or in striking, before contact. In football, the punter puts the last snap into his knee and foot as, or a shade after, he makes contact with the ball."10
In JKD, for example, strain energy is especially important for throwing hook punches. As we'll discuss in a future chapter, you never want to let your arm overtake your hip as you rotate into the punch. In a later chapter, we'll be referring to a "catch" you should feel on your shoulder as your hip momentarily rotates away from your arm. This creates tension, or strain, on the tendons of your chest and shoulder. You are stretching, or deforming, those tendons, so you can store more potential energy for the punch. The same goes for hook kicks. Your knee should never move ahead of your hip. Keeping the knee a hair behind the front hip increases the strain on the tendons of your leg at the hip. By keeping your leg stretched and rigid until the last minute, you'll be able to generate "snap" in your kicks.
This is also why flexibility is so important. The more you can stretch, or deform, the muscles and tendons, the more strain energy you can store. Of course, if the strain is too great, this can lead to pulled muscles, torn ligaments, and ruptured tendons. Again, flexibility will help minimize such injuries while increasing the potential energy and force of your kicks and punches.
ENERGY, WORK, AND MOMENTUM: GIVE A LITTLE TO GET A LITTLE
From our earlier explanation of impulse, remember that impulse is a change in momentum. In JKD, whenever we throw a punch or kick, we are looking to maximize the velocity and acceleration of our fist or foot. Recall the equation for impulse:
Ft = impulse
Also remember that this was derived from the following:
Force x time interval = mass x (final velocity — initial velocity)
It follows, then, that the greater the change in velocity, the greater the force production. How does this relate to work? Remember that work is the product of force and displacement. Greater displacement, or increased work, allows for a greater change in velocity. This is why the rear cross is often thought of as boxing's big gun. Your body has more room to rotate, and your fist has a greater distance to travel to the target. Your hand goes through a much larger displacement throwing a cross than it does throwing a jab. That larger displacement, or greater distance covered, means that more work is being done. It also means more force production.
In fighting applications, of course, there is always a compromise between power and speed. You don't want your hand to go through such a displacement that the moving target moves out of the way! You also don't want to wind up so your punches become telegraphic. But, depending on your body position, you can take advantage of doing more work to get more force. For example, coming out of a hook, you are naturally set up to throw a cross with greater room for hip rotation than usual. Or vice versa, after you've thrown a cross, you're set up to throw a hook (Figures 1.24-1.26). You've already rotated considerably clockwise, so you've got a lot of room from which to uncoil counterclockwise into the hook. Your hand travels a greater distance so you can pack more into your punch.11
NEWTON'S FIRST LAW OF MOTION: INERTIA
Before we progress any further, we must address the issue of inertia. As Newton's First Law of Motion states, an object continues in a state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.12 In the martial arts, then, this is especially important because the speed with which you initiate an action, either defensive or offensive, is crucial.
Obviously, this is where good form is so important. The less extraneous movement in your techniques, the faster you will be. This is why we always stress refinement of your skills. We are only born with so many fast twitch muscle fibers. But you can improve your speed by refining your technique. Even the smallest increments of refinement can pay off with exponentially increased speed.
Another way to overcome inertia is to manipulate the placement of your center of gravity. In our chapter on the straight lead, for instance, we'll discuss how a slight lean forward helps you, in effect, cheat inertia. By slightly offsetting your center of gravity towards the direction in which you want to move, you put gravity to work for you. You are essentially falling into a punch. This means more force production and less effort, a pretty good deal. The idea is very similar to a runner at the starting block. When a sprinter moves from the "on your mark" position to the "get set" position, he shifts his center of gravity up and to the furthest point forward within his base of support. This allows him to overcome inertia most efficiently. This is ideal for explosive movement in one direction, but not so great if you have to be ready to move in any direction. In the case of the straight lead, just prior to throwing it, you know you'll be moving linearly. Though not nearly to the degree that the runner does, you shift your line of gravity slightly forward. In his notes, Bruce wrote that this shifting of your center of gravity is to be used when you know you're in attack mode:
"For an attack, the center of gravity should imperceptibly be shifted to the front foot in order to allow the back
