Anatomy & Physiology

Anatomy & Physiology Study Guide

11.2 Levers The strength, quickness, or direction of movement produced by the contraction of a muscle can be altered by attaching the muscle to a lever. A lever is a rigid structure—such as a a bone—that moves on a fixed point called the fulcrum . When an applied force overcomes resistance or the weight of capacity, the lever moves. In the body, the lever is each bone, and each joint is a fulcrum. The joined force needed for movement is provided by muscles. The important thing about levers is that they can change the direction of an applied force, the strength and quickness of movement produced by an applied force, and the efficient strength of an applied force. There are three classes of levers: first- class, second-class, and third-class. An example of a first-class lever is a seesaw. In a second-class lever , the load is located between the applied force and the fulcrum. A familiar example is a loaded wheelbarrow. The most common levers in the body are third-class levers . In this lever system, a force is applied between the load and the fulcrum. 11.3 Origins and Insertions The place where the fixed end of a muscle connects to a bone, cartilage, or connective tissue is called the origin of the muscle. Where the movable end connects to another structure is called the insertion of the muscle. The origin is usually proximal to the insertion. When a muscle contracts, it produces a specific action or movement (flexion, extension, adduction, and so on). The decision as to which end is the origin and which is the insertion is typically based on movement from the anatomical position. 11.4 Accessory Muscles Large muscles of the limbs move over an extended range of motion and produce flexion or extension. Although these muscles cannot create powerful movements at full extension due to the relative positions of the articulating bones, smaller accessory muscles assist the movement until the larger muscle can perform at maximum efficiency. Initially, the smaller muscle produces maximum tension, while the larger muscle produces minimum tension. Based on their functions, muscles are described as one of three types. An agonist , or prime mover , is a muscle whose contraction is chiefly responsible for producing a particular movement. An antagonist is a muscle whose action opposes that of a particular agonist. Agonists and antagonists are functional opposites; if one produces flexion, the other will produce extension. A synergist helps a larger agonist work efficiently. Synergists may provide additional pull near the insertion or may stabilize the point of origin. 11.5 Skeletal Muscle Names Skeletal muscles are names using descriptive terms. Except for the platysma and the diaphragm, the complete names of all skeletal muscles include the term muscle. The designation of a muscle may include descriptive words about its location in the body, origin and insertion, fascicle organization, relative position, structural characteristics, and action.

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