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Chapter 8
The Muscular
System
I. Introduction
A. There are over 600
skeletal muscles in the
body.
B. 40-50% of your body
weight is skeletal muscle
C. Muscles, along with the
skeleton, determine the
form and contour of the
body
QUESTION….
Two people wear the exact same
size in clothing. One of the two has
15% body fat (meaning they have
85% muscle) and the other person
has 29% body fat. Who weighs
more?
 The person with the larger amount of muscle
percentage will weight more. Muscle density
is 1.06 g/ml and fat density is (about) 0.9
g/ml. Thus, one liter of muscle would weight
1.06 kg and one liter of fat would weight 0.9
kg. In other words, muscle is about 18%
more dense than fat.
D. Skeletal Muscles are organs
E. Function of muscles
 1. Movement
 2. Heat production
 3. Posture
II. How muscles are named
A. Muscles are named using one
of more of the following features
 1. Location (ie:subscapularis)
 2. Function (adductor muscle)
3. Shape (deltoid)
 4. Direction of the fibers or
muscle cells – (ie: “rectus” of
rectus abdominus means
“straight”)

5. Number of heads or
divisions (ie: biceps, triceps,
quadriceps)
 6. Size of the muscle (ie:
gluteus maximus, gluteus
medius, gluteus minimus)


7. Attachment of muscles
—point of attachment that
does not move when muscle
contracts
Insertion —point of attachment that
moves when muscle contracts
Origin
Location
 Brachialis
 Femoris
 Abdominal
 Dorsi
 Capitis
Slide 11
•
•
•
•
•
Arm
Leg
Abdomen
Back
Head
Function
 Adductor
 Abductor
 Supinator
 Flexor
 Extensor
•
•
•
•
•
Adducts
Abducts
Supinates
Flexes
Extends
Slide 12
Shape
 Deltoid
 Linea
 Trapezius
• Triangle
• Line
• trapezoid
Slide 13
Number of Heads
 Biceps
 Triceps
 Quadriceps
• Two
• Three
• Four
Slide 14
Direction of Fibers
 Oblique
 Rectus
 Transverse
 Circular
 Spiral
•
•
•
•
•
Diagonal
Straight
Transverse
Around
Spiral
Slide 15
Points of Attachment
Examples:
 sternocleidomastoid
• Sternum
• Clavicle
• Mastoid processes of the
temporal bone
Slide 16
Relative Size
 Major
 Maximus
 Minor
 Minimus
 Longus
 Brevis
 Latissimus
 Longissimus
 Magnus
 Vastus
Slide 17
•
•
•
•
•
•
•
•
•
•
Large
Largest
Small
Smallest
Long
Short
Very wide
Very long
Very large
Vast or huge
Examples:
 Flexor carpi radialis
 Brachioradialis
 Three headed arm muscle
 Straight upper leg muscle
 Flexor pollicus longus
•
•
•
•
•
Function/insertion/origin
Location/insertion
Triceps brachii
Rectus femoris
Long muscle that flexes the
thumb
 Vast side muscle of upper
leg
• Vastus lateralis
 Adductor brevis
• Short muscle that adducts the
leg
 Adductor magnus
• Largest muscle that adducts
the leg
Slide 18
III. Important
Skeletal Muscles
A. Muscles of facial
expression
 —unique in that at least one point of
attachment is to the deep layers of the skin
over the face or neck
1. Corrugator supercilii (AKA:
frontal muscle) – wrinkles
forehead when frowning, and
lifts eyebrows in surprise
2. Orbicularis oculi – encircles
the eye and closes it
3. Zygomaticus major – draws
the corner of the mouth
upward (smiling or laughing)
4. Obicularis oris – puckers the
mouth and presses the teeth to
inside cheek (commonly referred
to as the “kissing muscle”)
5. Buccinator – works with the
above muscle to produce the
kissing motion
B. Muscles of Mastication
(chewing)
1. Masseter – closes jaw
2. Temporalis – this works with
the above muscle to close the
jaw
3. Pterygoids – grates teeth
C. Muscles that move the head
 Muscles that move the head —paired
muscles on either side of the neck are
responsible for head movements




1. Sternocleidomastoid
2. Trapezius
3. Splenius capitis
4. Semispinalis capitis
C. Muscles of the Thorax
1. External intercostals –
elevated ribs for inhalation
2. Internal intercostals –
depresses ribs for exhalation
3. Diaphragm – enlarges thorax
causing inspiration. Singers train
this muscle so the viscera is pushing
down which gives the lungs more
room to expand. The can hold notes
longer and have to inhale less often.
D. Muscles of the Abdominal Wall
1. External obliques – outermost
layer
2. Internal obliques – middle layer
3. Transversus abdominus –
innermost layer
4. Rectus abdominus – midline
of the abdomen. “6 pack,
washboard stomach” Muscles
used to “push” in childbirth.
Aponeurosis
–layers of flat,
broad tendons
E. Muscles of the Back
 Muscles of the back —bend or stabilize the
back
 Allow for rotation
2 “cap” muscles
3 muscles that
move the scapula
4 muscles on
the scapula
2 serratus muscles
1 errector muscle
2 gluteal muscles
E. Muscles of the Upper Limb
1. Deltoid
2. Rotator cuff muscles (SITS
muscles):
 a. Supraspinatus
 b. Infraspinatus
 c. Teres minor
 d. Subscapularis
3. Pectoralis major
4. Opponens pollicus – thumb
muscles which allow thumb to be
drawn across the palm
5. Biceps brachii
6. Triceps brachii
F. Muscles of the Lower Limb
1. Gluteal group – posterior
buttocks
 a. Gluteus maximus
 b. Gluteus medius
 c. Gluteus minimus
 2.
 3.
 4.
 5.
Quadriceps group – anterior thigh
Hamstring group – posterior thigh
Gastrocnemius – calf muscles
Soleus – underneath gastrocnemius
IV. Other Muscles
“Involuntary
Muscles”
A. Cardiac
 1. The heart
 2. It is extremely striated with a series of
intercalated discs…..these are unique dark
bands where the plasma membrane of other
cardiac fibers come in contact with each
other. The nature of the fibers helps the heart
contract as a unit.
B. Smooth
 1. These form the muscles of the hollow
organs (such as the intestines)
 2. The cells of smooth muscles are tapered at
each end and have a single nucleus. They
have no striations which gives them the
appearance of being “smooth”.
V. Muscle Fiber Types
Each muscle contains both
types of muscle fibers in
varying proportions
A. Slow (red) fibers
 1. These contain large amounts of
myoglobin (oxygen storage molecules),
which give it its red color (red meat vs.
white meat in animals).
 2. “Slow” means they are slow to
fatigue
 3. Postural muscles that hold up a skeleton
for prolonged periods have greater amounts
of slow red fibers.
 4. Studies show that marathon runners have
a greater amount of red fibers than the
general population
B. Fast (white) fibers
 1. Muscle fibers that contain little
myoglobin (hence the word white)
 2. These muscles contract rapidly
 3. But the price of rapid contraction is rapid
depletion of ATP.
 4. Sprinters have a greater amount of white
fibers than the general population
VI. Muscle Contraction
A. Requires Energy
B. Comes from catabolism
 Requires these two ingredients
 1. Glucose (from food) – quick burst
fuel needed for muscle contraction
 2. Oxygen
a. Aerobic respiration – produces
maximum amount of energy using
oxygen
Anaerobic respiration – the process
that occurs when oxygen level is low.
Lactic acid builds up in the muscle
tissue during exercise and causes a
burning sensation…..then remains in
the muscle post-exercise and makes
muscles sore.
b.
VII. Muscle Fatigue
Loss of strength or
endurance
A. Physiological fatigue
1. Caused by a relative lack of
ATP due to:
 a. Depletion of oxygen or
glucose in the muscle fibers
 b. Inability to regenerate ATP
quick enough
c. Depletion of glycogen
 d. High levels of lactic acid
 e. Physiological fatigue rarely
happens – and if it does, the body
will just “shut” itself off

B. Psychological fatigue
1. This is what produces the
“exhausted” feeling
2. This will make you want to stop
muscle activity long before the
muscle has reached fatigue
C. Mind over matter
1. In physiological fatigue, we
cannot contract our muscles any
longer.
2. In psychological fatigue, we
simply will not contract our muscles
because we feel tired.
VIII. Problems
Frequently
Associated with
Muscles
A. Shoulder Joint Stability
1. The head of your humerus is
twice the size of the scapula cavity
that it rests in, which makes for
great mobility (that it why it is the
most mobile joint in the body)
2. This is also the reason it so
easily dislocates (it is very unstable)
3. The SITS muscles help to
strengthen this area
4. The muscles and tendons
in this area fuse and are called
the “rotator cuff”
B. Repetitive motions
1. Piano playing, meat cutting,
typing (secretaries)
2. This causes inflammation of
the tendon sheath that surrounds
the tendons (like in the wrist)
3. If the inflammation causes
compression on the median nerve in
the tunnel of the wrist, carpal tunnel
syndrome may result.
4. This produces tingling, muscle
weakness, and pain in the radial
side of the hand.
IX. Myopathies
Problems within
the muscles
A. Muscular Distrophy
 1. This is an X linked genetic
disorder…..so it effects boys more than
girls
 2. The first sign appears at age 3 with leg
weakness
 3. By age 5-10, it is severe and usually
fatal by the age of 21 die to heart
weakness and respiratory muscles
(Jerry’s Kids)
B. Hernias
1. Weakness in the abdominal
muscle so that abdominal organ
protrudes
2. If it becomes strangulated then
the mass cannot manipulate itself
back and blood flow is stopped to
that area.
3. At worst, gangrene can
occur
4. Usually requires
immediate surgical
intervention
X. Muscle Terms
A. Intramuscular injections
1. Shots given inside the muscle
2. If less than 2 ml – given in
deltoid
3. If greater than 2 ml – given in
gluteus maximus
B. Atrophy
1. Shrinkage of muscle mass
2. Usually caused by disease
or casting a bone break
C. Hypertrophy
1. Increase in muscle size
D. Peristalsis
1. The waves of action of
smooth muscle that cause the
movement of food down the
alimentary canal
E. Rigor mortis
1. Latin for “stiffness of death”
2. Skeletal muscles often stiffen
shortly after death
3. This is because at the time of
death the postural muscle fibers are
often in mid contraction.
4. But when death occurs, the
muscles cease to be stimulated so
they become stuck in the contracted
position.
5. Basically muscle fibers run out
of ATP required to turn off the
muscle contraction and relax it
F. Treppe Staircase Phenomenon
 1. Muscles contract more forcefully
after they have contracted a few times
(which is the reason you “warm up”
before activity)
 2. It works on the principle of
increasing the temperature of the muscle
to get the maximum amount of work out
of it
G. Muscle Tone
1. Continuous low level sustained
contraction by skeletal muscles
(ie:posture)
2. Less muscle tone is called
flaccid, more muscle tone than
normal is known as spastic
(disorder: movie – Awakenings)
H. Cramps
1. Painful muscle spasms
2. Can be symptoms of ion or
water imbalance (ie: leg
cramps)
I. Convulsions
1. Abnormal uncoordinated
muscular contractions
2. May result from seizure
J. Fibrillation
1. Flutter of muscle but no
movement
2. Happens in the heart
muscle during “heart attack”
K. Myopathies
1. Muscle disorders
L. Myoalgia
1. Muscle pain
M. Muscle strain
1. Overstretching or tearing
of muscle fibers
N. Muscle sprain
1. Muscle strain that occurs in
the area of a joint or a ligament
2. Takes weeks to heal because
soft tissue heals slower that hard
tissue (like bone)
Ankle Sprain
O. Myositis
1. Muscle inflammation
2. Subside within a few hours or
days
3. Could be caused by bacteria
in the system (like the flu) – that
is why you are “sore” when you
are sick with the flu
P. Myositis + tendon
inflammation
AKA: Charley horse