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Chapter 11
The Muscular System
HOW SKELETAL MUSCLES PRODUCE
MOVEMENT
Principles of Human Anatomy and Physiology, 11e
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Muscle
Attachment
Sites:
Origin and
Insertion
• Skeletal muscles shorten & pull on the bones they are attached to
• Origin is the bone that does not move when muscle shortens
(normally proximal)
• Insertion is the movable bone
• Fleshy portion of the muscle in between attachment sites = belly
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Lever Systems and Leverage
• Muscle acts on rigid rod (bone)
that moves around a
fixed point called a fulcrum
• Resistance is weight of body
part & perhaps an object
• Effort or load is work done
by muscle contraction
• Mechanical advantage
– the muscle whose attachment is farther from the joint
will produce the most force
– the muscle attaching closer to the joint has the greater
range of motion and the faster the speed it can
produce
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First - Class Lever
• Can produce mechanical
advantage or not depending on
location of effort & resistance
– if effort is further from fulcrum
than resistance, then a strong
resistance can be moved
• Head resting on vertebral column
– weight of face is the resistance
– joint between skull & atlas is
fulcrum
– posterior neck muscles provide
effort
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Second - Class Lever
• Similar to a wheelbarrow
• Always produce mechanical
advantage
– resistance is always closer to
fulcrum than the effort
• Sacrifice of speed for force
• Raising up on your toes
– resistance is body weight
– fulcrum is ball of foot
– effort is contraction of calf muscles
which pull heel up off of floor
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Third - Class Lever
• Most common levers in the body
• Always produce a mechanical
disadvantage
– effort is always closer to fulcrum than
resistance
• Favors speed and range of motion over
force
• Flexor muscles at the elbow
– resistance is weight in hand
– fulcrum is elbow joint
– effort is contraction of biceps brachii
muscle
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Coordination Within Muscle Groups
• Most movement is the result of several muscle working at the
same time
• Most muscles are arranged in opposing pairs at joints
– prime mover or agonist contracts to cause the desired
action
– antagonist stretches and yields to prime mover
– synergists contract to stabilize nearby joints
– fixators stabilize the origin of the prime mover
• scapula held steady so deltoid can raise arm
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HOW SKELETAL MUSCLES ARE NAMED
• The most important characteristics include the direction in
which the muscle fibers run, the size, shape, action,
numbers of origins, and location of the muscle, and the sites
of origin and insertion of the muscle
• Examples from Table 11.2
– triceps brachii -- 3 sites of origin
– quadratus femoris -- square shape
– serratus anterior -- saw-toothed edge
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Muscles of Facial Expression
• Orbicularis oculi closes
the eye
• Orbicularis oris puckers
the mouth
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Muscles that Move the Mandible
• Masseter, temporalis
Arise from skull & insert
on mandible
• Protracts, elevates or
retracts mandible
– Temporalis &
Masseter elevate the
mandible (biting)
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Muscles
that Move
the Head
• Sternocleidomastoid muscle
– arises from sternum & clavicle & inserts onto mastoid process of
skull
– contraction of one, laterally flexes the neck and rotates face in
opposite direction
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Muscles of Abdominal Wall
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Muscles of Abdominal Wall
• sheetlike muscles
– rectus abdominis = vertically oriented
– external & internal obliques
• wrap around body to form anterior body wall
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Muscles Used in Breathing
• Breathing requires a change in
size of the thorax
• During inspiration, thoracic
cavity increases in size
– external intercostal lift the
ribs
– diaphragm contracts &
dome is flattened
• During expiration, thoracic
cavity decreases in size
– internal intercostal used in
forced expiration
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Stabilizing the Pectoral Girdle
• Anterior thoracic muscles
– Subclavius extends from 1st
rib to clavicle
– Pectoralis minor extends from
ribs to coracoid process
– Serratus anterior extends
from ribs to inner surface of
scapula
• Posterior thoracic muscle
– Trapezius extends from skull
& vertebrae to clavicle &
scapula
– Rhomboideus extends from
thoracic vertebrae to vertebral
border of scapula
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Axial Muscles that Move the Arm
• Pectoralis major & Latissimus dorsi extend from body
wall to humerus.
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Muscles that Move the Arm
• Deltoid arises from acromion & spine of scapula & inserts on arm
– abducts, flexes & extends arm
• Rotator cuff muscles extend from scapula posterior to shoulder joint to
attach to the humerus
– supraspinatus & infraspinatus: above & below spine of scapula
– subscapularis on inner surface of scapula
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Flexors of the Forearm (elbow)
• Biceps brachii
– scapula to radial tuberosity
– flexes shoulder and elbow &
supinates hand
• Brachialis
– humerus to ulna
– flexion of elbow
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Extensors of the Forearm (elbow)
• Triceps brachii
– long head arises scapula
– medial & lateral heads
from humerus
– inserts on ulna
– extends elbow & shoulder
joints
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Muscle that Pronate & Flex
• Flexor carpi muscles
– radialis
– ulnaris
• Flexor digitorum
muscles
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Muscles that Supinate & Extend
• Extensors of wrist and
fingers
– extensor carpi
– extensor digitorum
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Muscles Crossing the Hip Joint
• Quadriceps femoris has 4 heads
– Rectus femoris crosses hip
– 3 heads arise from femur
• Vastus lateralis, intermedius
and medialis
– all act to extend the knee
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Adductor Muscles of the Thigh
• Adductor group of
muscle extends from
pelvis to linea aspera
on posterior surface
of femur
– adductor longus
– gracilis
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Muscles of the Butt & Thigh
• Gluteus muscles
– maximus, medius &
minimus
– maximus extends hip
– medius & minimus
abduct
• Hamstring muscles
– semimembranosus
(medial)
– semitendinosus (medial)
– biceps femoris (lateral)
– extend hip & flex knee
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Muscles of the Calf (posterior leg)
• muscles insert onto calcaneus
– gastrocnemius arises femur
• flexes knee and ankle
– soleus arise from leg
• flexes ankle
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Muscles of the Leg and Foot
• Anterior compartment of leg
– extensors of ankle & toes
• tibialis anterior
• extensor digitorum longus
• Shinsplints syndrome
– pain or soreness on anterior
tibia
– running on hard surfaces
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