- Muscle make up nearly half the body’s mass. The essential function of
muscle is contraction or shortening. This unique characteristic sets
muscle apart from other tissues in the body. All body movements depend
on the muscles. Thus, muscles can be viewed as the “machines” of the
Functions of the muscles
- Produces movement. All movements of the human body are result of muscular contraction.
- Maintaining posture. The skeletal muscles in the body maintain posture.
- Stabilizing joints. Presence of muscle tendons reinforces and stabilizes joints that have poorly fitting articulating surfaces.
- Generating heat. Heat is a by-product of muscle activity. This heat is essential in maintaining normal body temperature.
Types of muscles
- Also called: voluntary muscle, striated muscle
- This type of muscle attaches to the body’s skeleton. Because of their
attachment to the bony part of the body smoother contours of the body
are formed. Skeletal muscle fibers are cigar-shaped, multi-nucleate
cells and are the largest of the muscle fiber types. This is the only
muscle type that can be controlled consciously, thus it is a voluntary
muscle. Since its fibers appear to be striped it is known as striated
Summary of the characteristic of Skeletal Muscles:
- Subject to conscious control.
- Attaches to the body’s skeleton
- Soft and fragile.
- Its tissue can rapidly contract and with great force.
- Tires easily.
- Contraction is abrupt and rapid.
What is the reason why skeletal muscles are not ripped apart as they exert effort?
- Skeletal muscles do not rip apart after exerting effort because
thousands of skeletal muscle fibers are bundled together by connective
tissues which are responsible for providing strength and support to the
muscle as a whole.
Structure or Parts of Skeletal Muscle
- Endomysium – a delicate connective tissue sheath that encloses each skeletal muscle fiber.
- Perimysium – a coarse fibrous membrane that wraps the sheathed muscle fibers.
- Fascicle – bundle of fibers formed from group of sheathed muscle fibers wrapped by perimysium.
- Epimysium – a tough overcoat of connective tissue that bounds
together fascicles. This is the connective tissue that covers the entire
- Tendons – these are cordlike structures that are formed from
epimysia. These are composed of mostly collagenic fibers that can cross
rough bony projections. Aside from anchoring muscles, tendons are very
important in providing durability.
- Aponeuroses – these are sheet like structures that attaches muscles
indirectly to bones, cartilages or connective tissue coverings of each
- Also called: visceral muscles, non-striated muscles, involuntary muscles
- Smooth muscles, unlike skeletal muscles, have no striations. It is
controlled involuntarily, meaning to say individuals cannot consciously
regulate it. If skeletal muscles are found in the bones, smooth muscles
are found on the walls of hollow visceral organs such as the stomach,
urinary bladder and respiratory passages. The main function of smooth
muscles is to propel substances along a definite tract or pathway within
the body. These muscles have only one nucleus and are spindle-shaped.
Summary of the characteristics of smooth muscles
- Involuntary control.
- Found on the walls of the hollow visceral organs.
- Have no striations.
- Propels substances along a fixed tract inside the body.
- Have a single nucleus.
- Arranged in layers or sheets. Most often, it is arranged in two layers, one circular the other longitudinal.
- Alternately contracts and relaxes to change the shape and size of an organ.
- Contraction of these muscles is slow and sustained.
- Unlike the other two types of muscles mentioned above, cardiac muscle is only found in one place in the body – the heart. The function of the heart
as the pump – propelling blood into the blood vessels and to all
tissues of the body, is carried out because of the presence of cardiac
muscle. This muscle has similarities to skeletal muscles as it is
striated and smooth muscles as it is involuntary and cannot be
controlled consciously. Cardiac muscle fibers are branching cells joined
together by special junctions called intercalated discs.
Summary of the characteristics of cardiac muscle
- Only found in the heart.
- Involuntary control.
- Cushioned by small amounts of soft connective tissue.
- Arranged in spiral or figure 8-shaped bundles.
- Contracts at a steady rate set by the heart’s pacemaker.
Types of Muscles in the Body
|Location in the body
||Attached to bones
||Located in the walls of the hollow visceral organs with the exception of the heart
||Walls of the heart
|Muscle fiber shape and appearance
||Single, cigar shaped, very long, cylindrical, striated, multinucleated cells
||Singe, no striations, fusiform, cells have single nucleus
||Branching cell chains, uninucleate, with striations, intercalated discs
|Regulation of contraction
|Speed of contraction
||Slow to fast
||Yes, in some
Gross anatomy of Skeletal Muscles
Muscles in the head and neck
- Frontalis – covers the frontal bone. This muscle runs from the
cranial aponeurosis to the skin of the eyebrows where it inserts.
Frontalis muscle allows a person to raise his or her eyebrows and
wrinkle ones forehead.
- Orbicularis Oculi – the muscle fibers of this muscle run in circles
around the eyes. It is the presence of this muscle that an individual is
able to close his or her eyes, squint, blink and wink.
- Orbicularis Oris – also called the kissing muscle. This circular
muscle runs around the mouth. This is responsible for closing and
protruding ones mouth.
- Buccinator – this muscle runs across the cheek and inserts into
orbicularis oris. This muscle plays a vital role during chewing. It is
listed as a chewing muscle as it compresses the cheek to hold the food
between the teeth during chewing. When a person is whistling and blowing
a trumpet this muscle flattens the cheek.
- Zygomaticus – this muscle is referred to as the “smiling muscle” as it raises the corners of the mouth upward.
- Chewing muscles – aside from buccinators which is described above,
chewing muscles include the masseter and the temporalis muscle. Masseter
is the muscle covering the lower jaw and is responsible for closing the
jaw when chewing by elevating the mandible. The temporalis muscle, a
fan-shaped muscle, inserts into the mandible and acts as a synergist of
the masseter in closing the jaw.
- Neck muscles – muscles in the neck are the platysma and
sternocleidomastoid. Platysma is the sheet like muscle that covers the
neck anterolaterally. Its action is to pull the corners of the mouth
inferiorly which produces downward sag of the mouth. The other muscle of
the neck, the sternocleidomastoid, is found on each side of the neck.
The sternocleidomastoid muscles are two-headed muscles. Of the two heads
of each muscle, one arises from the sternum and the other arises from
the clavicle. When the sternocleidomastoid muscles contract together,
the neck flexes. If only one muscle of the sternocleidomastoid muscle
contracts, the head is rotated toward the opposite side.
- Pectoralis major – this muscle is a large fan-shaped muscle covering
the upper part of the chest. The adduction and the flexion of the
axilla is the action of this muscle. It forms the anterior wall of the
- Intercostals muscles – these muscles are located deep between the
ribs. The external intercostals muscles help raise the rib cage for
breathing air in, making it a vital structure in breathing. The internal
intercostals ribs, on the other hand, help air to move out the lungs
when an individual exhales forcibly.
- Muscles of the abdominal girdle – muscles of the abdominal girdle
are subdivided into two categories – the anterior and posterior
abdominal muscles. The anterior muscles serve as reinforcement of the
body trunk. Both of these muscles are suitable for containing and
protecting the abdominal contents.
Anterior Abdominal Muscles
- Rectus abdominis muscle – this paired strap-like muscle is the most
superficial muscle of the abdomen. The MAIN FUNCTION of the rectus
abdominis muscle is to flex the vertebral column. During defecation and
childbirth, this muscle compresses the abdominal contents. Aside from
that, it is also involved in forced breathing.
- External Oblique muscle – this paired muscle makes up the lateral
walls of the abdomen. The MAIN FUCNTION of this muscle is to flex the
vertebral column like the rectus abdominis muscle. However, they also
rotate the trunk and bend it laterally.
- Internal oblique muscle – this paired muscle serves the same function as that of the external oblique muscles.
- Trapezius muscles – these muscles are the most superficial muscles
of the posterior and upper trunk. They serve as the antagonists of the
sternocleidomastoids and they can elevate, depress, adduct ad stabilize
the scapula. When seen together, the trapezius muscle forms a diamond or
kite-shaped muscle mass.
- Latissimus Dorsi muscle – this is a large and flat pair of muscle
that covers the entire lower back. Latissimus dorsi muscle extends and
adducts the humerus. This muscle plays a vital role in bringing down the
arm in a power stroke when swimming or striking a blow.
- Erector Spinae muscle – this group of muscle is a prime movers of
back extension. Aside from acting as powerful back extensors, erector
spinae muscle also helps control the action of bending over at the
waist. When an injury to back structure occurs, these muscles go into
spasms which are a common source of lower back pain. Each erector spinae
muscle is composed of three muscle columns that cover the entire length
of the vertebral column. The three muscle columns are longissimus,
iliocostalis and spinalis.
- Deltoid muscles – these are fleshy and triangle-shaped muscles that
form the round shape of the shoulders. They are bulky and have been the
most common and most favorite injection site. The deltoid muscles are
the prime movers of arm abduction.
Muscles of the Upper Limb
Muscles of the upper limbs are divided into three groups.
- First group – muscles arising from the shoulder girdle and cross the
shoulder joint to insert into the humerus. These muscles move the arm.
- Second group – muscles that encloses the humerus and insert on the
forearm bones. These muscles cause movement at the elbow joint.
- Third group – includes muscles of the forearm that cause their movement.
Muscles of the Humerus that Act on the forearm
- Biceps Brachii – this muscle bulges when the elbow is flexed. It is a
powerful prime mover for the flexion of the forearm and acts to
supinate the forearm.
- Brachialis – this muscle plays an essential in elbow flexion. It lies deep to the biceps muscle.
- Brachioradialis – this is a fairly weak muscle. It arises on the humerus and inserts into the distal forearm.
- Triceps brachii – this is the only muscle fleshing out the posterior
humerus. It is the powerful prime mover of elbow extension. Other term
for this muscle is the “boxer’s mucle” as it can deliver a straight arm
knock-out punch. This muscle is also the antagonist of the biceps
Muscles of the Lower Limb
Characteristic of the muscles of the lower limb:
- These muscles cause movement at the hip, knee and foot joints.
- They are one of the largest and strongest muscles in the body.
- These muscles play a vital role in walking and balancing the body.
Muscles causing movement at the hip joint
- Gluteus Maximus Muscle – this is a superficial hip muscle that
shapes the buttocks. Gluteus maximus muscle is a very powerful hip
extensor that is responsible in bringing the thigh to a straight line.
- Gluteus medius Muscle – this is a hip abductor muscle. Gluteus
medius muscle is very important in stabilizing the pelvis when an
individual is walking. When more than 5 ml of medication is administered
intramuscularly, the gluteus medius muscle is used. However, the medial
part of each buttock overlies the large sciatic nerve, hence; this area
must be used carefully. Because of this reason, the fleshy gluteus
maximus would be a better choice. In cases, where the gluteus medius is
used for IM injection, the nurse should divide the buttocks into four
equal parts mentally. The upper outer quadrant is a very safe site for
an IM injection.
- Iliopsoas – this muscle is composed of two muscles – the iliacus and
psoas major. Iliopsoas is a prime mover of hip flexion and it also acts
to keep the upper body from falling backward when a person is standing
- Adductor muscles – as their name indicate, these muscles adduct or
press the thighs together. However, since gravity does most of the work
for them, these muscles tend to become flabby very easily.
Muscles causing movement at the knee joint
- Hamstring group – these muscles form the muscle mass of the
posterior thigh. This group is composed of three muscles – the biceps
femoris, semimembranosus and semitendinosus muscles.
- Sartorius – this thin and strap-like muscle is not very significant
because it is a very weak flexor of the thigh. However, this muscle is
the most superficial muscle of the thigh. The Sartorius muscle is often
referred to as the tailor’s muscle because it acts as a synergist to
bring about the cross-legged position in which old-time tailor’s is
- Quadriceps group – four muscles compose this muscle group. Namely,
the rectus femoris and the three vastus muscles. These muscles flesh out
the anterior thigh. Quadriceps group acts to extend the knee
Muscles causing movement at the ankle and foot
- Tibialis anterior – this is a superficial muscle of the anterior leg that acts to dorsiflex and invert the foot.
- Extensor digitorum longus – this muscle inserts into the phalanges
of toes 2 to 5. It is a prime mover of toe extension and a dorsiflexor
of the foot.
- Fibularis muscles – there are three fibularis muscles namely, the
longus, brevis and tertius. These muscles are found on the lateral part
of the leg and working as a group, they are responsible for the plantar
flexion and the eversion of the foot.
- Gastrocnemius – this muscle forms the curved half of the posterior leg. It is a prime mover for plantar flexion of the foot.
- Soleus – this muscle lies deep to gastrocnemius. It has no effect on
knee movement but it is a string plantar flexor of the foot.
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