Chapter 10
THE MUSCULAR SYSTEM
I. INTRODUCTION
A. The muscular system specifically concerns skeletal muscles and associated connective tissue that
make individual muscle organs.
B. This chapter discusses how skeletal muscles produce movement and describes the principal skeletal
muscles.
II. HOW SKELETAL MUSCLES PRODUCE MOVEMENT
A. Muscle Attachment Sites: Origin and Insertion
1. Skeletal muscles produce movements by exerting force on tendons, which in turn pull on
bones or other structures, such as skin.
2. Most muscles cross at least one joint and are attached to the articulating bones that form the
joint.
3. When such a muscle contracts, it draws one articulating bone toward the other.
a. The attachment to the stationary bone is the origin.
b. The attachment to the movable bone is the insertion.
B. General Principles
A. Tendons: Attach muscles to bones
1. Aponeurosis: A very broad tendon
B. Muscles
1. Origin or head: Muscle end attached to more stationary of two bones
2. Insertion: Muscle end attached to bone with greatest movement
3. Belly: Largest portion of the muscle between origin and insertion
4. Synergists: Muscles that work together to cause a movement
5. Prime mover (agonist): Plays major role in accomplishing movement
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6. Antagonist: A muscle working in opposition to agonist
7. Fixators: Stabilize joint/s crossed by the prime mover
C. Coordinated Action of Muscle Groups
1. The movement of a muscle is its action; muscles seldom act alone.
2. The prime mover (agonist) is the muscle producing the most force.
3. A synergist is a muscle that aids the prime mover.
4. The antagonist is a muscle that opposes the prime mover; an antagonistic pair of muscles act on
opposite sides of a joint.
5. A fixator is a muscle that prevents the movement of a bone.
B. Lever Systems and Leverage
1. Bones serve as levers and joints serve as fulcrums.
2. The lever is acted on by two different forces: resistance (load) and effort.
3. Levers are categorized into three types – first-class (ERF), second-class (FRE), and thirdclass (FER) – according to the position of the fulcrum, effort, and resistance on the lever.
C. Leverage, the mechanical advantage gained by a lever, is largely responsible for a muscle’s strength
and range of motion (ROM), i.e., the maximum ability to move the bones of a joint through an arc.
Levers and Biomechanics of the Joints
1. A lever is an elongated, rigid object that rotates around a fixed point called the fulcrum.
2. The function of a lever is to confer an advantage.
3. When an effort applied to one point on the lever overcomes a resistance at some other
point, rotation occurs.
4. The part of a lever from the fulcrum to the point of effort is called the effort arm, and
the part from the fulcrum to the point of resistance is the resistance arm.
5. The mechanical advantage of a lever is the ratio of its output force to its input force.
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6. In a first-class lever, the fulcrum is in the middle; in a second-class lever, the resistance
is in the middle; and in a third-class lever, the effort is applied between the fulcrum and the
resistance.
7. Muscle acts on rigid rod (bone) that moves around a fixed point called a fulcrum
8. Resistance (load) is weight of body part & perhaps an object
9. Effort or load is work done by muscle contraction
10. Mechanical advantage
a. the muscle whose attachment is farther from the joint will produce the most force
b. the muscle attaching closer to the joint has the greater range of motion and the
faster the speed it can produce
D. Classes of Lever Systems
1. First class - Fulcrum between load and effort
a. Can produce mechanical advantage or not depending on location of effort & resistance
b. if effort is further from fulcrum than resistance, then a strong resistance can be
moved
c. Head resting on vertebral column
1) weight of face is the resistance
2) joint between skull & atlas is fulcrum
3) posterior neck muscles provide effort
2. Second class - Load between fulcrum and effort
a. Similar to a wheelbarrow
b. Always produce mechanical advantage
– resistance is always closer to fulcrum than the effort
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Sacrifice of speed for force
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Raising up on your toes
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– resistance is body weight
– fulcrum is ball of foot
– effort is contraction of calf muscles which pull heel up off of floor
3. Third class - Effort applied between fulcrum and load
E. Effects of Fascicle Arrangement
1. Skeletal muscle fibers (cells) are arranged within the muscle in bundles called fasciculi.
2. The muscle fibers are arranged in a parallel fashion within each bundle, but the arrangement
of the fasciculi with respect to the tendons may take one of four characteristic patterns:
parallel, fusiform, pennate, and circular.
3. Fascicular arrangement is correlated with the power of a muscle and the range of motion.
D. Coordination Within Muscle Groups
1. Most movements are coordinated by several skeletal muscles acting in groups rather than
individually, and most skeletal muscles are arranged in opposing (antagonistic) pairs at
joints.
2. A muscle that causes a desired action is referred to as the prime mover (agonist); the
antagonist produces an opposite action.
3. Most movements also involve muscles called synergists, which serve to steady a movement,
thus preventing unwanted movements and helping the prime mover function more efficiently.
4. Some synergist muscles in a group also act as fixators, which stabilize the origin of the prime
mover so that it can act more efficiently.
5. Under different conditions and depending on the movement and which point is fixed, many
muscles act, at various times, as prime movers, antagonists, synergists, or fixators.
III. HOW SKELETAL MUSCLES ARE NAMED
A. The names of most of the nearly 700 skeletal muscles are based on several types of characteristics.
B. These characteristics may be reflected in the name of the muscle.
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C. 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.
IV. PRINCIPLE SKELETAL MUSCLES
V. DISORDERS: HOMEOSTATIC IMBALANCES
A. Running Injuries
1. Most running injuries involve the knee. Other commonly injured sites are the calcaneal
(Achilles) tendon, medial aspect of the tibia, hip area, groin area, foot and ankle, and back.
2. Running injuries are frequently related to faulty training techniques.
3. Running injuries can be treated initially (first 2-3 days) with rest, ice, compression, and
elevation (RICE therapy). Alternating moist heat and ice massage may be used as a followup treatment. Sometimes, nonsteroidal anti-inflammatory drugs (NSAIDS) or local injections
of corticosteroids are needed; an alternate fitness program is necessary to keep active during
the recovery period followed by careful rehabilitative exercise.
B. Compartment Syndrome
1. Skeletal muscles in the limbs are organized in units called compartments.
2. In compartment syndrome, some external or internal pressure constricts the structures within
a compartment, resulting in damaged blood vessels and subsequent reduction of the blood
supply to the structures within the compartment.
3. Without intervention, nerves suffer damage, and muscle develop scar tissue that results in
permanent shortening of the muscles, a condition called contracture.
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