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 leg and foot freedom for the shortest, fastest and most explosive lunge."13
You'll see this idea of cheating inertia again and again. In the footwork chapter, we'll discuss certain moves that are aided by a slight shift in weight. For example, when reversing direction with a side step, if you lean slightly with the upper body in the reverse direction and then sidestep with both feet momentarily off the ground, you fall towards the desired direction (Figure 1.27). If you were moving in one direction, left for instance, remember this is a form of linear inertia. To move right, then, you've got to impose a force. You won't have to work so hard to impose that force if you let gravity do a lot of the work for you.
The same principle applies to defensive moves like bobbing and weaving. When you weave, a slight lean at the waist sets the majority of your body weight into motion. Again, gravity will be pulling on you and helping you along. In the section on kicking, you'll see that a slight lean forward and the placement of your body weight in the front foot will help you generate the torque needed to get your leg up quickly to kick.
The law of inertia applies to both linear and rotational motion. In both cases, mass is directly proportional to the degree of an object's inertia. The more massive an object, the more force is required to move it. Think of heavyweights versus featherweights and notice how much faster the smaller fighters are. It takes a lot more energy and force to move around 200 pounds as opposed to 126 pounds. This explains how smaller fighters are still able to generate tremendous power. They are capable of accelerating their body weight better than larger fighters. Remember that force increases with acceleration. For every fighter there is an optimal balance between weight, force, and speed. Greater weight, or mass, may mean more force, but at some point, too much will compromise speed, and consequently, force. Too little weight may mean not enough force behind punches or enough muscle to move body parts fast enough to accelerate adequately.
In rotational, or angular motion, there is another factor besides mass that impacts inertia. With angular motion, an object rotates about an axis. In JKD, our angular punches include hooks and uppercuts. The most important thing to remember about rotary inertia is that the greater the distance between the rotating object and its axis of rotation, the greater the rotational inertia. This is also referred to as the radial distribution of mass14 (Figures 1.28—1.29).
To illustrate this, think of a figure skater going into a spin. To increase his spin rate, he draws in his arms and legs, decreasing rotary inertia, or resistance. This increases angular velocity. To slow down and come out of the spin, he spreads his arms and legs out, increasing rotary inertia. His body mass is distributed farther away from the axis of rotation. There are notes in the Tao that address this principle in relation to other sports:
"After momentum in a throwing or elliptical striking movement has been generated by a long radius and a long arc in the swingg, the speed may be increased without applying additional force by suddenly shortening the radius of the arc. This effect is seen in the 'pull-in' at the last of the arc in the hammer throw, in the backward thrust against the forward leg by the batter in baseball, and so on. Snapping a towel or a whip are common examples of the same 'shortened lever' principle."15
The same rule applies to fighters throwing hook punches. The tighter your hook, the closer your arm is to your body, the faster you can turn into the punch. And since hooks are usually for close quarters work, you want to make yourself small anyway. Keeping your hooks tight makes you more elusive, faster, and more powerful. This is all related to the radial distribution of mass. And once again, developing your punches so as to minimize motion and maximize efficiency is a matter of refining proper technique.