Читать книгу Jeet Kune Do - Teri Tom - Страница 11

This isn't a biomechanics textbook, and it's beyond the scope of this volume to go into the finer details of that science, but an understanding of some basic principles will give you a much better understanding of Jeet Kune Do. On so many fronts, Bruce Lee was light years ahead of his time, and this arena is no exception. Breaking down professional sports to a science may be commonplace these days, but in Lee's time, that kind of analysis was only just beginning. While he may have surveyed many other arts, what he chose to incorporate into his own repertoire was very specific and considered, resulting in an art essentially limited to 4 or 5 punches, 3 or 4 kicks, and a few grappling techniques.

We'll touch on some of the strategic elements later on, but first, we must learn to perform them correctly. And what will help you refine these techniques to perfection is a basic understanding of why Bruce Lee chose to do things a certain way. A lot of people neglect this aspect of the art and move on to application. But failing to build a strong technical foundation is like trying to drive without learning how to put the key in the ignition. It makes no sense, and you'll end up going nowhere. Sloppy technique makes for sloppy application.

It may seem like splitting hairs to break these tools down to a science, but in any sport, knowing how to refine your skills will give you a competitive edge that could make all the difference. For sprinters, a millisecond means victory or defeat. The martial arts should be no different. Honing your skills will result in more speed, more power, and more successful application of those skills. While it's true the number of fast twitch muscle fibers you have is genetically predetermined, you can level the playing field somewhat and do the best with what you've got by refining your technique.

Also if you're in this for the long haul, knowing the basics of biomechanics can save your body a lot of wear and tear. Proper technique and knowledge of what makes those techniques sound can increase your longevity. I always look to Ted Wong as a great example of this. He's honed these skills for over thirty years. As of this writing, he's 70 years old and while almost all of his contemporaries have fallen by the wayside or declined markedly in their performance, he still spars with blokes less than half his age and complains of few aches and pains. He'll tell you that when he first started, before he'd refined his technique, how his body hurt so much he seriously contemplated quitting the martial arts altogether. He also hits a lot harder than he did in the 1960's. If you compare footage of him then and now, you'll see that he can attribute this longevity to analysis and improvement of his skills over the years.

So how do we go about refining technique? The first step is to know why a specific technique is performed the way it is. In this book, we are going to show you why a Bruce Lee punch or kick was so effective. True, the man was born with some serious fast-twitch muscle fibers. But he was also relentless in the refinement of his technique.

Before we can fully describe each technique, though, we need to know some basic principles of biomechanics. We'll briefly explain some of these laws and then we'll see how they apply to all JKD techniques. Later, as we discuss the specifics of each technique, you may want to refer back to this section.

BIOMECHANICS AND FORCE

The term biomechanics basically is the science of forces and how they affect humans. A force is either a push or a pull that can act externally on an object in the environment (i.e. throwing a punch on a heavy bag is a pushing force on the bag) or internally within a system or object (i.e. muscles create pulling forces around joints causing movement of your limbs).

TYPES OF MOTION

Motion or movement may be defined as a change in position. There are three kinds of motion. Linear motion is defined as having all points on a body or object move the same distance in the same direction and at the same time. Simply put for our purposes, this is movement in a straight line. For example, jumping up and down is linear. The trajectory of a straight punch is linear. On the other hand, the trajectory of a hook is circular (Figures 1.1-1.3). This is called angular motion and occurs when all points on a body or object move about the same axis.¹ Finally, we have general motion, which is a combination of both linear and angular motion. Most human movement falls under this category, and in fact, all of our JKD techniques combine elements of both. All JKD kicks and punches include linear vertical, linear horizontal, and angular components.

FORCE PRODUCTION

In layman's terms, velocity and speed are interchangeable. It is important to note, however, that speed is merely distance traveled over time. Velocity is a measure of both speed and direction and is represented in physics studies as a vector quantity. Vector quantities are drawn as arrows and show you two things—how fast and in what direction. The length of the arrow represents speed—for example, 1 centimeter might represent 20 km/hr. And direction of the arrow represents direction of the object. If an object is moving in two directions, you can draw a triangle and find the sum of the two vectors by solving for what we call the resultant using the parallelogram rule.

Velocity is important because a change in direction and/or speed is called acceleration and in force production, acceleration is the name of the game. In most sports, success depends on generating the most acceleration before applying force to another object. In tennis, the object of service technique is to generate maximum acceleration of the racquet head at impact. In the baseball or golf swing, again, the object is to maximize acceleration of the bat or club when you strike the ball. The same goes for throwing punches. The object is not only to move your fist or foot at a high velocity, but to have that velocity maximally increase at the point of impact. This is known as Newton's Second Law of Motion which states that the acceleration produced by a net force on a body is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the body.² In mathematical terms, it is expressed as:

Force = mass x acceleration

From the equation, we see that force production increases with acceleration. We also see that an increase in mass of the object that is accelerating also increases force. This is central to all the techniques that comprise JKD, which I affectionately call "the art of how to best throw your weight around." For every punch and every kick, you should be asking yourself how you can get as much of your body weight into the technique without compromising balance and mobility.

In the following chapters, we'll explain what Bruce Lee believed to be the best ways to maximize acceleration and body weight into each technique. We'll explain how a 135-pound Bruce Lee could generate such incredible power. Bruce himself wrote in the Tao:

"The principle is to preserve the maximum acceleration up to the last instant of contact. Regardless of distance, the final phase of a movement should be the fastest."³

IMPULSE AND SNAPPINESS

Related to the equation for Newton's Second Law is an equation that accounts for a change in velocity—and in the case of throwing punches, a change in direction. Momentum is defined as the following equation:

Momentum = mass x velocity

To change an object's momentum, the velocity must change as well. When you throw a punch, the punch does not continue forever in the same direction unless you're Mr. Fantastic. No, you have to retract your hand at some point. This requires a change in direction. For straight punches, this means throwing the punch straight out, hitting the target, and then bringing the hand back to the on-guard position. We've already established that force is a product of mass and acceleration. But there's another variable here—how long do we apply force to an object to maximize force production?

Let's use the equation for Newton's Second Law and for acceleration, we'll use the average acceleration. This is the initial velocity (v_j) (which in the case of our punch is when the hand starts moving towards the target) minus the final velocity (v_f) (the velocity at impact) divided by the time duration of force application:

Remember Newton's Second Law is expressed as:

Force = mass x acceleration

If we substitute average acceleration into the equation, we have:

Multiply both sides of the equation by the time interval:

Force x time interval = mass x (final velocity — initial velocity)

This product of force and the time interval is what we call impulse and is expressed as:

Impulse = Ft

What does this have to do with JKD? In Bruce Lee's words, this is what we'd call the equation for "snappiness." You'll see quite a few references to snappiness in Lee's writings, as he was distinguishing between a forceful punch and a push. If your hand stays in contact with the target too long, it becomes a mere push.

This is why: from the equation, you can see that force is inversely related to time. The more time spent during force application, the less force is required to cause a change in momentum. When you throw a punch, at some point, you are going to have to stop and retract the hand. When you stop your fist, this is a change in momentum. The time that it takes for you to change that momentum is inversely related to the amount of force required to make that change.

A good example would be that of the shoulder roll versus running straight into a punch. To decrease the amount of force coming your way, you roll with a punch. You move in the same direction with the punch to increase the amount of time you're in contact with it. This lessens the impact. On the other hand, say you run into a punch. It takes a lot less time, and therefore, a lot more force, to stop you in your tracks. I wouldn't recommend trying this one out.

Another example would be the difference between training gloves and fight gloves. Training gloves have a lot more padding. This lessens the impact of punches because it takes more time to for your fist to connect with its target. Fight gloves, have less padding and therefore require less time to change the momentum of your fist, and less impact time means a greater force is required to change that momentum.

In general, Bruce favored a punching depth of about 2—4 inches past the target. This was just enough to penetrate the target without devolving into a push:

"All punches should end with a snap several inches behind the target. Thus, you punch through the opponent yet end the punch with a snap." ⁴

This goes for all punches and not just straight ones. Even with angular punches like hooks and uppercuts, you move straight through the target for only a few inches before leaving the target with a tearing motion as your hand continues to travel along its angular path.

PROJECTILE MOTION

In discussions of martial arts technique, you hear a lot about torque, force, mass, acceleration, and stability. But you never hear about projectile motion. Yet this is a concept central to most JKD techniques, and it has to do with footwork. A projectile is any object that has been thrown or dropped into the air and once in the air, the only force acting on it, barring significant air resistance, is gravity. A lot of the time in JKD, the projectile is you! Every time you push step or push off, you are momentarily—even if it's only for a millisecond—in the air. Your toes might still be barely touching the ground, but the majority of your body weight is airborne. Every time you throw a straight punch, and almost any time you throw any punch, your body itself becomes a projectile giving you more force production by allowing you to throw your body weight into it.

So let's look a little closer at projectile motion. Once you've thrown an object—in this case, your body—into the air, that's it, you cannot change directions in midair. The only force acting on you at this point is gravity, which we know to have an acceleration of 9.81 m/s/s downward or —9.81 m/s/s.

I won't bore you with the derivation of the equations for projectile motion, but there is an excellent explanation of it in McGinnis' Biomechanics of Sport and Exercise for those of you who are interested.⁵

For our purposes, just knowing what the equation is for vertical velocity of a projectile should be enough:

Where:

Now, if you look at this equation carefully, it should look familiar. Remember from algebra class:

y = mx + b

It's that trusty parabolic equation where —9.8 m/s/s is the slope of the line. Again, we won't go into all the mathematical details here. Just know that whenever you push off or launch yourself into the air, even if it's just for a fraction of a second, your body is following a parabolic pathway. And at any point on that path, you have both a vertical and horizontal velocity. The horizontal displacements for each time interval, by the way, are equal, creating that symmetrical parabolic path.

What does all of this have to do with JKD? Well, first we mentioned footwork. There are three things that determine what kind of parabola we have: time spent airborne, peak height, and horizontal distance covered (also known as horizontal displacement). In his discussions of footwork, Bruce stresses the importance of small steps. The reason? You'll be able to shift direction much faster. Yes, there's even an equation for this:

Horizontal displacement = initial horizontal velocity x flight time

So you can see the bigger your step, the longer time you spend in the air. And remember once you've launched yourself into the air, you are at the mercy of gravity. You cannot change your direction until you come back down. So the less push off you give yourself, the less time you'll spend in the air, and the less distance you'll cover—small steps. There will be times, of course, when the situation will call for you to cover greater distance with your footwork, but in general, keep those steps small and controlled.

Projectile motion is not only used to explain shiftiness, though. It's a law that is central to punching power. As you'll see in our discussion of the punches, when you push off with the back leg, you always want to hit the target before your front foot hits the ground. The reason is easily explained by projectile motion. Remember, force is a product of mass and acceleration. Acceleration is a change in velocity. At any point on the projectile parabola, you have both a vertical and horizontal velocity, and you are accelerating towards the ground. If you wait until you stop and hit the ground, you will no longer be accelerating towards the target. You no longer have a velocity in the direction of the target and you've missed out on using all the body weight that gravity was pulling on. How are you going to produce force for that punch now?

If you do hit the target before you and your front foot land, you take advantage of throwing all your body weight into the punch. You have both horizontal velocity and gravity on your side. You're accelerating, baby.

With footwork, with or without an accompanying punch, it's best to minimize your time in flight so that you will stay close to the ground and mobile. So how high should you push off? It's been found in studies with shot-putters that the resultant velocity increases—and, thus, acceleration and force increase—when the shot is released at an angle of less than 45°.⁶ To maximize horizontal velocity and minimize "hang time," then, the same goes for you when you "release" yourself as a projectile from the ground. This has to do with air resistance. The higher you launch yourself, the more you are actually held up, to some degree, by air resistance. This is exactly what we don't want. What we're aiming for is to cover as much ground as possible with the least amount of time in the air. You'll have to experiment with this on your own to find what is most efficient for you.

WORK AND POWER

From our discussion of impulse, you know that the impulse is the product of force and time. We can also measure another variable—the distance an object moves when a force is applied to it. The product of this distance and the force is known as work and the equation is:

Work = force x distance

Whenever you step and slide and cover a certain amount of ground, work is being done. Whenever you throw a kick, your muscles contract, producing forces that pull on your tendons and bones. This causes your leg to move through space. Your leg covers a specific distance. That's work.

Crucial to the biomechanics of the martial arts is the rate at which you can do work. We call this power. In layman's terms, we often interchange this with the word force and we will do so in this book, but in "biomechanicspeak," they are two very different things. The equation for power is as follows:

Power, as you can imagine, is very important to us. It doesn't really help us to throw a kick if it's so slow it never reaches the target. Just think of Bruce Lee and how fast he moved his limbs through space. He was power personified.

KINETIC ENERGY

In the realm of biomechanics, energy is defined as the capacity to do work. As you may recall from high school physics, mechanical energy comes in two forms: 1) kinetic energy, which is energy of motion and 2) potential energy, which is energy due to position.

When an object moves, its motion gives it an ability to do work. The movement gives it kinetic energy. If you hit a heavy bag, your moving fist has kinetic energy, the ability to displace the bag. Kinetic energy is determined by an object's mass and velocity. The mathematical equation is:

Kinetic energy = ½ (mass x velocity²)

This equation makes measuring kinetic energy much easier than measuring force, as we often know the mass and velocity of objects. Measuring acceleration is not always so easy.⁷

In the case of hitting the heavy bag, the equation makes sense. The faster you hit the bag, the more capacity you have for moving it. And the more body weight—or mass—you put behind your punch, the more you'll displace it.

POTENTIAL ENERGY: THE ENERGY OF GOOD FORM

In scientific terms, potential energy is often defined as energy of position. How appropriate! Throughout Bruce Lee's writings, you'll see reference after reference to good form, alignment, position. In the following chapters, we'll be spending a lot of time describing the proper stance, and the mechanics of each technique. Some of this is strategic, of course, but the underlying principle is that we are trying to create the most potential energy without sacrificing efficiency or safety (i.e. mobility, stability, etc.)

There are two types of potential energy that we'll be discussing repeatedly. The first is gravitational potential energy. Elevating objects against gravity requires work. So once an object is elevated, it has additional potential energy. Just as we explained in the case of projectile motion, we want to use gravity to our advantage as often as we can. The equation for this is:

Gravitational potential energy = mass x gravitational acceleration x height

So, in our example of the straight lead and projectile motion, when you push off, you temporarily elevate yourself above the ground. In that airborne position, you have more gravitational potential energy to direct in your punch. This is essentially what Jack Dempsey described as the "falling step." By "falling," you allow gravity to take you downward and into the punch. In his book, Championship Fighting, Dempsey uses the analogy of a sled to explain gravitational potential energy:

"In a sense, the boy and his sled are falling objects. But the slope of the hill prevents them from falling straight down. Their fall is deflected to the angle of the hill. The direction of their weight-in-motion is on a slant. And when they reach the level plain at the bottom of the hill, they will continue to slide for a while. However, the direction of their slide on the plain—the direction of their weight-in-motion—will be straight out, at a right angle to the straight-down pull of gravity."⁸

We'll 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 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.

TORQUE

Since we've just been discussing rotary inertia, now would be a good time to introduce the subject of torque. Torque is a force that is specifically rotational and results in a turning effect. Mathematically, it is represented by the equation:

Torque = lever arm x force

Just as the velocity of a rotating object is dependent on a distance variable, torque depends on the distance from the line of force to the axis of rotation. This distance is called the lever arm. It is also referred to as the moment arm or the perpendicular distance. A good example of this would be a door swinging on its hinges. If you exert a force on the door by pushing it on the side of the door close to the hinges—the axis of rotation—notice how hard you have to push to open the door. If, on the other hand, you push the door at the point furthest from the hinges—where door knobs and door handles are always placed—notice how relatively easy it is to open the door. In accordance with our equation, you can produce the same amount of torque with a large force and a small perpendicular distance, or a small amount of force and a greater perpendicular distance.

The angular force, or torque, produced by a hook punch is very similar. In any sport, torque, particularly hip rotation, is an elemental component of technique. Think of the golf swing, the tennis forehand, baseball's tabletop swing, and the football pass. All involve some kind of hip rotation, which is initiated by force produced by the body.

In our discussion of rotary inertia, recall that there is a trade-off between mass, velocity, and force production. Similarly, with torque there is a trade-off between the lever arm, force, and angular force. When throwing a hook, then, the tighter the hook, the more force must be applied to generate a certain amount of torque. When throwing a loose hook, where the arm is more extended, you can potentially throw a punch with just as much angular force and less effort. For strategic purposes, though, even though it requires a tremendous amount of energy, it is usually more advantageous to throw tight hooks for reasons we've already outlined—speed, explosiveness, and evasion. However, there will be times, when a loose hook, with its increased lever arm, increased torque, and whip-like action, will be an effective choice.¹⁶ Torque is not just important to the angular punches like hooks and uppercuts. Hip rotation is crucial to all punches—including straight ones—and all kicks, as we'll see in upcoming chapters. The increased perpendicular distance in kicking is one of the reasons why kicks can generate so much more power than punches. The distance from your hips and the axis of rotation to your foot is much greater than the distance between the axis and your hand. However, most people move their upper body limbs much faster than their lower limbs because the legs carry so much more mass. It's the old mass-versus-speed balancing act.

In addition to the torque generated by hip rotation, there is also a kind of torque that is very important to kicking. It is generated by what we call a force couple, which consists of two forces acting in opposite directions. To illustrate a force couple, think of a book lying flat on a tabletop. If you were to push the book to the right at the lower left corner and simultaneously push it to the left at the top right corner, the book would spin counterclockwise. The two forces are moving in opposite directions and are noncolinear. (If they were colinear, you would be pushing at both lower corners in opposite directions and the book wouldn't move).

What do force couples have to do with kicking? Well, to get your front leg off the ground quickly you'll actually need to generate a bit of torque. We'll get into more detail in the kicking chapter, but what you're essentially doing is pushing slightly upward with the back foot and pulling by first digging into the ground with the front foot. At the same time, you are redirecting your center of gravity from a forward more upright position to your back leg and downward. You are doing the same thing the book does on the tabletop. Your limbs are rotating about your center of gravity as you shift from having the weight in the front foot to placing it in the back. The push-pull action helps you generate the torque that makes it possible to get your leg up into kicking position (Figures 1.30—1.34)

BALANCE AND STABILITY

As we've seen throughout Bruce Lee's writings, balance is a key component of JKD and one of what Bruce termed its "underlying ingredients."¹⁷ An object is said to be balanced or in stable equilibrium if its line of gravity falls within its base of support.¹⁸ Stability refers to the degree to which an athlete can resist having his balance disturbed. In any fighting situation, balance and stability are important for so many reasons. If you are unbalanced in any way, it is difficult to be in a position to either attack or evade. Without stability, it is impossible to generate adequate force in punches and kicks, and it's a lot easier to be knocked down. Before we further explore these definitions, let's define a few other terms first.

We keep referring to center of gravity throughout this chapter, so let's define it. The center of gravity of an object is that point on an object around which its weight is evenly distributed. We can think of this as that area of the body where most of our mass is concentrated. For our purposes, this is almost always at some location at the core, or trunk, of the body—basically, anywhere on the body that is not a limb. Our limbs, however, carry quite a bit of weight, and when they shift, so does our center of gravity. For example, if you raise your hands above your head, your center of gravity, while still located at some location at your core, shifts up. When you move your right arm out to your side, your center of gravity shifts to the right. When you weave to the left, your center of gravity moves slightly to the left. When you duck into a crouch and bend your knees, you lower your center of gravity (Figure 1.35).

The next term we need to define is the base of support. In sports biomechanics, this is the area on the ground defined by the athlete's point of contact. In our case, this would be the area determined by our foot position.¹⁹ If you were to draw a line from an object's center of gravity straight down to the ground, that line should fall within the base of support. We say the object is balanced. This is what we call the line of gravity. If, however, the line falls outside the base of support, we say the object is unbalanced.

This is a fundamental element of all JKD techniques. One of the most common mistakes among students just learning to throw the straight lead is that they allow their center of gravity to overtake their base of support. They think that because it is a linear punch, they must throw their weight forward. This is partially true, but as we'll see in a subsequent chapter, this has more to do with hurtling your entire body weight forward via projectile motion. To maintain balance, though, you can never let your trunk overtake your feet. A good way to test this is to stop yourself after throwing a punch—it works for the cross, too—and look down at the floor. If you see that you're overlooking your knee, you're okay. But if you find yourself looking at a point on the floor that is in front of your knee, you're in trouble.

The same is also true for defensive moves like the bob and weave. A lot of beginners start out by weaving too far to either side. Their trunks sway outside the base of support making the move awkward and unbalanced. You can use the same test for this. Weave to the left and stop. Look down. Are you looking straight down at your knee? If so, you're okay. If you're looking at a point to the left of your left knee, then you're unbalanced. Take it down a notch and minimize your movement.

All JKD techniques require some transfer of weight from one point within the base of support to another. In throwing a hook punch, for example, we often start with more of our weight in the front foot, at the front of our support base, and then shift that weight to the back foot, at the back of the support base, creating a pulling action. You can generate a lot of force while keeping the line of gravity within the base of support.

The real challenge of balance in JKD, though, is maintaining balance and stability over a constantly shifting base of support. From Commentaries on the Martial Way.

"Movingproperly means carrying out the necessary movement without loss of balance. Until balance is regained, the boxer is ineffective in both attack and defense. Therefore in all movement, balance must be retained."²⁰

Related to the idea of balance is the property of stability. As we mentioned earlier, this is the degree of resistance required to disturb one's balance. There are three variables that affect stability: the height of the object's center of gravity, the size of the base of support, and the object's weight. And, yes, there is an equation to represent this:

Toppling force x moment arm of toppling force = object's weight x moment arm of object

Technically, it's not the width of the base of support that determines stability. More accurately, it's the horizontal distance between the line of gravity and the edge of the base of support in the direction of the toppling force that determines stability.²¹ This is the moment arm of the object. The "toppling force" is the force required to unbalance the object. The moment arm of the toppling force is dependent on the object's center of gravity. The higher the object's center of gravity, the longer the moment arm is for toppling the object. Remember, a longer moment arm requires less force to produce torque. Therefore, the lower the object's center of gravity, the shorter the moment arm of the toppling force, and a greater force is required to unbalance the object.

This is a pretty technical explanation of stability. What's important to know is that lowering your center of gravity and widening the base of support usually result in greater stability.

So we've already established that widening the base of support, or in our case, widening the JKD stance, increases stability. But remember Bruce's quote about "movement without loss of balance." Movement. In JKD, your base of support is constantly shifting. As we'll see in the next chapter, the JKD stance is designed to strike a balance between stability and mobility. The wider your stance the more stable and less mobile you'll be. A narrower stance means less stability but more mobility. There is always a trade-off between the two.

In certain situations, you'll want to lower your center of gravity and widen your base of support. In close quarters, for example, when you duck, you widen your stance and lower yourself to the ground. Not only does this get you out of harm's way, but you've also hunkered down so that follow up blows are less likely to knock you over. Similarly, if a grappler shoots in to throw you, you'll go into a wrestler's crouch. Again, this widens your stance and lowers your center of gravity. In this position it will take a lot more force to topple you.

Widening the base of support, though, is not just for defensive maneuvers. As we'll see in the next section, whenever you apply a force, an equal and opposite force comes back at you. The more stable your stance, the less likely you'll be thrown off balance by the opposing reaction force.²² Examples of widening the base of support to apply force can be found in almost all sports. When a pitcher throws a ball, he takes that giant step after the wind up just before releasing the ball. If he didn't, he wouldn't be able to apply as much force to the ball without being thrown off balance. Try hitting a heavy bag while standing on one foot. That's a pretty narrow base of support. You're likely to be knocked off balance. Now stand in a regular stance and hit the bag. You've widened the base of support and can absorb the opposing force of the bag while maintaining stability.

NEWTON'S THIRD LAW OF MOTION: MINIMIZING WEAR AND TEAR

In our analysis of stability, we've just introduced the concept of action and reaction. This is more formally known as Newton's Third Law of Motion, which states:

"To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal and directed to contrary parts."²³

Going back to our heavy bag example, remember how a stable stance keeps us balanced after we've hit it. When we apply force to the bag, it sends an equal and opposite force right back at us. If our stance lacks stability, we'll be knocked off balance. Think of the recoil of a rifle. The force the rifle exerts on the bullet causes an equal force that acts on the rifle, causing it to kick. The rifle, however, does not move with the same acceleration over the same distance as the bullet, though, because it is so much more massive than the bullet.

In our heavy bag example, then, notice the jarring effect you feel when you hit it. That's the same amount of force that you applied to the bag coming at you. In JKD, we're often in the business of hitting things. Over days, weeks, years, decades, that's a lot of hitting! And if you're connecting, that's a lot of force coming back at you—every time you make contact with a target.

Because of all that pounding, you'll want to construct a stance and develop techniques that help you absorb that reactionary force with the least wear and tear on your body. If you plan to stay in the martial arts for any real length of time, it's in your best interest to adopt practices that minimize the physical stress you'll incur.

This leads us right into the next chapter, where balance, stability, mobility, force production, potential energy, and martial arts longevity all begin—the JKD stance.