The site at which a muscle begins is its origin, and the site at which it ends is its insertion. Each muscle contracts to produce a specific action, or movement. In general, the origin remains stationary--attached to a bone or other structure that doesn't move--whereas the insertion moves. In most cases, the origin is proximal to the insertion. For example, consider the gastrocnemius muscle (in the calf), which extends from the distal portion of the femur muscle to the calcaneus. As Figure 11-2b shows, when the gastrocnemius contracts, it pulls the calcaneus toward the knee. As a result, we say that the gastrocnemius muscle has its origin at the femur and its insertion at the calcaneus. The "Origins and Insertions" section of the Martini FAP Interactive CD-ROM illustrates these features for many of the body's major muscles.

The decision as to which end is the origin and which is the insertion is usually based on movement from the anatomical position. Part of the fun of studying the muscular system is that you can actually do the movements and think about the muscles involved. As a result, laboratory discussions of the muscular system tend to resemble disorganized aerobics classes.

When the origins and insertions cannot be determined easily on the basis of common movement or position, other rules are used. If a muscle extends between a broad aponeurosis and a narrow tendon, the aponeurosis is the origin and the tendon is the insertion. If several tendons are at one end and just one is at the other, the muscle has multiple origins and a single insertion. These simple rules cannot cover every situation. Knowing which end is the origin and which is the insertion is ultimately less important than knowing where the two ends attach and what the muscle accomplishes when it contracts.

Most muscles originate at a bone, but some originate at a connective tissue sheath or band. Examples are intermuscular septa (components of the deep fascia that may separate adjacent skeletal muscles), the interosseous membranes of the forearm or leg, and the fibrous sheet that spans the obturator foramen of the pelvis.