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PART I FOUNDATION CONCEPTS


3.3 Demonstrate the different types of muscle contractions, and compare and contrast them in terms of force produced.


3.4 Summarize the anatomical composition and physiological function of the motor unit. 3.5 Identify the various types of muscle architecture and fiber types, and relate each to their potential for generating force and movement.


3.6 Name the various connective tissue structures found in the system of muscles, and define their role in generating force, providing structure, and resisting stretch.


3.7 Discuss the relationship between muscle length and speed of contraction and their ability to produce tension.


3.8 Compare and contrast active and passive muscle insufficiency. 3.9 Outline the neural structures involved with motor output, including the sensory receptors specific to muscle tissue.


3.10 Analyze the changes in the muscular system associated with aging and inactivity.


Note to Students: It is recommended that you review the pretest questions before reading the chapter to better understand the critical elements of the text. As you complete each section, stop and answer the corresponding questions to test your comprehension. The pretest does not cover all the material in each chapter and should not be used as a sole means of self-assessment of knowledge.


The skeletal system is the fundamental structure of the human body and the lever system on which muscles act to move or stabilize the body. Muscles generate mobility as they act against external forces, such as gravity, fric- tion, and loads, to move body segments through daily activities. They produce stabilizing forces to hold the body or body segments stationary against external forces or loads. Stabilization of a proximal body segment can provide the anchor for distal movement by the extremi- ties. For example, as a person plays the violin, the trunk and scapular muscles stabilize the proximal portion of the body as the arms and fi ngers perform intricate distal movements. If the person were unable to hold the trunk relatively still, it would be increasingly diffi cult to move the arms, hands, and fi ngers with the speed and precision needed to play the instrument.


This chapter begins with an overview of the general structural and mechanical properties of skeletal muscles. It covers the interrelationship between these muscle structures and the nervous system and identifi es factors that infl uence the speed and force of muscle contractions. The responsiveness of the neuromuscular system, muscle fi ber types, muscle length, and joint angles all are contrib- uting factors to the quantity and quality of the contraction produced. This introduction to the characteristics and general function of skeletal muscle provides a background for the study of individual muscle groups in subsequent chapters. Understanding skeletal muscle structures and their function creates a foundation for insight into how muscles can facilitate recovery and optimal function or


how they can contribute to injury and dysfunction. Figure 3.1 shows the major skeletal muscles of the body.


MUSCLE ACTION TERMINOLOGY


Specifi c terminology is used to describe how a muscle or group of muscles functions during the performance of an action. The term agonist is used to describe the muscle that is contracting to initiate a specifi c movement. During wrist fl exion, the fl exor carpi ulnaris and fl exor carpi radialis are considered the agonists to produce the movement. The muscle (or muscles) that has the oppo- site action of the agonist is labeled the antagonist. The antagonist does not contract during the agonist activation but has the potential to move the joint in the oppo- site direction of the agonist. For example, during wrist fl exion, the wrist extensors are the antagonists of the movement. The label would be reversed if wrist extension were the desired movement: The wrist extensors would be the agonists, and the fl exors would be the antagonists. If the agonist and the antagonist contract at the same time, they produce cocontraction (stabilization). These simultaneous contractions provide stability to body seg- ments, such as when trunk core muscles contract to provide a stable base for movement of the extremities. When several muscles act together to produce a move- ment, they are considered synergists. The fl exor carpi ulnaris and fl exor carpi radialis act as synergist muscles when they contract together to produce wrist fl exion in a neutral position without deviation toward the radius or ulna. Table 3.1 summarizes the various roles muscles can play during movement.


Muscles have an internal connection to the central nervous system (CNS), providing the means to gener- ate movements with the exact speed, force, and distance needed for a wide variety of movements. Through this movement system, the body is able to accomplish actions


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