5
Skeletal Muscles Are Grouped Based on
Location and Action 168
• Skeletal muscles are attached to bones with tendons. The
attachment of a muscle to a stationary bone is its origin, the
attachment to a movable bone is its insertion, and the part
of the muscle in between is the belly, as shown.
• Muscles usually work in pairs that have opposing actions.
The muscle acting as the agonist (prime mover) contracts
to produce the desired movement, and the antagonist
relaxes. The synergist works with the prime mover to cause
the action and help reduce unnecessary movement, and
the fixator stabilizes the origin of the prime mover so it can
work more efficiently.
• The principal skeletal muscles are grouped according to
region. Their names indicate specific characteristics, which
are based on descriptive categories such as direction of
fibers, location, size, number of origins, shape, origin and
insertion points, and action.
Movable bones
Key Terms
actin 169
• intercalated disc 167
• sarcoplasmic reticulum 158
aerobic respiration 163
• intermediate filament 165
• skeletal muscles 154
agonist 169
• isometric contraction 164
• slow oxidative (SO) fiber 165
anaerobic respiration 163
• isotonic contraction 164
• smooth muscle 155
antagonist 169
• lactic acid 163
• striated 154
autorhythmic 167
• latent period 163
• synergist 169
belly 168
• motor unit 164
• temporal summation 164
cardiac muscle tissue 155
• multi-unit smooth muscle tissue 166
• tetanus 164
contraction period 163
• muscle tone 164
• thick filament 158
creatine phosphate 163
• myofibril 156
• thin filament 158
dense body 165
• myofilament 158
• transverse tubule (T-tubule) 158
depolarization 160
• myoglobin 163
• tropomyosin 169
fascicle 157
• myosin 169
• troponin 169
fast glycolytic (FG) fiber 165
• neuromuscular junction 160
• twitch 163
fast oxidative-glycolytic (FOG) fiber 165
• origin 168
• visceral (single-unit) muscle tissue 166
fixator 169
• prime mover 169
• wave summation 164
glycogen 163
• relaxation period 163
glycolysis 163
• repolarization 160
insertion 168
• sarcomere 156
Relationships of muscles to bones • Figure 6.12
Tendon
INSERTION
of triceps
brachii on
ulna
Elbow joint
Ulna
Shoulder joint
Scapula
ORIGIN of
triceps brachii
from scapula
and humerus
BELLY
of triceps
brachii
muscle
ORIGIN of
biceps brachii
from scapula
Tendons
BELLY
of biceps
brachii
muscle
STATIONARY
BONE - For
movement of the
arm, this is the
humerus.
Tendon
INSERTION
of biceps
brachii on
radius
Radius
Key Terms 187
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