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MUSCLES
General Considerations on Muscles
The muscular system consists of all the muscles of the body. The disciplined related to the study of
muscles is myology. Musculus (muscle) is derived from the word mus-mouse; musculus- little mouse. All
skeletal muscles are composed of one specific type of muscle tissue. These muscles move the skeleton,
therefore, move the body parts.
There are three muscle types:
 Skeletal striated muscle is voluntary somatic muscle that makes up the gross skeletal muscles that
compose the muscular system, moving or stabilizing bones and other structures (e.g., the eyeballs).
Striated muscles are innervated by the somatic nervous system.
 Cardiac striated muscle is involuntary visceral muscle that forms most of the walls of the heart and
adjacent parts of the great vessels, such as the aorta, and pumps blood.
 Smooth muscle (unstriated muscle) is involuntary visceral muscle that forms part of the walls of
most vessels and hollow organs (viscera), moving substances through them by coordinated sequential
contractions (pulsations or peristaltic contractions). Non-striated and cardiac muscle are innervated by
the autonomic nervous system.
All skeletal muscles, commonly referred to simply as “muscles,” have fleshy, reddish, contractile portions
(one or more heads or bellies) composed of skeletal striated muscle. Some muscles are fleshy throughout, but
most also have white non-contractile portions (tendons), composed mainly of organized collagen bundles, that
provide a means of attachment. When referring to the length of a muscle, both the belly and the tendons are
included. In other words, a muscle's length is the distance between its attachments.
Most skeletal muscles are attached directly or indirectly to bones, cartilages, ligaments, or fascias or to
some combination of these structures. Some muscles are attached to organs (the eyeball, for example), skin
(such as facial muscles), and mucous membranes (intrinsic tongue muscles). Muscles are organs of
locomotion (movement), but they also provide static support, give form to the body, and provide heat.
The architecture and shape of muscles vary. The tendons of some muscles form flat sheets, or
aponeuroses, that anchor the muscle to the skeleton (usually a ridge or a series of spinous processes) and/or to
deep fascia (such as the latissimus dorsi muscle of the back), or to the aponeurosis of another muscle (such as
the oblique muscles of the anterolateral abdominal wall).
Muscle terminology
Many terms provide information about a structure's shape, size, location, or function or about the
resemblance of one structure to another.
 Most muscles are named on the basis of their function or the bones to which they are attached. The
abductor digiti minimi muscle, for example, abducts the little finger.
 Some muscles have descriptive names to indicate their main characteristics. The deltoid muscle,
which covers the point of the shoulder, is triangular, like the symbol for delta, the fourth letter of the
Greek alphabet. The suffix -oid means “like”; therefore, deltoid means like delta.
 Other muscles are named on the basis of their position (medial, lateral, anterior, posterior) or length
(brevis, short; longus, long).
 Some muscles are named according to their shape—the piriformis muscle, for example, is pear shaped
(L. pirum, pear + L. forma, shape or form).
 Other muscles are named according to their location. The temporal muscle is in the temporal region
(temple) of the cranium (skull).
Muscles may be described or classified according to their shape, for which a muscle may also be named:
 Flat muscles have parallel fibers often with an aponeurosis—for example, the external oblique (broad flat
muscle). The sartorius is a narrow flat muscle with parallel fibers.
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 Pennate muscles are feather-like (L. pennatus, feather) in the arrangement of their fascicles, and may be
unipennate, bipennate, or multi-pennate—for example, the extensor digitorum longus (unipennate), the rectus
femoris (bipennate), and deltoid (multi-pennate).
 Fusiform muscles are spindle shaped with a round, thick belly (or bellies) and tapered ends—for
example, biceps brachii.
 Convergent muscles arise from a broad area and converge to form a single tendon—for example, the
pectoralis major.
 Quadrate muscles have four equal sides (L. quadratus, square)—for example, the rectus abdominis,
between its tendinous intersections.
 Circular or sphincteral muscles surround a body opening or orifice, constricting it when contracted—
for example, orbicularis oculi (closes the eyelids).
 Multi-headed or multi-bellied muscles have more than one head of attachment or more than one
contractile belly, respectively. Biceps muscles have two heads of attachment (e.g., the biceps brachii), triceps
muscles have three heads (e.g., triceps brachii), and the digastric and gastrocnemius muscles have two
bellies.
Contraction of muscles
Skeletal muscles function by contracting; they pull and never push. When a muscle contracts and
shortens, one of its attachments usually remains fixed while the other (more mobile) attachment is pulled
toward it, often resulting in movement. Attachments of muscles are commonly described as the origin and
insertion; the origin is usually the proximal end of the muscle, which remains fixed during muscular
contraction, and the insertion is usually the distal end of the muscle, which is movable. However, this is not
always the case. Some muscles can act in both directions under different circumstances. For example, when
doing pushups, the distal end of the upper limb (the hand) is fixed (on the floor) and the proximal end of the
limb and the trunk are being moved. Whereas the structural unit of a muscle is a skeletal striated muscle fiber,
the functional unit of a muscle is a motor unit, consisting of a motor neuron and the muscle fibers it controls.
When a motor neuron in the spinal cord is stimulated, it initiates an impulse that causes all the muscle fibers
supplied by that motor unit to contract simultaneously.
Functions of muscles
Muscles serve specific functions in moving and positioning the body.
A prime mover (agonist) is the main muscle responsible for producing a specific movement of the body. It
contracts concentrically to produce the desired movement, doing most of the work (expending most of the
energy) required. A fixator steadies the proximal parts of a limb through isometric contraction while
movements are occurring in distal parts. A synergist complements the action of a prime mover. An
antagonist is a muscle that opposes the action of another muscle. The same muscle may act as a prime mover,
antagonist, synergist, or fixator under different conditions.
Nerves and arteries to muscles
Variation in the nerve supply of muscles is rare; it is a nearly constant relationship. In the limb, muscles of
similar actions are generally contained within a common fascial compartment and share innervation by the
same nerves. The blood supply of muscles is not as constant as the nerve supply and is usually multiple.
Arteries generally supply the structures they contact.
Muscles of the Face and the Scalp
The facial muscles (muscles of facial expression) move the skin and change facial expressions to convey
mood. Most muscles attach to bone or fascia and produce their effects by pulling the skin.
The occipitofrontalis is a flat digastric muscle which elevates the eyebrows and produce transverse wrinkles
across the forehead. This gives the face a surprised look.
Several muscles alter the shape of the mouth and lips during speaking as well as during such activities as
singing, whistling, and mimicry. The shape of the mouth and lips is controlled by a complex threedimensional group of muscular slips, which include the following:
 Elevators, retractors, and evertors of the upper lip.
 Depressors, retractors, and evertors of the lower lip.
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
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The orbicularis oris, the sphincter around the mouth.
The buccinator in the cheek.
The platysma (G. flat plate) is a broad, thin sheet of muscle in the subcutaneous tissue of the neck. It helps
depress the mandible and draw the corners of the mouth inferiorly.
The function of the eyelids (L. palpebrae) is to protect the eyeballs from injury and excessive light. The
eyelids also keep the cornea moist by spreading the tears. The orbicularis oculi closes the eyelids and
wrinkles the forehead vertically. The muscles of the nose may provide evidence of breathing behaviors.
Cutaneous (sensory) innervation of the face and anterosuperior part of the scalp is provided primarily by the
trigeminal nerve (CN V), whereas motor innervation to the facial muscles is provided by the facial nerve (CN
VII).
Muscles of the Neck
The sternocleidomastoid (SCM) muscle is a broad, strap-like muscle that has two heads: The
rounded tendon of the sternal head attaches to the manubrium, and the thick fleshy clavicular head attaches to
the superior surface of the clavicle. Bilateral contractions of the SCMs will cause extension of the elevating
the chin. Acting unilaterally, the SCM laterally flexes the neck (bends the neck sideways) and rotates the head
so the ear approaches the shoulder of the ipsilateral (same) side.
Trapezius is a large, flat triangular muscle that covers the posterolateral aspect of the neck and thorax.
The trapezius provides a direct attachment of the pectoral girdle to the trunk. This large, triangular muscle
covers the posterior aspect of the neck and the superior half of the trunk. It was given its name because the
muscles of the two sides form a trapezium (G. irregular four-sided figure). The trapezius assists in suspending
the upper limb.
Muscles of the Pectoral and Scapular Regions
Four anterior axioappendicular muscles (thoracoappendicular or pectoral muscles) move the pectoral
girdle: pectoralis major, pectoralis minor, subclavius, and serratus anterior.
The pectoralis major is a large, fan-shaped muscle that covers the superior part of the thorax. It has
clavicular and sternocostal heads. It produces powerful adduction and medial rotation of the arm when acting
together. The pectoralis minor lies in the anterior wall of the axilla where it is almost completely covered by
the much larger pectoralis major. The subclavius lies almost horizontally when the arm is in the anatomical
position. This small, round muscle is located inferior to the clavicle and affords some protection to the
subclavian vessels and the superior trunk of the brachial plexus if the clavicle fractures. The serratus anterior
overlies the lateral part of the thorax and forms the medial wall of the axilla (L. serratus, a saw).
The posterior axioappendicular muscles (superficial and intermediate groups of extrinsic back muscles)
attach the superior appendicular skeleton (of the upper limb) to the axial skeleton (in the trunk). The posterior
shoulder muscles are divided into three groups:
 Superficial posterior axioappendicular (extrinsic shoulder) muscles: trapezius and latissimus dorsi.
 Deep posterior axioappendicular (extrinsic shoulder) muscles: levator scapulae and rhomboids.
 Scapulohumeral (intrinsic shoulder) muscles: deltoid, teres major, and the four rotator cuff muscles.
The name latissimus dorsi (L. widest of back) was well chosen because the muscle covers a wide area of the
back. This large, fan-shaped muscle passes from the trunk to the humerus and acts directly on the
glenohumeral joint and indirectly on the pectoral girdle (scapulothoracic joint).
The six scapulohumeral muscles (deltoid, teres major, supraspinatus, infraspinatus, subscapularis,
and teres minor) are relatively short muscles that pass from the scapula to the humerus and act on the
glenohumeral joint. The deltoid is a thick, powerful, coarse-textured muscle covering the shoulder and
forming its rounded contour. As its name indicates, the deltoid is shaped like the inverted Greek letter delta
(Δ).
Muscles of the Arm & the Hand
Of the four major arm muscles, three flexors (biceps brachii, brachialis, and coracobrachialis) are in
the anterior (flexor) compartment, supplied by the musculocutaneous nerve, and one extensor (triceps
brachii) is in the posterior compartment, supplied by the radial nerve. The ulnar nerve supplies the flexor
carpi ulnaris and flexor digitorum profundus (media half) and hypothenar muscles. The biceps brachii is the
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flexor of the arm. The brachialis is the main flexor of the forearm. The coracobrachialis helps flex and
adduct the arm and stabilize the glenohumeral joint. The triceps brachii is the main extensor of the forearm.
There are 17 muscles crossing the elbow joint, some of which act on the elbow joint exclusively,
whereas others act at the wrist and fingers.
The flexor muscles of the forearm are in the anterior (flexor-pronator) compartment of the forearm and are
separated from the extensor muscles of the forearm by the radius and ulna and, in the distal two thirds of the
forearm, by the interosseous membrane that connects them. The extensor muscles of the forearm are in the
posterior (extensor-supinator) compartment of the forearm, and all are innervated by branches of the radial
nerve. The intrinsic muscles of the hand are located in five compartments.
Muscles of the Gluteal Region, Back, the Leg & the Foot
The large anterior compartment of the thigh contains the anterior thigh muscles, the flexors of the hip
and extensors of the knee. The pectineus is a flat quadrangular muscle located in the anterior part of the thigh.
The iliopsoas is the chief flexor of the thigh, the most powerful of the hip flexors. The sartorius, the “tailor's
muscle” (L. sartus, patched or repaired), is long and ribbon-like. It passes lateral to medial across the
superoanterior part of the thigh. The sartorius, the longest muscle in the body.
The quadriceps femoris (L., four-headed femoral muscle) forms the main bulk of the anterior thigh
muscles and collectively constitutes the largest and one of the most powerful muscles in the body. It covers
almost all the anterior aspect and sides of the femur. The quadriceps femoris (usually shortened to quadriceps)
is the great extensor of the leg.
The rectus femoris received its name because it runs straight down the thigh (L. rectus, straight). The
names of the three large vastus muscles (vasti) indicate their position around the femoral shaft; vastus
lateralis, medialis and intermedius. The muscles of the medial compartment of the thigh comprise the
adductor group.
Gluteal and posterior thigh regions
The gluteus maximus is the most superficial gluteal muscle. It is the largest, heaviest, and most
coarsely fibered muscle of the body. The main actions of the gluteus maximus are extension and lateral
rotation of the thigh. The smaller gluteal muscles, gluteus medius and gluteus minimus, are fan shaped, and
their fibers converge in the same manner. The pear-shaped piriformis (L. pirum, a pear), obturator internus
and the superior and inferior gemelli (L. geminus, small twin) are the other muscles of the region, as well as
the quadratus femoris and the obturator externus. The posterior thigh muscles include the hamstring
muscles: (1) semitendinosus, (2) semimembranosus, and (3) biceps femoris (long head). The four muscles
in the anterior compartment of the leg are the tibialis anterior, extensor digitorum longus, extensor
hallucis longus, and fibularis tertius. The lateral compartment of the leg is the smallest (narrowest) of the
leg compartments. The lateral compartment contains the fibularis longus and brevis muscles.
The posterior compartment of the leg (plantarflexor compartment) is the largest of the three leg
compartments. The superficial group of calf muscles (muscles forming prominence of “calf” of posterior leg)
includes the gastrocnemius, soleus, and plantaris. The gastrocnemius and soleus share a common tendon,
the calcaneal tendon, which attaches to the calcaneus. The calcaneal tendon (L. tendo calcaneus, Achilles
tendon) is the most powerful (thickest and strongest) tendon in the body. Collectively these two muscles make
up the three-headed triceps surae (L. sura, calf). These muscles raise heel during walking; flex the leg at the
knee joint. Four muscles make up the deep group in the posterior compartment of the leg: popliteus, flexor
digitorum longus, flexor hallucis longus, and tibialis posterior.
Of the 20 individual muscles of the foot, 14 are located on the plantar aspect, 2 are on the dorsal aspect,
and 4 are intermediate in position.
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