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Bones and Skeletal Muscles Assignment

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Describe the relationship between bones and skeletal muscles in producing body movements.
Define lever and fulcrum, and compare the three types of levers based on location of the fulcrum,
effort, and load.
Identify the types of fascicle arrangements in a skeletal muscle, and relate the arrangements to
strength of contraction and range of motion.
Explain how the prime mover, antagonist, synergist, and fixator in a muscle group work together to
produce movement.
Muscle Attachment Sites: Origin and Insertion
Those skeletal muscles that produce movements do so by exerting force on tendons, which in turn pull on
bones or other structures (such as skin). Most muscles cross at least one joint and are usually attached to
articulating bones that form the joint (Figure 11.1a).
Figure 11.1
Relationship of skeletal muscles to bones.
Muscles are attached to bones by tendons at their origin and insertion. Skeletal muscles produce movements by
pulling on bones. Bones serve as levers, and joints act as fulcrums for the levers. Here the lever–fulcrum principle is
illustrated by the movement of the forearm. Note where the load (resistance) and effort are applied in (b).
In the limbs, the origin of a muscle is usually proximal and the insertion is usually distal.
John Gibb/Imagineering.
Where is the belly of the muscle that extends the forearm located?
When a skeletal muscle contracts, it pulls one of the articulating bones toward the other. The two articulating
bones usually do not move equally in response to contraction. One bone remains stationary or near its original
position, either because other muscles stabilize that bone by contracting and pulling it in the opposite direction
or because its structure makes it less movable. Ordinarily, the attachment of a muscle's tendon to the stationary
bone is called the origin (OR i jin); the attachment of the muscle's other tendon to the movable bone is called‐ ‐
the insertion (in SER shun). A good analogy is a spring on a door. In this example, the part of the spring
attached to the frame is the origin; the part attached to the door represents the insertion. A useful rule of thumb
is that the origin is usually proximal and the insertion distal; the insertion is usually pulled toward the origin.
The fleshy portion of the muscle between the tendons is called the belly (body), the coiled middle portion of
the spring in our example. The actions of a muscle are the main movements that occur when the muscle
contracts. In our spring example, this would be the closing of the door. Certain muscles are also capable
of reverse muscle action (RMA). This means that during specific movements of the body the actions are
reversed and therefore the positions of the origin and insertion of a specific muscle are switched.
Muscles that move a body part often do not cover the moving part. Figure 11.1b shows that although one of the
functions of the biceps brachii muscle is to move the forearm, the belly of the muscle lies over the humerus,
not over the forearm. You will also see that muscles that cross two joints, such as the rectus femoris and
sartorius of the thigh, have more complex actions than muscles that cross only one joint.
Anatomy Overview: Skeletal Muscle
Anatomy Overview: The Muscular System Cross Section of Skeletal Muscle
Lever Systems and Leverage
In producing movement, bones act as levers, and joints function as the fulcrums of these levers. A lever is a
rigid structure that can move around a fixed point called a fulcrum, symbolized by . A lever is acted on
at two different points by two different forces: the effort (E), which causes movement, and the load
or resistance, which opposes movement. The effort is the force exerted by muscular contraction; the load is
typically the weight of the body part that is moved or some resistance that the moving body part is trying to
overcome (such as the weight of a book you might be picking up). Motion occurs when the effort applied to the
bone at the insertion exceeds the load. Consider the biceps brachii flexing the forearm at the elbow as an object
is lifted (Figure 11.1b). When the forearm is raised, the elbow is the fulcrum. The weight of the forearm plus
the weight of the object in the hand is the load. The force of contraction of the biceps brachii pulling the
forearm up is the effort.
The relative distance between the fulcrum and load and the point at which the effort is applied determine
whether a given lever operates at a mechanical advantage or a mechanical disadvantage. For example, if the
load is closer to the fulcrum and the effort farther from the fulcrum, then only a relatively small effort is
required to move a large load over a small distance. This is called a mechanical advantage. If, instead, the
load is farther from the fulcrum and the effort is applied closer to the fulcrum, then a relatively large effort is
required to move a small load (but at greater speed). This is called a mechanical disadvantage. Compare
chewing something hard (the load) with your front teeth and the teeth in the back of your mouth. It is much
easier to crush the hard food item with the back teeth because they are closer to the fulcrum (the jaw or
temporomandibular joint) than are the front teeth. Here is one more example you can try. Straighten out a
paper clip. Now get a pair of scissors and try to cut the paper clip with the tip of the scissors (mechanical
disadvantage) versus near the pivot point of the scissors (mechanical advantage).
Levers are categorized into three types according to the positions of the fulcrum, the effort, and the load:
1. The fulcrum is between the effort and the load in first class levers (Figure 11.2a). (Think EFL.)
Scissors and seesaws are examples of first class levers. A first class lever can produce either a
mechanical advantage or mechanical disadvantage depending on whether the effort or the load is closer
to the fulcrum. (Think of an adult and a child on a seesaw.) As we've seen in the preceding examples, if
the effort (child) is farther from the fulcrum than the load (adult), a heavy load can be moved, but not
very far or fast. If the effort is closer to the fulcrum than the load, only a lighter load can be moved, but
it moves far and fast. There are few first class levers in the body. One example is the lever formed by
the head resting on the vertebral column (Figure 11.2a). When the head is raised, the contraction of the
posterior neck muscles provides the effort E, the joint between the atlas and the occipital bone (atlanto
occipital joint) forms the fulcrum , and the weight of the anterior portion of the skull is the
load .
2. The load is between the fulcrum and the effort in second class levers (Figure 11.2b). (Think ELF.)
Second class levers operate like a wheelbarrow. They always produce a mechanical advantage because
the load is always closer to the fulcrum than the effort. This arrangement sacrifices speed and range of
motion for force; this type of lever produces the most force. This class of lever is uncommon in the
human body. An example is standing up on your toes. The fulcrum is the ball of the foot. The
load is the weight of the body. The effort (E) is the contraction of the muscles of the calf, which raise
the heel off the ground.
3. The effort is between the fulcrum and the load in third class levers (Figure 11.2c). (Think FEL.) These
levers operate like a pair of forceps and are the most common levers in the body. Third class levers
always produce a mechanical disadvantage because the effort is always closer to the fulcrum than the
load. In the body, this arrangement favors speed and range of motion over force. The elbow joint, the
biceps brachii muscle, and the bones of the arm and forearm are one example of a third class lever
(Figure 11.2c). As we have seen, in flexing the forearm at the elbow, the elbow joint is the
fulcrum , the contraction of the biceps brachii muscle provides the effort E, and the weight of the
hand and forearm is the load .
Animation: Contraction and Movement
Effects of Fascicle Arrangement
Recall from Chapter 10 that the skeletal muscle fibers (cells) within a muscle are arranged in bundles known
as fascicles (FAS i kuls). Within a fascicle, all muscle fibers are parallel to one another. The fascicles,‐ ‐
however, may form one of five patterns with respect to the tendons: parallel, fusiform (spindle shaped, narrow
toward the ends and wide in the middle), circular, triangular, or pennate (shaped like a feather) (Table 11.1).
Table 11.1 Arrangement of Fascicles
Fascicles parallel to longitudinal axis of muscle; terminate at either end in flat tendons.
Example: Sternohyoid muscle (see Figure 11.8a)
Fascicles nearly parallel to longitudinal axis of muscle; terminate in flat tendons; muscle tapers toward
tendons, where diameter is less than at belly.
Example: Digastric muscle (see Figure 11.8a)
Fascicles in concentric circular arrangements form sphincter muscles that enclose an orifice (opening).
Example: Orbicularis oculi muscle (see Figure 11.4a)
Fascicles spread over broad area converge at thick central tendon; gives muscle a triangular appearance.
Example: Pectoralis major muscle (see Figure 11.3a)
Short fascicles in relation to total muscle length; tendon extends nearly entire length of muscle.
Fascicles arranged on only one side of tendon.
Example: Extensor digitorum longus muscle (see Figure 11.22b)
Fascicles arranged on both sides of centrally positioned tendons.
Example: Rectus femoris muscle (see Figure 11.20a)
Fascicles attach obliquely from many directions to several tendons.
Example: Deltoid muscle (see Figure 11.10a)
Fascicular arrangement affects a muscle's power and range of motion. As a muscle fiber contracts, it shortens
to about 70% of its resting length. The longer the fibers in a muscle, the greater the range of motion it can
produce. However, the power of a muscle depends not on length but on its total cross sectional area, because a
short fiber can contract as forcefully as a long one. So the more fibers per unit of cross sectional area a muscle
has, the more power it can produce. Fascicular arrangement often represents a compromise between power and
range of motion. Pennate muscles, for instance, have a large number of short fibered fascicles distributed over
their tendons, giving them greater power but a smaller range of motion. In contrast, parallel muscles have
comparatively fewer fascicles, but they have long fibers that extend the length of the muscle, so they have a
greater range of motion but less power.
CLINICAL CONNECTION Intramuscular Injections
An intramuscular (IM) injection penetrates the skin and subcutaneous layer to enter the muscle itself.
intramuscular injections are preferred when prompt absorption is desired, when larger doses than can be
given subcutaneously are indicated, or when the drug is too irritating to give subcutaneously. the common
sites for intramuscular injections include the gluteus medius muscle of the buttock (see Figure 11.3b),
lateral side of the thigh in the midportion of the vastus lateralis muscle (see Figure 11.3a), and the deltoid
muscle of the shoulder (see Figure 11.3b). muscles in these areas, especially the gluteal muscles in the
buttock, are fairly thick, and absorption is promoted by their extensive blood supply. to avoid injury,
intramuscular injections are given deep within the muscle, away from major nerves and blood vessels.
intramuscular injections have a faster speed of delivery than oral medications but are slower than
intravenous infusions.
Coordination among Muscles
Movements often are the result of several skeletal muscles acting as a group. Most skeletal muscles are
arranged in opposing (antagonistic) pairs at joints—that is, flexors–extensors, abductors–adductors, and so on.
Within opposing pairs, one muscle, called the prime mover or agonist (= leader), contracts to cause an action
while the other muscle, the antagonist (anti = against), stretches and yields to the effects of the prime mover.
In the process of flexing the forearm at the elbow, for instance, the biceps brachii is the prime mover, and the
triceps brachii is the antagonist (see Figure 11.1a). The antagonist and prime mover are usually located on
opposite sides of the bone or joint, as is the case in this example.
With an opposing pair of muscles, the roles of the prime mover and antagonist can switch for different
movements. For example, while extending the forearm at the elbow against resistance (i.e., lowering the load
shown in Figure 11.2c), the triceps brachii becomes the prime mover, and the biceps brachii is the antagonist.
If a prime mover and its antagonist contract at the same time with equal force, there will be no movement.
Figure 11.2 Lever structure and types of levers.
Levers are divided into three types based on the placement of the fulcrum, effort, and load (resistance). John
Which type of lever produces the most force?
Sometimes a prime mover crosses other joints before it reaches the joint at which its primary action occurs.
The biceps brachii, for example, spans both the shoulder and elbow joints, with primary action on the forearm.
To prevent unwanted movements at intermediate joints or to otherwise aid the movement of the prime mover,
muscles called synergists (SIN er jists; syn = together; ergon = work) contract and stabilize the intermediate
joints. As an example, muscles that flex the fingers (prime movers) cross the intercarpal and radiocarpal joints
(intermediate joints). If movement at these intermediate joints were unrestrained, you would not be able to flex
your fingers without flexing the wrist at the same time. Synergistic contraction of the wrist extensor muscles
stabilizes the wrist joint and prevents unwanted movement, while the flexor muscles of the fingers contract to
bring about the primary action, efficient flexion of the fingers. Synergists are usually located close to the prime
Some muscles in a group also act as fixators, stabilizing the origin of the prime mover so that the prime mover
can act more efficiently. Fixators steady the proximal end of a limb while movements occur at the distal end.
For example, the scapula is a freely movable bone that serves as the origin for several muscles that move the
arm. When the arm muscles contract, the scapula must be held steady. In abduction of the arm, the deltoid
muscle serves as the prime mover, and fixators (pectoralis minor, trapezius, subclavius, serratus anterior
muscles, and others) hold the scapula firmly against the back of the chest (see Figure 11.14a, b). The insertion
of the deltoid muscle pulls on the humerus to abduct the arm. Under different conditions—that is, for different
movements—and at different times, many muscles may act as prime movers, antagonists, synergists, or
In the limbs, a compartment is a group of skeletal muscles, their associated blood vessels, and associated
nerves, all of which have a common function. In the upper limbs, for example, flexor compartment muscles are
anterior, and extensor compartment muscles are posterior.
CLINICAL CONNECTION Benefits of Stretching
The overall goal of stretching is to achieve normal range of motion of joints and mobility of soft tissues
surrounding the joints. For most individuals, the best stretching routine involves static stretching, that is,
slow sustained stretching that holds a muscle in a lengthened position. The muscles should be stretched to
the point of slight discomfort (not pain) and held for about 30 seconds. Stretching should be done after
warming up to increase the range of motion most effectively.
1. Improved physical performance. A flexible joint has the ability to move through a greater range
of motion, which improves performance.
2. Decreased risk of injury. Stretching decreases resistance in various soft tissues so there is less
likelihood of exceeding maximum tissue extensibility during an activity (i.e., injuring the soft
3. Reduced muscle soreness. Stretching can reduce some of the muscle soreness that results after
4. Improved posture. Poor posture results from improper position of various parts of the body and
the effects of gravity over a number of years. stretching can help realign soft tissues to improve and
maintain good posture.
Explain seven features used in naming skeletal muscles.
The names of most of the skeletal muscles contain combinations of the word roots of their distinctive features.
This works two ways. You can learn the names of muscles by remembering the terms that refer to muscle
features, such as the pattern of the muscle's fascicles; the size, shape, action, number of origins, and location of
the muscle; and the sites of origin and insertion of the muscle. Knowing the names of a muscle will then give
you clues about its features. Study Table 11.2 to become familiar with the terms used in muscle names.
Table 11.2 Characteristics Used to Name Muscles
Rectus Parallel to midline Rectus abdominis 11.10c
Transverse Perpendicular to midline Transversus abdominis 11.10c
Oblique Diagonal to midline External oblique 11.10a
Maximus Largest Gluteus maximus 11.20c
Minimus Smallest Gluteus minimus 11.20d
Longus Long Adductor longus 11.20a
Brevis Short Adductor brevis 11.20b
Latissimus Widest Latissimus dorsi 11.15b
Longissimus Longest Longissimus capitis 11.19a
Magnus Large Adductor magnus 11.20b
Major Larger Pectoralis major 11.10a
Minor Smaller Pectoralis minor 11.14a
Vastus Huge Vastus lateralis 11.20a
Deltoid Triangular Deltoid 11.15b
Trapezius Trapezoid Trapezius 11.3b
Serratus Saw toothed Serratus naterior 11.14b
Rhomboid Diamond shaped Rhomboid amjor 11.15d
Orbicularis Circular Orbicularis couli 11.4a
Pectinate Comblike Pectineus 11.20a
Piriformis Pear shaped Piriformis 11.20d
Platys Flat Platysma 11.4c
Quadratus Square, four sided Quadratus efmoris 11.20d
Gracilis Slender Gracilis 11.20a
Flexor Decreases joint angle Flexor carpi radialis 11.17a
Extensor Increases joint angle Extensor carpi ulnaris 11.17d
Abductor Moves bone away from midline Abductor pollicis longus 11.17e
Adductor Moves bone closer to midline Adductor longus 11.20a
Levator Raises or elevates body part Levator scapulae 11.14a
Depressor Lowers or depresses body part Depressor labii inferioris 11.4b
Supinator Turns palm anteriorly Supinator 11.17c
Pronator Turns palm posteriorly Pronator teres 11.17a
Sphincter Decreases size of an opening External anal sphincter 11.12
Tensor Makes body part rigid Tensor fasciae latae 11.20a
Rotator Rotates bone around longitudinal axis Rotatore 11.19b
Biceps Two origins Biceps brachii 11.16a
Triceps Three origins Triceps brachii 11.16b
Quadriceps Four origins Quadriceps femoris 11.20a

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