Dem Bones, Dem Bones Gonna ...
Introduction to Torque
Get a move on, Charley!
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We contract our muscles by sending messages to them in the form of electrical impulses via our nerves. Like telephone messages to our neighbors, the impulses travel along communication lines to fibers in our muscles.
When muscle contracts, it tends to pull its ends toward its center -- it pulls in on itself. But the pulling doesn't dictate what movement will occur or even that it will happen. The most you can say is that a muscle tends to do what it could do. It doesn't always fulfill its tendencies, because opposing tendencies may be at work to prevent it.
Because of the contraction, bones attached to the ends of the muscle tend to be pulled toward each other, so the bones, and their attached mass, tend to rotate. That is, forces are applied through the muscles, and the weight of the attached mass (or load) swings about the connecting points, the joints of the body. The skeletal muscles are the machines that produce the action of the body.
Contraction of each muscle produces a force, which, applied to the attached bones, results in torque. The action tends to change the angular relationship among the bones. Torque is force times distance, where the distance is understood to be the length of a lever arm from its point of rotation (fulcrum) to the point of application of the force. The action of the force is presumed to be at right angles to the lever arm, so the force tends to rotate the arm around its fulcrum. If F is a force at right angles to a weightless moment arm at a distance, d, from the fulcrum, then the torque, T, is the product
T = Fd.
The amount of the applied torque depends on the length of the lever arm d and the applied force F. A torque advantage is gained either by increasing the force, or by increasing the length of the lever, or both.
Several muscles may be attached to any of the bones, and torque can be applied to the bones individually by each muscle, each tending to produce a different rotation. For example, one muscle (the tensor fasciae latae) that attaches to the lower leg tends to swing it out to the side, while another (the quadriceps) tends to extend it, and a third (the hamstrings) tends to bend it. The resulting action on the leg, called the resultant, is the sum of the torques produced by the separate muscles.
Though the average person may not understand the lever, even a child can experience the feeling. We apply torque one way or another dozens of times per day. For example, we use torque to open a door or turn the lid on a jar. We apply torque when using a screwdriver or a crowbar, when we pry open a can of paint, lift an object, turn the lid of a peanut butter jar, or perform many other everyday tasks. We also apply torque when we roll on the floor, spin on our toes, or perform somersaults, not to mention when we walk and talk.
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We apply torque every time we move. There are three ways to apply the torque, and they define three classes of levers, identified as first, second, and third class levers, differentiated according to the alignment of the elements making up the lever, namely:
1. The force you apply (or the effort you make).
2. An opposing force such as a weight, which is called the load.
3. The pivot point, or fulcrum of the action.
You can record yoyr mileage with a pedometer.
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In the first class lever, the weight and force are on opposite sides of the fulcrum, as follows.
In this form of torque, a small force can be used to advantage over a heavy weight if a sufficiently long force arm (lever arm) can be used. Examples include scissors, crowbars, and teeter-totters.
An example of this lever in the human body is the muscle known as the triceps that attaches to the bones in the arm (the ulna and the humerus) and tends to extend the arm. When the triceps is made to contract, it pulls on the short side of the ulna from its pivot point at the elbow and tends to rotate the longer side to straighten the arm.
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In the second class lever, the load is between the fulcrum and the force. In this case, too, a smaller effort can be used to advantage over a larger weight. An example of this lever is a wheelbarrow.
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The third class lever is the same as the second-class lever except that the applied force is between the load and the fulcrum. In this case, there is no force advantage. In fact, a larger force is actually needed to move a smaller weight, so there is a force disadvantage. The value of the lever lies in the gain in speed of movement of the weight. In a dynamic situation, the levers (bones) are in motion, rotating about their joints and also being translated by the action of other bones. For these levers, the weight end, i.e., the end of the longer weight arm, moves through a greater distance than does the end of the lever arm. So it has to move with greater speed. Speed advantage is gained at the cost of force advantage.
Common examples of this lever class include the inside door handle of a car, the coiled spring on a screen door, a pair of fingernail clippers, and tweezers.
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The concept of a center of gravity has proved to be very useful for the study of motions of the body, but it can be troublesome to calculate as well as to understand. One reason is that it depends on the idea of a center point of the body, or what is called a balance point, which moves around as the body moves and may not even be in the body.
For a regular object, like a tennis ball or a basketball, it's fairly easy to see that the center of gravity coincides with the geometric center of the object. You have the sense that, if you place the ball on a flat surface, it will remain stationary -- that it won't tumble or roll -- because opposite parts of the ball are at the same distance from the center and neutralize or balance each other. The net torque about the center is zero.
But the term also applies to irregular objects, like tables and chairs or, more to the point, like the human body, which is not only irregular but also changes its configuration as it moves. So how do you determine the geometric center or point of balance of a body? In particular, where is the center for your own body, and what happens to it when you move?
Center of gravity is intimately involved with torque. You can think of an object as an aggregate of small parts distributed about its center of gravity and pulled down by gravity. If you could put a fulcrum at the object's center of gravity, all of the torques about it would cancel each other out and the object would be in balance, meaning it wouldn't rotate. And if you applied a force through the center of gravity, the object would move in a straight line; there would be no twisting or rotating, such as you get when you push open a door.
The location of an object's center of gravity determines whether or not the object is stable. If you're in a standing posture, for example, the center of gravity is a point somewhere in the middle of your body just above your pelvis. This is the point where your entire weight might be thought to be concentrated, and it defines your center of gravity for the standing position. You remain stable in this posture, because your center of gravity lies inside, or above, the supporting structure of your legs. That is, you're able to hold your balance, because the rotating effect of gravity on one side of your body counters the effect on the other side, so gravity only pulls you against the support and doesn't cause you to spin, or tumble.
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Since torque is produced by the contraction of muscles, which are attached to bones, you can see that many torques at various levels are applied in the performance of any skill. In golf, for instance, you apply torque to grip the club, using many muscles of the hands, wrists, and arms. You apply torque through the legs, first to establish a stance and then to carefully rotate the torso into a backswing, after which you generate torque for a forward drive to and through the ball. Each segment of the dynamics engages every muscle and creates detailed torques that add up to the gross torque that produces the large, obvious movements.
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