Chapter 6 The Muscular System 191 Copyright Goodheart-Willcox Co., Inc. Muscles of the Lower Limb While the structure of the upper limb lends itself well to activities that involve large ranges of motion, the lower limb is well designed for its primary jobs of standing and walking. Running, jumping, kicking, climbing, skipping, hopping, and dancing are just a few of the additional capabilities of the lower limb. The lower limb includes the joints of the hip, knee, and ankle, along with numerous joints in the foot. This section includes the major muscles of the hip, knee, and ankle, but omits a number of small muscles that play assistive roles. The table in Figure 6.23 outlines the locations and primary functions of the major muscles of the lower limb. Figure 6.24 shows these muscles. Muscles Acting at the Hip The hip is surrounded by large, powerful muscles that make it an extremely stable joint but also tend to limit its range of motion. The large, superficial gluteus maximus, extending from the sacrum and iliac to the gluteal tuberosity of the femur, is the primary extensor muscle of the hip. This muscle also contributes to lateral rotation of the leg. It is usually activated only when the hip is in flexion, stretching the muscle slightly, as during cycling or climbing stairs. Just deep to gluteus maximus is the gluteus medius. Connecting the posterior ilium to the greater trochanter of the femur, this muscle is responsible for abduction and medial rotation of the leg. It also serves the important function of stabilizing the pelvis during the support phase of walking and running or when standing on one leg. The primary flexor of the hip is the iliopsoas complex, consisting of the iliacus (originating from the iliac fossa and sacrum) and the psoas (originating from the 12th thoracic and all of the lumbar vertebrae). These muscles join to insert on the lesser trochanter of the femur. Adduction at the hip is the function of the adductor muscles, which extend from the pubis to the linea aspera of the femur. The adductors are active during the swing phase of gait to bring the foot under the body’s center of gravity. The longest muscle in the human body is the sartorius, a thin strap muscle that originates on the anterior superior iliac spine and inserts on the proximal tibia. Sartorius assists with flexion and lateral rotation of the thigh at the hip. names: supraspinatus, infraspinatus, teres minor, and subscapularis. Supraspinatus, named after its origin in the supraspinous fossa of the scapula, assists with arm abduction. Infraspinatus, named after its origin in the infraspinous fossa of the scapula, contributes to lateral rotation of the humerus. Teres minor, originating on the lateral posterior scapula, also laterally rotates the humerus. Emanating from the subscapular fossa or the scapula, subscapularis medially rotates the humerus. Muscles Acting at the Elbow and Wrist The arm muscles enable strong, controlled movements in sports such as gymnastics, rowing, and archery, as well as fast, powerful movements in weightlifting, boxing, and throwing. The biceps brachii and triceps brachii are superficial muscles on the anterior and posterior aspects of the arm, respectively, that are typically well developed in bodybuilders. One head of the biceps originates on the superior side of the glenoid fossa, and the other originates on the coracoid process. Both heads insert on the radial tuberosity, enabling the biceps to assist with supination and flexion of the forearm. The three heads of the triceps brachii cover the posterior aspect of the arm, with origins on the inferior glenoid fossa and entire humeral shaft. The insertion is on the olecranon. The triceps contracts to extend the arm at the elbow. Other anterior arm muscles include the brachialis and brachioradialis, which also contribute to flexion of the forearm. The brachialis connects the humerus and ulna, and the brachioradialis connects the radius and humerus. There are nine muscles that cross the wrist, and ten muscles contained entirely within the hand, many of which branch to several of the fingers. The names of many of these muscles describe their functions as well as their locations. For example, the flexor carpi ulnaris causes flexion at the wrist and crosses the wrist on the ulnar (little finger) side. The extensor digitorum superficialis and extensor digitorum profundus cause extension of the digits, or fingers. SELF CHECK 1. What is sacrificed at the shoulder to allow greater range of motion? 2. Which four muscles make up the rotator cuff?