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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