Chapter 6 The Muscular System 171 Copyright Goodheart-Willcox Co., Inc. The versatility of the arrangements of human muscles in agonist and antagonist pairs around joints enables the different movements of the human body. These versatile arrangements also help to stabilize joints and maintain body posture. Heat Production Everyone knows that vigorous exercise is typically accompanied by an increase in body temperature and sweating. This happens because the working muscles generate heat. Even when you are not exercising, the muscles, comprising approximately 40% of body mass, generate heat, and this heat helps maintain normal body temperature. How does this happen? Muscles require energy in the form of adenosine triphosphate (ATP) to function. You may recall from your study of Chapter 2 that ATP is generated within muscle cells. The ATP is then released to provide energy when the muscle is stimu- lated, generating heat in the process. A muscle can also lengthen while developing tension. Suppose someone were to place in your hands a weight that was too heavy for you to hold in position. At first, your biceps would develop tension in an effort to hold the weight in place. But because the weight is too heavy to manage, you would lower the weight, and your biceps would lengthen. This type of action is known as an eccentric contraction, or lengthening, contraction (Figure 6.4B). In this case, the force of gravity (not the triceps) acting on the weight causes the weight to lower. In a third scenario (Figure 6.4C), you “flex” the muscles in your arm, developing tension in both the biceps and triceps, but there is no movement. This is called an isometric contraction of both the biceps and triceps. During an isometric contraction, no change in muscle length occurs. SECTION 6.1 REVIEW Mini-Glossary agonist role played by a skeletal muscle in causing a movement antagonist role played by a skeletal muscle acting to slow or stop a movement aponeurosis a flat, sheetlike fibrous tissue that connects muscle or bone to other tissues concentric contraction a type of contraction that results in shortening of a muscle contractility the ability to contract or shorten eccentric contraction a type of contraction that results in lengthening of a muscle elasticity the ability to return to normal length after being stretched endomysium a fine, protective sheath of connective tissue that surrounds a skeletal muscle fiber epimysium the outermost sheath of connective tissue that surrounds a skeletal muscle extensibility the ability to be stretched fascicle a bundle of muscle fibers irritability the ability to respond to a stimulus isometric contraction a type of contraction that involves no change in muscle length muscle fiber an individual skeletal muscle cell perimysium a connective tissue sheath that envelops each primary bundle of muscle fibers sarcolemma the delicate membrane that surrounds each striated muscle fiber Review Questions 1. Starting with a muscle fiber and working from the inside out, name each part of the skeletal muscle structure. 2. Describe the role of each type of muscle tissue (cardiac, smooth, and skeletal). 3. Explain the difference between irritability and contractility. 4. What is the difference between extensibility and elasticity? 5. Give three examples of how you use isometric contractions during a typical day. 6. What do you think would happen if your antagonist muscles no longer functioned?