At the conclusion of this chapter, the student should be able
- 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
- 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
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.
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.
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