Skeletal muscles do not work in isolation. Each muscle is attached to bony processes or sheets of connective tissue. All voluntary muscles are somehow associated with the skeletal system. The muscles to bone attachment at which the muscle begins is called the origin and the site at which it ends is it's insertion. The origin is usually proximal and attaches to the less moveable bone, while the insertion is distal and attaches to the more moveable bone. Therefore, when a muscle contracts, the insertion moves toward the origin. For example, the deltoid muscle in the shoulder of the humerus. When this muscle contracts, the more move able it's insertion on the humerus. When this muscle contracts, the more moveable insertion moves toward the origin and pulls the humerus toward the scapula. Almost all skeletal muscles either originate or insert on the skeleton and many like the deltoid, have both their origin and insertion on bone. Contraction of the deltoid flexes the shoulder. It is the job of another muscle or group of muscles to extend the shoulder or to produce an opposite action. For example, the teres minor is one of the muscles that extends the shoulder. Similarly, abductors and adductors work in opposition of each other. The concept of muscles working to produce movement in opposite directions allows dogs to move their joints in both directions.