170 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. developed is constant throughout the muscle, tendons, and attachment sites. Skeletal Muscle Contraction Although the term contraction (which implies short- ening) is commonly used to mean that tension has developed in a muscle, muscles do not always shorten when they develop tension. When a skeletal muscle develops tension, one of three actions can occur: the muscle can shorten, remain the same length, or actually lengthen. The familiar large muscle groups—the biceps and triceps, on the anterior and posterior sides of your upper arm—provide examples of these three different types of tension. When the biceps muscle develops tension and shortens, your hand moves up toward your shoulder (Figure 6.4A). This is called a concentric contraction, or shortening, contraction of the biceps. The biceps is performing the role of agonist, or prime mover, and the opposing muscle group, the triceps, is playing the role of antagonist. The antagonist muscles may be completely relaxed or may develop a slight amount of tension, depending on the requirements of the movement. contractility. Two of these—extensibility, the ability to be stretched, and elasticity, the ability to return to normal length after being stretched—are common not just to muscle, but to many types of biological tissues. As an example, when a muscle group, such as the hamstrings (on the posterior side of the thigh), are stretched over a period of time, the muscles lengthen, and the range of motion at the hip increases, making it easier to touch the toes. The stretched muscles do not return to resting length immediately, but shorten over a period of time. Another characteristic common to all muscle is irritability, or the ability to respond to a stimulus. Muscles are routinely stimulated by signals from the nerves that supply them. Muscles can also be mechanically stimulated, such as by an external blow to a muscle. A muscle’s response to all forms of stimulus is contraction. Contractility, the ability to contract or shorten, is the one behavioral characteristic unique to muscle tissue. Most muscles have a tendon attaching to a bone at one end and another tendon attaching to a different bone at the other end. When a muscle contracts, it pulls on the bones to which it is attached. This pulling force is called a tensile force, or tension. The amount of tension A B C Eccentric contraction Isometric contractions Triceps relaxed (antagonist) Triceps relaxed Tension in triceps Tension in biceps (agonist) Tension in biceps Tension in biceps Concentric contraction © Body Scientific International Figure 6.4 Muscle contraction. A—In concentric contraction, the agonist biceps contracts and the antagonist triceps relaxes. B—The biceps is eccentrically contracting (lengthening) while serving as a brake to control the downward motion of the weight. C—In isometric contractions, both the biceps and triceps develop tension, but neither muscle shortens, and there is no motion.