Skeletal muscles do not work in isolation. When a muscle is attached to the skeleton, the nature and site of the connection will determine the force, speed, and range of the movement produced. These characteristics are interdependent, and the relationships can explain a great deal about the general organization of the muscular and skeletal systems.

The force, speed, or direction of movement produced by contraction of a muscle can be modified by attaching the muscle to a lever. A lever is a rigid structure—such as a board, a crowbar, or a bone—that moves on a fixed point called the fulcrum. A child's teeter-totter, or seesaw, provides a familiar example of lever action. In the body, each bone is a lever, and each joint is a fulcrum. Levers can change (1) the direction of an applied force, (2) the distance and speed of movement produced by an applied force, and (3) the effective strength of an applied force.

Classes of Levers

Three classes of levers are found in the human body. The seesaw is a first-class lever. In such a lever, the fulcrum lies between the applied force (AF) and the resistance (R).  The body has few first-class levers. One, involved with extension of the neck, is shown in Figure 11-2a.

In a second-class lever (Figure 11-2b), the resistance is located between the applied force and the fulcrum. A familiar example is a loaded wheelbarrow. The weight of the load is the resistance, and the upward lift on the handle is the applied force. Because in this arrangement the force is always farther from the fulcrum than the resistance is, a small force can balance a larger weight. That is, the effective force is increased. Notice, however, that when a force moves the handle, the load moves more slowly and covers a shorter distance. The body has few second-class levers. In performing plantar flexion, the calf muscles use a second-class lever (Figure 11-2b).

Third-class levers are the most common levers in the body. In this lever system, a force is applied between the resistance and the fulcrum (Figure 11-2c). An example is a ladder that you raise to lean it against a building. The fulcrum is the base of ladder, in contact with the ground. Force is applied where you grasp the ladder, and the resistance is the weight of the ladder between your hands and the free end. The effect is the reverse of that for a second-class lever: Speed and distance traveled are increased at the expense of effective force. In the example shown (the biceps brachii muscle, which flexes the elbow), the resistance is six times farther from the fulcrum than is the applied force. The effective force is reduced to the same degree. The muscle must generate 180 kg of tension at its attachment to the forearm to support 30 kg held in the hand. However, the distance traveled and the speed of movement are increased by at the same 6 : 1 ratio: The load will travel 45 cm when the point of attachment moves 7.5 cm.

Although not every muscle operates as part of a lever system, the presence of levers provides speed and versatility far in excess of what we would predict on the basis of muscle physiology alone. Skeletal muscle fibers resemble one another closely, and their abilities to contract and generate tension are quite similar. Consider a skeletal muscle that can contract in 500 msec and shorten 1 cm while it exerts a 10-kg pull. Without using a lever, this muscle would be performing efficiently only when moving a 10-kg weight a distance of 1 cm. But, by using a lever, the same muscle operating at the same efficiency could move 20 kg a distance of 0.5 cm, 5 kg a distance of 2 cm, or 1 kg a distance of 10 cm.

Why does a pennate muscle generate more tension than does a parallel muscle of the same size?

Which type of muscle would you expect to be guarding the opening between the stomach and the small intestine?

The joint between the occipital bone of the skull and the first cervical vertebra (atlas) is part of which type of lever system?