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Answer & Explanation:Part A:What is motor unit and why do they vary in size?Part B: First describe the common action, using the correct terms from the descriptions in figure 11.2 in your book. Then find out what the origins and insertions of the muscles are and figure out how the two complement each other in performing the particular movement you have identified that they accomplish in common. In other words, how do their motion and relative effect differ? Finally, try to explain why you need both of them. Do not concern yourself about other things that either of the muscles do, just those that affect the common action.The two muscles: Teres major/latissimus dorsiChapter 11 – The Muscular System.pdf
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CHAPTER 11 | THE MUSCULAR SYSTEM
“The Muscular System” from Anatomy and Physiology by OpenStax College is available under a
Creative Commons Attribution 3.0 Unported license. © 2013, Rice University.
11 | THE MUSCULAR
SYSTEM
Figure 11.1 A Body in Motion The muscular system allows us to move, flex and contort our bodies. Practicing yoga,
as pictured here, is a good example of the voluntary use of the muscular system. (credit: Dmitry Yanchylenko)

Introduction
Chapter Objectives
After studying this chapter, you will be able to:
•
•
•
•
•
Describe the actions and roles of agonists and antagonists
Explain the structure and organization of muscle fascicles and their role in generating force
Explain the criteria used to name skeletal muscles
Identify the skeletal muscles and their actions on the skeleton and soft tissues of the body
Identify the origins and insertions of skeletal muscles and the prime movements
Think about the things that you do each day—talking, walking, sitting, standing, and running—all of these activities require
movement of particular skeletal muscles. Skeletal muscles are even used during sleep. The diaphragm is a sheet of skeletal
muscle that has to contract and relax for you to breathe day and night. If you recall from your study of the skeletal system
and joints, body movement occurs around the joints in the body. The focus of this chapter is on skeletal muscle organization.
The system to name skeletal muscles will be explained; in some cases, the muscle is named by its shape, and in other cases
it is named by its location or attachments to the skeleton. If you understand the meaning of the name of the muscle, often it
will help you remember its location and/or what it does. This chapter also will describe how skeletal muscles are arranged to
accomplish movement, and how other muscles may assist, or be arranged on the skeleton to resist or carry out the opposite
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movement. The actions of the skeletal muscles will be covered in a regional manner, working from the head down to the
toes.
11.1 | Interactions of Skeletal Muscles, Their Fascicle
Arrangement, and Their Lever Systems
By the end of this section, you will be able to:
• Compare and contrast agonist and antagonist muscles
• Describe how fascicles are arranged within a skeletal muscle
• Explain the major events of a skeletal muscle contraction within a muscle in generating force
To move the skeleton, the tension created by the contraction of the fibers in most skeletal muscles is transferred to
the tendons. The tendons are strong bands of dense, regular connective tissue that connect muscles to bones. The bone
connection is why this muscle tissue is called skeletal muscle.
Interactions of Skeletal Muscles in the Body
To pull on a bone, that is, to change the angle at its synovial joint, which essentially moves the skeleton, a skeletal muscle
must also be attached to a fixed part of the skeleton. The moveable end of the muscle that attaches to the bone being pulled
is called the muscle’s insertion, and the end of the muscle attached to a fixed (stabilized) bone is called the origin. During
forearm flexion—bending the elbow—the brachioradialis assists the brachialis.
Although a number of muscles may be involved in an action, the principal muscle involved is called the prime mover,
or agonist. To lift a cup, a muscle called the biceps brachii is actually the prime mover; however, because it can be assisted
by the brachialis, the brachialis is called a synergist in this action (Figure 11.2). A synergist can also be a fixator that
stabilizes the bone that is the attachment for the prime mover’s origin.
Figure 11.2 Prime Movers and Synergists The biceps brachii flex the lower arm. The brachoradialis, in the forearm,
and brachialis, located deep to the biceps in the upper arm, are both synergists that aid in this motion.
A muscle with the opposite action of the prime mover is called an antagonist. Antagonists play two important roles in
muscle function: (1) they maintain body or limb position, such as holding the arm out or standing erect; and (2) they control
rapid movement, as in shadow boxing without landing a punch or the ability to check the motion of a limb.
For example, to extend the knee, a group of four muscles called the quadriceps femoris in the anterior compartment of
the thigh are activated (and would be called the agonists of knee extension). However, to flex the knee joint, an opposite or
antagonistic set of muscles called the hamstrings is activated.
As you can see, these terms would also be reversed for the opposing action. If you consider the first action as the knee
bending, the hamstrings would be called the agonists and the quadriceps femoris would then be called the antagonists. See
Table 11.1 for a list of some agonists and antagonists.
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CHAPTER 11 | THE MUSCULAR SYSTEM
Agonist and Antagonist Skeletal Muscle Pairs
Agonist
Antagonist
Triceps brachii: in the
Biceps brachii: in the anterior
posterior compartment
compartment of the arm
of the arm
Movement
The biceps brachii flexes the forearm, whereas the
triceps brachii extends it.
Hamstrings: group of three
muscles in the posterior
compartment of the thigh
Quadriceps femoris:
group of four muscles
in the anterior
compartment of the
thigh
The hamstrings flex the leg, whereas the quadriceps
femoris extend it.
Flexor digitorum superficialis
and flexor digitorum
profundus: in the anterior
compartment of the forearm
Extensor digitorum: in
the posterior
compartment of the
forearm
The flexor digitorum superficialis and flexor digitorum
profundus flex the fingers and the hand at the wrist,
whereas the extensor digitorum extends the fingers
and the hand at the wrist.
Table 11.1
There are also skeletal muscles that do not pull against the skeleton for movements. For example, there are the muscles
that produce facial expressions. The insertions and origins of facial muscles are in the skin, so that certain individual
muscles contract to form a smile or frown, form sounds or words, and raise the eyebrows. There also are skeletal muscles
in the tongue, and the external urinary and anal sphincters that allow for voluntary regulation of urination and defecation,
respectively. In addition, the diaphragm contracts and relaxes to change the volume of the pleural cavities but it does not
move the skeleton to do this.
Exercise and Stretching
When exercising, it is important to first warm up the muscles. Stretching pulls on the muscle fibers and it also results
in an increased blood flow to the muscles being worked. Without a proper warm-up, it is possible that you may either
damage some of the muscle fibers or pull a tendon. A pulled tendon, regardless of location, results in pain, swelling,
and diminished function; if it is moderate to severe, the injury could immobilize you for an extended period.
Recall the discussion about muscles crossing joints to create movement. Most of the joints you use during exercise
are synovial joints, which have synovial fluid in the joint space between two bones. Exercise and stretching may also
have a beneficial effect on synovial joints. Synovial fluid is a thin, but viscous film with the consistency of egg whites.
When you first get up and start moving, your joints feel stiff for a number of reasons. After proper stretching and
warm-up, the synovial fluid may become less viscous, allowing for better joint function.
Patterns of Fascicle Organization
Skeletal muscle is enclosed in connective tissue scaffolding at three levels. Each muscle fiber (cell) is covered by
endomysium and the entire muscle is covered by epimysium. When a group of muscle fibers is “bundled” as a unit within
the whole muscle by an additional covering of a connective tissue called perimysium, that bundled group of muscle fibers
is called a fascicle. Fascicle arrangement by perimysia is correlated to the force generated by a muscle; it also affects the
range of motion of the muscle. Based on the patterns of fascicle arrangement, skeletal muscles can be classified in several
ways. What follows are the most common fascicle arrangements.
Parallel muscles have fascicles that are arranged in the same direction as the long axis of the muscle (Figure 11.3). The
majority of skeletal muscles in the body have this type of organization. Some parallel muscles are flat sheets that expand at
the ends to make broad attachments. Other parallel muscles are rotund with tendons at one or both ends. Muscles that seem
to be plump have a large mass of tissue located in the middle of the muscle, between the insertion and the origin, which is
known as the central body. A more common name for this muscle is belly. When a muscle contracts, the contractile fibers
shorten it to an even larger bulge. For example, extend and then flex your biceps brachii muscle; the large, middle section
is the belly (Figure 11.4). When a parallel muscle has a central, large belly that is spindle-shaped, meaning it tapers as it
extends to its origin and insertion, it sometimes is called fusiform.
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Figure 11.3 Muscle Shapes and Fiber Alignment The skeletal muscles of the body typically come in seven different
general shapes.
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CHAPTER 11 | THE MUSCULAR SYSTEM
Figure 11.4 Biceps Brachii Muscle Contraction The large mass at the center of a muscle is called the belly.
Tendons emerge from both ends of the belly and connect the muscle to the bones, allowing the skeleton to move. The
tendons of the bicep connect to the upper arm and the forearm. (credit: Victoria Garcia)
Circular muscles are also called sphincters (see Figure 11.3). When they relax, the sphincters’ concentrically arranged
bundles of muscle fibers increase the size of the opening, and when they contract, the size of the opening shrinks to the point
of closure. The orbicularis oris muscle is a circular muscle that goes around the mouth. When it contracts, the oral opening
becomes smaller, as when puckering the lips for whistling. Another example is the orbicularis oculi, one of which surrounds
each eye. Consider, for example, the names of the two orbicularis muscles (orbicularis oris and oribicularis oculi), where
part of the first name of both muscles is the same. The first part of orbicularis, orb (orb = “circular”), is a reference to a
round or circular structure; it may also make one think of orbit, such as the moon’s path around the earth. The word oris
(oris = “oral”) refers to the oral cavity, or the mouth. The word oculi (ocular = “eye”) refers to the eye.
There are other muscles throughout the body named by their shape or location. The deltoid is a large, triangular-shaped
muscle that covers the shoulder. It is so-named because the Greek letter delta looks like a triangle. The rectus abdomis
(rector = “straight”) is the straight muscle in the anterior wall of the abdomen, while the rectus femoris is the straight muscle
in the anterior compartment of the thigh.
When a muscle has a widespread expansion over a sizable area, but then the fascicles come to a single, common
attachment point, the muscle is called convergent. The attachment point for a convergent muscle could be a tendon, an
aponeurosis (a flat, broad tendon), or a raphe (a very slender tendon). The large muscle on the chest, the pectoralis major, is
an example of a convergent muscle because it converges on the greater tubercle of the humerus via a tendon. The temporalis
muscle of the cranium is another.
Pennate muscles (penna = “feathers”) blend into a tendon that runs through the central region of the muscle for its
whole length, somewhat like the quill of a feather with the muscle arranged similar to the feathers. Due to this design,
the muscle fibers in a pennate muscle can only pull at an angle, and as a result, contracting pennate muscles do not move
their tendons very far. However, because a pennate muscle generally can hold more muscle fibers within it, it can produce
relatively more tension for its size. There are three subtypes of pennate muscles.
In a unipennate muscle, the fascicles are located on one side of the tendon. The extensor digitorum of the forearm
is an example of a unipennate muscle. A bipennate muscle has fascicles on both sides of the tendon. In some pennate
muscles, the muscle fibers wrap around the tendon, sometimes forming individual fascicles in the process. This arrangement
is referred to as multipennate. A common example is the deltoid muscle of the shoulder, which covers the shoulder but has
a single tendon that inserts on the deltoid tuberosity of the humerus.
Because of fascicles, a portion of a multipennate muscle like the deltoid can be stimulated by the nervous system to
change the direction of the pull. For example, when the deltoid muscle contracts, the arm abducts (moves away from midline
in the sagittal plane), but when only the anterior fascicle is stimulated, the arm will abduct and flex (move anteriorly at the
shoulder joint).
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The Lever System of Muscle and Bone Interactions
Skeletal muscles do not work by themselves. Muscles are arranged in pairs based on their functions. For muscles attached to
the bones of the skeleton, the connection determines the force, speed, and range of movement. These characteristics depend
on each other and can explain the general organization of the muscular and skeletal systems.
The skeleton and muscles act together to move the body. Have you ever used the back of a hammer to remove a nail
from wood? The handle acts as a lever and the head of the hammer acts as a fulcrum, the fixed point that the force is applied
to when you pull back or push down on the handle. The effort applied to this system is the pulling or pushing on the handle
to remove the nail, which is the load, or “resistance” to the movement of the handle in the system. Our musculoskeletal
system works in a similar manner, with bones being stiff levers and the articular endings of the bones—encased in synovial
joints—acting as fulcrums. The load would be an object being lifted or any resistance to a movement (your head is a load
when you are lifting it), and the effort, or applied force, comes from contracting skeletal muscle.
11.2 | Naming Skeletal Muscles
By the end of this section, you will be able to:
• Describe the criteria used to name skeletal muscles
• Explain how understanding the muscle names helps describe shapes, location, and actions of various muscles
The Greeks and Romans conducted the first studies done on the human body in Western culture. The educated class of
subsequent societies studied Latin and Greek, and therefore the early pioneers of anatomy continued to apply Latin and
Greek terminology or roots when they named the skeletal muscles. The large number of muscles in the body and unfamiliar
words can make learning the names of the muscles in the body seem daunting, but understanding the etymology can help.
Etymology is the study of how the root of a particular word entered a language and how the use of the word evolved over
time. Taking the time to learn the root of the words is crucial to understanding the vocabulary of anatomy and physiology.
When you understand the names of muscles it will help you remember where the muscles are located and what they do
(Figure 11.5, Figure 11.6, and Table 11.2). Pronunciation of words and terms will take a bit of time to master, but after you
have some basic information; the correct names and pronunciations will become easier.
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CHAPTER 11 | THE MUSCULAR SYSTEM
Figure 11.5 Overview of the Muscular System On the anterior and posterior views of the muscular system
above, superficial muscles (those at the surface) are shown on the right side of the body while deep muscles (those
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underneath the superficial muscles) are shown on the left half of the body. For the legs, superficial muscles are shown
in the anterior view while the posterior view shows both superficial and deep muscles.
Figure 11.6 Understanding a Muscle Name from the Latin
Mnemonic Device for Latin Roots
Latin or Greek
Translation
Example
Mnemonic Device
ad
to; toward
ADvance toward your goal
ab
away from
n/a
sub
under
SUBmarines move under water.
ductor
something that moves
A conDUCTOR makes a train move.
anti
against
If you are antisocial, you are against engaging in social
activities.
epi
on top of
n/a
apo
to the side of
n/a
longissimus
longest
“Longissimus” is longer than the word “long.”
longus
long
long
brevis
short
brief
maximus
large
max
medius
medium
“Medius” and “medium” both begin with “med.”
minimus
tiny; little
mini
rectus
straight
To RECTify a situation is to straighten it out.
multi
many
If something is MULTIcolored, it has many colors.
uni
one
A UNIcorn has one horn.
bi/di
two
If a ring is DIcast, it is made of two metals.
tri
three
TRIple the amount of money is three times as much.
quad
four
QUADruplets are four children born at one birth.
externus
outside
EXternal
internus
inside
INternal
Table 11.2
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CHAPTER 11 | THE MUSCULAR SYSTEM
Anatomists name the skeletal muscles according to a number of criteria, each of which describes the muscle in some
way. These include naming the muscle after its shape, its size compared to other muscles in the area, its location in the body
or the location of its attachments to the skeleton, how many origins it has, or its action.
The skeletal muscle’s anatomical location or its relationship to a particular bone often determines its name. For
example, the frontalis muscle is located on top of the frontal bone of the skull. Similarly, the shapes of some muscles are
very distinctive and the names, such as orbicularis, reflect the shape. For the buttocks, the size of the muscles influences
the names: gluteus maximus (largest), gluteus medius (medium), and the gluteus minimus (smallest). Names were given
to indicate length— brevis (short), longus (long)—and to identify position relative to the midline: lateralis (to the outside
away from the midline), and medialis (toward the midline). The direction of the muscle fibers and fascicles are used to
describe muscles relative to the midline, such as the rectus (straight) abdominis, or the oblique (at an angle) muscles of the
abdomen.
Some muscle names indicate the number of muscles in a group. One example of this is the quadriceps, a group of
four muscles located on the anterior (front) thigh. Other muscle names can provide information as to how many origins a
particular muscle has, such as the biceps brachii. The prefix bi indicates that the muscle has two origins and tri indicates
three origins.
The location of a muscle’s attachment can also appear in its name. When the name of a muscle is based on the
attachments, the origin is always named first. For instance, the sternocleidomastoid muscle of the neck has a dual origin
on the sternum (sterno) and clavicle (cleido), and it inserts on the mastoid process of the temporal bone. The last feature
by which to name a muscle is its action. When muscles are named for the movement they produce, one can find action
words in their name. Some examples are flexor (decreases the angle at the joint), extensor (increases th …
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