This is not to say that all life depends on muscle tissue. Some large organisms get by very nicely without it; we call them plants. But life as we live it would be impossible, because virtually all of our dynamic interactions with the environment involve muscle tissue.

Muscle cells are specialized to contract. The individual cells in muscle tissue are tied together, primarily by collagen fibers. When the muscle cells contract, they pull on those fibers the way a group of people might pull on a rope. The pull, called tension, is an active force--energy must be expended to produce it. Tension is applied to some object, whether a rope, a rubber band, or a book on a tabletop.

Tension applied to an object tends to pull the object toward the source of the tension. However, before movement can occur, the applied tension must overcome the object's resistance, a passive force that opposes movement. The amount of resistance can depend on the weight of the object, its shape, friction, and other factors. When the applied tension exceeds the resistance, the object moves. In contrast, compression, or a push applied to an object, tends to force the object away from the source of the compression. Again, no movement can occur until the applied compression exceeds the resistance of the object. Muscle cells can use energy to shorten and generate tension but not to lengthen and generate compression.

As we noted in Chapter 6, collagen fibers cannot withstand compression; when compressed, they simply bend out of the way. But collagen fibers can resist considerable tension, so when a muscle pulls on collagen fibers, those fibers transmit the force and pull on something else. What happens next depends largely on what the collagen fibers are attached to and on how the muscle cells are arranged. We will encounter several examples in this chapter as well as in Chapters 11, 20, and 24.