Chapter 3

At the conclusion of this chapter, the student should be able to:

• 1. Describe the structure and properties of the whole muscle, fast- and slow-twitch muscle fiber, and the myofibril.
• 2. Explain how the relationship of the muscle’s line of pull to the joint axis affects the movement produced by the muscle.
• 3. Describe the relationship between the skeletal muscle’s fiber arrangement and its function.
• 4. Define the roles a muscle may play (agonist, antagonist, and synergist), and explain the cooperative action of muscles in controllingjoint actions by naming and ex-plainingthemuscle roles in a specified movement.
• 5. Define the types of muscular contraction (concentric, eccentric, and static), and name and demonstrate each type of action.
• 6. Demonstrate an understanding of the influence of gravity and other external forces on muscular action by correctly analyzing several movement patterns in which these forces influence the muscular action.
• 7. Describe the various methods of studying muscle action, citing the advantages and disadvantages of each method.
• 8. State the force–velocity and length–tension relationships of muscular contraction, and explain the significance of these relationships in static and dynamic movements.
• 9. Identify the muscle groups active in a variety of motor skills.

Body parts are moved by external or internal forces. The internal force responsible for the movement and positioning of the bony segments of the body is the action of skeletal muscles. These muscles are able to serve this function because they can contract, they are attached to the bones, and they cross a joint. In addition, they are constructed of bundles of striated muscle fibers, which differ in both structure and function from the highly specialized cardiac muscle and from the smooth muscle of blood vessels, digestive organs, and urogenital organs.

### Properties of Muscular Tissue

The properties of striated muscle tissue are extensibility, elasticity, and contractility. The first two enable a muscle to be stretched like an elastic band and, when the stretching force is discontinued, to return again to its normal resting length. Tendons, which are simply continuations of the muscle’s connective tissue, also possess these properties. Contractility, the ability to shorten and produce tension at its ends, is a unique property possessed by muscle tissue only. The average muscle fiber can shorten to approximately one-half its resting length. It can also be stretched until it is approximately one-half again as long as its resting length. The range between the maximal and minimal lengths of a muscle fiber is known as the amplitude of its action. The elongation varies proportionately with the length of the fiber and inversely with its cross section.

### The Muscle Fiber

A single muscle cell is a threadlike fiber about 1 to 20 inches in length and containing the cell nucleus, mitochondria (important in cell metabolism), myoglobin (similar to hemoglobin), and glycogen (form of sugar). In addition, microscopic examination has also revealed that ...

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