Muscles are composed of elongated cells called muscle fibers. Muscle fibers use ATP generated through cellular respiration (Chapter 6) to generate force and change length during a contraction. This conversion of chemical energy into muscle work underlies much of the energy cost of animal movement discussed in Chapter 40.

A force is a push or pull by one object interacting with another object. The forces generated by skeletal muscles, for example, act as pulling forces at specific sites of the skeleton. The work performed by a muscle is equal to force times length change. For example, work is increased when either the force produced or the distance the skeleton is moved increases. Similarly, a muscle that produces a pulling force twice as much as another muscle but moves the skeletal segment only half the distance does the same work as that other muscle.

Because muscles only exert pulling forces, pairs of muscles are arranged to produce movements in two opposing directions at specific joints of the skeleton. For example, muscles pull on the bones of your leg and foot to swing them forward when you kick a ball. Opposing sets of muscles contract to swing your leg back and flex your knee before and after the kick.

Muscles can shorten very quickly, and they can produce large forces for their weight. The forces they produce can be many times an animal’s body weight, allowing animals to move quickly. Some small muscles contract extremely quickly: The muscles of many flying insects contract 200 to 500 times per second. Even in vertebrate animals, muscles can contract as many as 50 to 100 times per second. Other muscles, such as the closing muscle of a clam, contract slowly, but can do so for prolonged periods without tiring. To see how muscles produce forces and contract to change length, we now consider how muscle fibers are organized at the molecular level.