Chapter 6 The Muscular System 183 Copyright Goodheart-Willcox Co., Inc. cross bridges connections between the heads of myosin filaments and receptor sites on the actin filaments dorsiflexion movement of the top of the foot toward the lower leg eversion movement in which the sole of the foot is rolled outward extension movement that returns a body segment to anatomical position in the sagittal plane fast-twitch a type of muscle that contracts quickly flexion forward movement of a body segment away from anatomical position in the sagittal plane hyperextension backward movement of a body segment past anatomical position in the sagittal plane insertion the site of a muscle’s attachment to a bone that tends to move when the muscle contracts inversion movement in which the sole of the foot is rolled inward lateral rotation outward (lateral) movement of a body segment in the transverse plane medial rotation inward (medial) movement of a body segment in the transverse plane motor neuron a nerve that stimulates skeletal muscle tissue motor unit a single motor neuron and all of the muscle fibers that it stimulates neuromuscular junction the link between an axon terminal and a muscle fiber opposition the act of touching any of your four fingers to your thumb this movement enables grasping of objects origin the site of a muscle’s attachment to a relatively fixed structure parallel fiber architecture a muscle fiber arrangement in which fibers run mostly parallel to each other along the length of the muscle pennate fiber architecture a muscle fiber arrangement in which each fiber attaches obliquely to a central tendon plantar flexion downward movement of the foot away from the lower leg pronation medial rotation of the forearm (palm down) sarcomeres units composed of actin and myosin that contract inside the muscle fiber slow-twitch type of muscle that contracts slowly and is resistant to fatigue supination lateral rotation of the forearm (palm up) synaptic cleft the tiny gap that separates the axon terminal from the muscle fiber tetanus a sustained, maximal level of muscle tension that occurs with high-frequency stimulation Review Questions 1. Explain the role of acetylcholine in muscle contractions. 2. What is measured to determine muscular power? 3. Discuss the differences between a large and small motor unit and their functions. 4. Describe parallel and pennate fiber patterns. 5. What is the difference between pronation and supination of the hand and the foot? 6. What are the directions of movement for the sagittal, frontal, and transverse planes? 7. Describe the difference between abduction and adduction. 8. Which fiber types do you think contribute to each of the following: muscular strength, power, and endurance? Explain your reasoning. 9. Is the strongest athlete the fastest? Why or why not? 10. Why do temperature and humidity increase the rate of muscle fatigue? 11. Do you think a soccer player has more fast-twitch or slow-twitch muscle fibers? Why? 12. Which joints in the upper and lower limbs can perform flexion and extension? SECTION 6.3 Major Skeletal Muscles Objectives • Identify the locations and functions of the muscles of the head and neck. • Identify the locations and functions of the trunk muscles. • Identify the locations and functions of the muscles of the upper limb. • Identify the locations and functions of the muscles of the lower limb. Key Terms agonist-antagonist pairs diaphragm linea alba rectus sheath rotator cuff