When many people pull on a rope, the amount of tension produced is proportional to the number of people pulling. In a skeletal muscle fiber, the amount of tension generated during a contraction depends on the number of pivoting cross-bridges in all the sarcomeres along all the myofibrils. The number of cross-bridges that can form depends on the degree of overlap between thick filaments and thin filaments. When the muscle fiber is stimulated to contract, only myosin heads in the zone of overlap can bind to active sites and produce tension. The tension produced by the entire muscle fiber can thus be related to the structure of an individual sarcomere.

When the sarcomeres are as short as they can be, the thick filaments are jammed against the Z lines. Although cross-bridge binding can occur, the myosin heads cannot pivot, and no tension is produced (Figure 10-11a). Even if the sarcomeres are somewhat longer, the thin filaments that extend across the center of the sarcomere collide with or overlap the thin filaments of the opposite side (Figure 10-11b). This disruption of the normal arrangement of thick and thin filaments interferes with the binding of cross-bridges to active sites. As a result, little tension is produced when the muscle fiber is stimulated.

Within the optimal range of sarcomere lengths (Figure 10-11c), the maximum number of cross-bridges can form and the tension produced is highest. Any further increase in sarcomere length reduces the tension produced by reducing the size of the zone of overlap and the number of potential cross-bridge interactions (Figure 10-11d). When the zone of overlap is reduced to zero, thin and thick filaments cannot interact (Figure 10-11e). The muscle fiber cannot produce any tension, and a contraction cannot occur. Such extreme stretching of a muscle fiber is normally prevented by the titin filaments in the muscle fiber, which tie the thick filaments to the Z lines, and by the surrounding connective tissues, which limit the degree of muscle stretch.

In summary, muscle fibers contract most forcefully when stimulated over a narrow range of resting lengths. The normal range of sarcomere lengths in the body (Figure 10-11) is 75 to 130 percent of the optimal length. The arrangement of skeletal muscles, connective tissues, and bones normally prevents the extreme compression or excessive stretching that corresponds to Figure 10-11a or 10-11e. For example, straightening your elbow stretches your biceps brachii muscle, but the bones and ligaments of the elbow end this movement before the muscle fibers stretch too far.

During normal movements, your muscle fibers perform over a broad range of intermediate lengths; the tension produced varies with the initial length of the muscle fibers. During an activity such as walking, in which muscles contract and relax cyclically, muscle fibers are stretched to a length very close to "ideal" before they are stimulated to contract. Some muscles must contract over a large range of resting lengths. When they contract at inefficient resting lengths, these muscles are generally assisted by the contractions of other muscles whose lengths are closer to ideal. (In Chapter 11, we will discuss the mechanical principles involved.)

 How would a drug that interferes with cross-bridge formation affect muscle contraction?

 What would you expect to happen to a resting skeletal muscle if the sarcolemma suddenly became very permeable to Ca²⁺?

 Predict what would happen to a muscle if the motor end plate failed to produce acetylcholinesterase.