Muscles work in pairs to produce movement at the joints (see Display 20-1 for a description of various types of movement). As one muscle, the prime mover, contracts, an opposing muscle, the antagonist, must relax. For example, when the biceps brachii on the anterior surface of the upper arm contracts to flex the arm, the triceps brachii on the posterior surface must relax (Fig. 20-2). When the arm is extended, these actions are reversed. In a given movement, the point where the muscle is attached to a stable part of the skeleton is the origin; the point where a muscle is attached to a moving part of the skeleton is the insertion.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.