Sample Chapter 10 from the Textbook (35559.0K) - McGraw-Hill

310 PART 2 Support and Movement
10.1 General Principles of Skeletal
Muscle Anatomy
Origins of biceps
brachii on scapula
Belly of
biceps brachii
Scapula
Learning Outcomes
After reading this section, you should be able to
A. Define the following and give an example of each: origin,
insertion, agonist, antagonist, synergist, fixator, and
prime mover.
B. Explain how fasciculus orientation determines muscle
shape and list examples of muscles that demonstrate
each shape.
C. Recognize muscle names based on specific nomenclature
rules.
D. Explain each of the three classes of levers in the body
and give a specific example of each class.
Most skeletal muscles extend from one bone to another and cross
at least one joint. Muscle contraction causes most body movements
by pulling one of the bones toward the other across a movable
joint. Some muscles are not attached to bone at both ends. For
example, some facial muscles attach to the skin, which moves as
the muscles contract.
The two points of attachment of each muscle are its origin
and its insertion. The origin, also called the fixed end, is usually
the most stationary, proximal end of the muscle. Some muscles
have multiple origins. For example, the triceps brachii has three
origins that converge to form one muscle. In the case of multiple
origins, each origin is also called a head. The insertion, also called
the mobile end, is usually the distal end of the muscle attached to the
bone undergoing the greatest movement. The part of the muscle
between the origin and the insertion is the belly (figure 10.1). At the
attachment point, each muscle is connected to bone by tendons.
Tendons may be long and cablelike; broad and sheetlike (called
aponeuroses; ap′ō-noo-rō′sēz); or short and almost nonexistent.
The action of a muscle is the movement accomplished when it
contracts. Muscles are typically grouped so that the action of one
muscle or group of muscles is opposed by that of another muscle or
group of muscles. For example, the biceps brachii flexes (bends) the
elbow, and the triceps brachii extends the elbow. A muscle that
accomplishes a certain movement, such as flexion, is called the agonist
(ag′ō-nist). A muscle acting in opposition to an agonist is called an
antagonist (an-tag′ō-nist). For example, when flexing the elbow,
the biceps brachii is the agonist, whereas the triceps brachii, which
relaxes and stretches to allow the elbow to bend, is the antagonist.
When extending the elbow, the muscles’ roles are reversed; the triceps
brachii is the agonist and the biceps brachii is the antagonist.
Most joints in the body have agonist and antagonist groups or pairs.
Muscles also tend to function in groups to accomplish specific
movements. For example, the deltoid, biceps brachii, and pectoralis
major all help flex the shoulder. Furthermore, many muscles are
members of more than one group, depending on the type of movement
being produced. For example, the anterior part of the deltoid
muscle functions with the flexors of the shoulder, whereas the posterior
part functions with the extensors of the shoulder. Members
Extension
Ulna
Radius
Flexion
Insertion of
biceps brachii on
radial tuberosity
Tendon
Origins of triceps
brachii on
scapula and
humerus
Humerus
Insertion of triceps
brachii on olecranon
process
FIGURE 10.1 Muscle Attachment
Muscles are attached to bones by tendons. The biceps brachii has two heads,
which originate on the scapula. The triceps brachii has three heads, which
originate on the scapula and the humerus. The biceps brachii inserts onto
the radial tuberosity and onto nearby connective tissue. The triceps brachii
inserts onto the olecranon process of the ulna.
of a group of muscles working together to produce a movement are
called synergists (sin′er-jistz): the biceps brachii and the brachialis
are synergists in elbow flexion. Among a group of synergists, if
one muscle plays the major role in accomplishing the movement,
it is called the prime mover. The brachialis is the prime mover in
flexing the elbow. Fixators are muscles that hold one bone in place
relative to the body while a usually more distal bone is moved. The
origin of a prime mover is often stabilized by fixators, so that its
action occurs at its point of insertion. For example, the muscles of
the scapula act as fixators to hold the scapula in place while other
muscles contract to move the humerus.
Muscle Shapes
The shape and size of any given muscle greatly influence the degree
to which it can contract and the amount of force it can generate.
Muscles come in a wide variety of shapes, which can be grouped
into five classes based on arrangement of the fasciculi (bundles of
muscle fibers that can be distinguished by the unaided eye; see
section 9.3): circular, convergent, parallel, pennate, and fusiform.
Muscles can also have specific shapes, such as quadrate, rhomboidal,
trapezium, or triangular (table 10.1).
Circular muscles, such as the orbicularis oris and orbicularis
oculi, have their fasciculi arranged in a circle around an opening and
act as sphincters to close the opening. Examples of circular muscles
are those that surround the eyes, called the orbicularis oculi, and
those that surround the mouth, called the orbicularis oris.
Convergent muscles have fascicles that arrive at one common
tendon from a wide area, creating muscles that are triangular in
shape. Having fibers that lie side by side can result in muscles with