The Skeletal Muscle System

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The Skeletal Muscle System

• Module 6

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• A skeletal muscle attaches to bone via a tendon.
• Typically, a muscle attaches to one bone that
  moves significantly and another that stays
  relatively stationary. We can define the parts of
  a muscle based on these bones.Origin - The
  point at which a muscle's tendon attaches to the
  more stationary boneInsertion - The point at
  which a muscle's tendon attaches to the moveable
  boneBelly - The largest part of the muscle,
  which actually contains the muscle cells

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• The place that the tendon attaches to the
  relatively stationary bone is called the origin,
  or head, of the muscle.
• The point at which the tendon attaches to the
  moving bone is called the insertion. In the
  limbs, the origins are usually more proximal and
  the insertions are more distal. In the trunk of
  the body, the origins are usually more medial,
  and the insertions are more lateral.
• There are a few exceptions to these general
  rules, but they are both good guides to follow.
  The muscle belly is largest part of the muscle,
  and it is between the origin and insertion.

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• The most important thing to remember about muscle
  action is that muscles never work individually.
  They always work in groups of two or more.
• Figure 6.1 illustrates how two muscles, the
  biceps brachii and the triceps brachii, work
  together to flex and extend the forearm.
• Remember from the previous module that a relaxed
  muscle can be extended (stretched out) without
  much effort, since the myosin heads have released
  the actin myofilaments.

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• The first drawing illustrates how these two
  muscles flex the forearm.
• To flex the forearm, the radius and ulna must be
  pulled towards the humerus.
• To do this, the biceps brachii contracts.
• The triceps brachii is attached to the ulna.
• If the radius and ulna are going to move towards
  the humerus, the triceps must be relaxed.
  Otherwise, it would fight the motion caused by
  the contraction of the biceps. Now remember,
  muscles do not extend themselves.
• They can only contract. However, if the triceps
  relaxes, the contraction of the biceps will pull
  on the radius and ulna. As long as the triceps is
  relaxed, the force provided by the biceps will
  passively extend the triceps. Thus, to flex the
  elbow, the biceps contracts, and the triceps
  relaxes so that it can be passively extended.

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• In the second drawing, we show how these same two
  muscles can extend the forearm.
• To do that, the triceps contracts.
• This pulls the radius and ulna away from the
  humerus.
• Once again, however, for this to happen smoothly,
  the biceps cannot be contracted.
• Otherwise, it would fight the motion of the
  radius and ulna.
• Thus, the biceps relaxes and is passively
  extended by the motion of the radius and ulna.

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• This is typical of how skeletal muscles work.
  They work as partners which are opposite of one
  another. When the first partner contracts, the
  second relaxes and motion in one direction is
  caused.
• Motion in the other direction is caused when the
  two partners switch jobs, and the first relaxes
  while the second contracts. In reality, this is
  an oversimplification.
• Muscles tend to work in whole groups, depending
  on the joints involved.

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• There may be a group of muscles that creates
  motion in one direction and then another group of
  muscles which create the opposite motion.
• The motion requires the synchronization of all of
  these muscles.
• When muscles work together to create the same
  motion, they are called synergists.
• If one muscle is more important than the others
  in creating that motion, it is called the prime
  mover.
• When a muscle works opposite of another muscle,
  it is called an antagonist. The triceps brachii,
  then, is the antagonist of the biceps brachii,
  and the biceps brachii is the antagonist of the
  triceps brachii.

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• The last general principle you need to understand
  is that muscles tend to work as levers in the
  body.
• Remember, a muscle can only contract. It cannot
  extend on its own. Thus, muscles pull they do
  not push.
• A muscle, then, creates a pull on a lever made
  out of bone.
• A lever consists of a rigid bar that rotates
  around a fixed point called the fulcrum. When a
  force (the effort) is applied, a weight (the
  resistance) is moved.
• There are actually three types of levers, each of
  which has a different arrangement of the fulcrum,
  effort, and resistance. You find those three
  lever types in the human body as well.

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• The best example of a first-class lever is a
  see-saw.
• The fulcrum is in the middle.
• One child's weight is the effort which lifts the
  other child (the resistance) up. Thus, the
  fulcrum is between the effort and resistance.
• The posterior neck muscles which tilt your head
  up are an example of such a lever in your body.
• The muscles pull down on the back of the head.
  The joint between the skull and the first
  cervical vertebra is the fulcrum, and the head is
  the resistance. Since the resistance is near the
  fulcrum, only a small effort is needed to pull a
  large resistance.

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• A common example of a second-class lever is the
  wheelbarrow.
• In this kind of lever, the resistance is between
  the fulcrum and the effort.
• In the foot, the calf muscles pull on the heel.
  The ball of the foot acts as the fulcrum.
• Using this second-class lever, the calf muscles
  can lift the entire weight of the body, but only
  a short distance.
• When you stand on your tiptoes, then, you are
  using this second-class lever so that one set of
  muscles lifts your entire body.

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• The third-class lever is actually the most common
  lever in the body. In a third-class lever, the
  effort is between the fulcrum and the resistance.
  
• Depending on how you use a shovel, it can act as
  such a lever.
• When you hold the handle of the shovel still and
  lift the contents of the shovel with your other
  hand, you are using the shovel as a third class
  lever.
• One of the many examples of such a lever in your
  body is your forearm.
• When the biceps brachii pulls on your radius, the
  elbow acts as the fulcrum, and the contents in
  the hand (the resistance) are lifted.
• This kind of lever actually requires a much
  greater force to be used as the effort, but the
  resistance can be lifted much farther than it can
  in the other two types of levers.

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An Overview of the Skeletal Muscle System

• There are 640 skeletal muscles in the body.
• Superficial skeletal muscles - These are the
  muscles on the surface of the muscle system.
  There are deeper muscles, but if you were to
  peel back a person's skin, these are the
  muscles you would see.
• Although this is a great figure to show you the
  incredible design that exists in the body, there
  is just too much information in the figure to
  discuss the muscles individually.
• Thus, we want to show you this figure, and we do
  expect you to be able to identify each muscle in
  the figure. However, we will actually discuss the
  muscles in the next few sections, where we
  concentrate on individual parts of the body.

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Muscle Names

• The names of the muscles can appear to be quite
  strange. However, if you learn a few general
  principles about how muscles are named, you will
  find it easier to learn those names. In general,
  muscles are named according to one or more of the
  following seven criteria
• Muscle size
• Muscle shape
• Location of the muscle
• Orientation of the muscle's fascicles
• Muscle origin and insertion
• Number of heads (origins) on the muscle
• Muscle function

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• Unfortunately, there are no set rules.
• Some muscles are named based on shape alone,
  while others are named based on location alone.
• Still others are named for both their location
  and size.
• The best way for you to see how this works is to
  give you a few examples.

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• Look, for example at the brachialis muscle in the
  top right of Figure 6.3.
• This muscle is on the arm. Term brachium refers
  to the upper arm. Thus, the brachialis is named
  solely for the location of the muscle.
• Thus, some muscle names mix naming methods. For
  example, notice the biceps brachii pointed out in
  the anterior view.
• However, to point out that this muscle is on the
  arm, it is called the biceps brachii. Biceps
  refers to the fact that it has two heads.
• Brachii refers to the fact that it is on the
  arm. The bone over which the muscle is located
  can also be used in the name.
• For example, the tibialis anterior muscle
  illustrated in the anterior view of Figure 6.3 is
  located over the tibia on the anterior side.

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• Muscle names also can blend location and size.
• Gluteus maximus - the first part of its name
  refers to the fact that it is on the buttock,
  because gluteus refers to the buttock.
• The second part of the name indicates that it is
  the largest gluteus muscle. There is another
  buttock muscle, the gluteus minimus , which is
  the smallest buttock muscle. Finally, there is a
  gluteus medius , whose size is in between the
  two. The terms longus and brevis are also
  used to indicate long and short, respectively.

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• Muscle shape is often used as a sole criterion
  for naming. The deltoid muscle gets its name from
  the fact that it is triangular, and the Greek
  letter delta is a triangle.
• The trapezius muscle is shaped like a trapezoid.
  
• A quadratus muscle is rectangular, while a
  rhomboidal muscle is in the shape of a rhombus.

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• The orientation of a muscle's fascicles (bundles
  of muscle fibers) will play a role in naming as
  well.
• Notice the rectus abdominis muscle in the
  anterior view.
• The term rectus means straight. Thus, the
  fascicles of this muscle run straight down the
  abdomen.
• The external oblique muscle pointed out in the
  anterior view gets its name from the fact that
  the muscle fascicles lie oblique (neither
  perpendicular nor parallel) to the body's
  longitudinal axis.

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• Some muscles are also named according to their
  origin and insertion.
• The brachioradialis muscle illustrated in the
  anterior view has its origin in the arm and its
  insertion in the radius.
• The sternocleidomastoid muscle in the anterior
  view of has its origin in the sternum and
  clavicle and its insertion in the mastoid process
  of the temporal bone.

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• Finally, some muscles are named based on their
  action.
• The masseter muscle shown in the anterior view
  is a muscle involved in mastication (chewing).
• The thigh adductors shown in the anterior view
  adduct the thighs.

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The Major Muscles of the Head and Face

• In the top part of the figure, you can see
  several of the muscles involved in facial
  expression and head movement.
• For example, the sternocleidomastoid muscle is
  the prime mover of a group of muscles (including
  the posterior triangle, an array of muscles)
  which rotate and flex the head.
• Now remember, you have to be familiar with the
  terms of motion to be able to understand this
  module. If you don't understand the motion terms
  we use (rotation, flexion, etc.), review the
  final section of Module 4.

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• The sternocleidomastoid muscles are found on both
  sides of the neck.
• If the muscle on just one side of the neck
  contracts, the head rotates so that the chin
  moves upwards towards the opposite side.
• If the muscles on both sides contract together,
  the head is flexed, bringing the chin closer to
  the chest.
• ry turning your head to the right. Can you feel
  the left sternocleidomastoid muscle contract? Try
  lying down flat on your back. Without moving your
  back, lift up your head. Can you feel both
  sternocleidomastoid muscles contract?

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• The frontalis and occipitalis muscles are often
  considered two bellies of a single muscle, called
  the occipitofrontalis muscle.
• They elevate the eyebrows and wrinkle the
  forehead.
• The orbicularis oculi circles the orbit and is
  used to close the eye, as in winking.
• Another circular muscle, the orbicularis oris
  (or' is), surrounds the lips and is used to purse
  them.

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• The buccinator,or cheek muscle, makes rapid
  changes in the volume of the oral cavity.
• For example, when you fill your mouth with water
  and squirt it out, you are using the buccinator.
• This muscle also pulls on the corner of the
  mouth. Thus, the buccinator and the orbicularis
  oris are often called the kissing muscles.

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• The zygomaticus major elevates and draws the
  corner of the mouth laterally. The zygomaticus
  minor elevates the upper lip.
• When you smile, then, you use these two muscles.
• On the other hand, the platysma pulls the corners
  of the mouth down into a frown. It is also used
  to wrinkle the skin of the neck.
• You can look into a mirror while you smile,
  frown, and wrinkle the skin on your neck to see
  these muscles contract.

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• The other muscles in the figure represent some of
  the major muscles which control
  mastication.Mastication - The process of
  chewing

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• The temporalis muscle elevates the mandible. It
  also can be used to retract the mandible.
• The masseter also elevates the mandible, and it
  can be used to protract the mandible (which means
  to push the lower jaw forward).
• The lateral pterygoid (ter' ih goyd) depresses
  (lowers) the mandible.
• Thus, it is used to open the mouth. The medial
  pterygoid elevates the mandible (along with the
  masseter and temporalis) in order to close the
  mouth. These muscles all work together to make
  the motions which enable you to grind your food
  with your teeth.

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Major Muscles of the Anterior Chest and Abdominal
Wall
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• The sternocleidomastoid muscle is labeled in this
  figure.
• The deltoid muscle packs three different
  functions into one muscle.
• The anterior muscle fibers in the deltoid flex
  the arm. This means that the arm is raised up in
  the position associated with sleepwalking.
• The lateral fibers abduct the arm, bring them
  straight out from the body.
• Finally, the posterior fibers extend the arm,
  which brings it from a flexed position back to
  the anatomical position.

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• The pectoralis major muscle acts on the arm. It
  adducts, flexes, and rotates the arm medially.
• Adduction brings the arm outstretched to the side
  back to anatomical position.
• Flexion, as mentioned before, puts the arm in a
  sleepwalker's position, and medial rotation
  twists the upper arm towards the trunk.
• The pectoralis minor, on the other hand, does not
  act on the arm. Instead, it elevates the ribs and
  depresses the scapula.

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• The other muscles are the major muscles of the
  abdominal wall.
• They flex and rotate the vertebral column. They
  can also compress the abdomen.
• This is useful in vomiting, defecation,
  urination, and childbirth, because these muscles
  powerfully compress the contents of the abdomen.
• In a relatively muscular person with little fat,
  you can see a thin line that runs down the center
  of the abdominal wall.
• It starts near the end of the sternum and runs
  through the navel to the pubis. This is called
  the linea alba, or white line. It is not
  muscle, and that's why it is labeled in blue ink.
  Instead, this is a band of connective tissue that
  binds all of the abdominal muscles.

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• On either side of the linea alba, you will find
  the rectus abdominis.
• This straight muscle is covered with a sheath of
  connective tissue.
• Once again, the sheath is not a muscle, so it is
  labeled in blue.
• On the left side of the figure, the sheath has
  been removed to reveal the rectus abdominis. This
  muscle flexes the vertebral column. You can try
  this motion by taking a bow.
• Lateral to the rectus abdominis, you will find
  the external oblique muscle. It flexes the
  vertebral column but can also rotate it.
• The internal oblique does the same job, but it
  lies deep to the external oblique. Beneath the
  internal oblique, you will find the transversus
  abdominis. This muscle does not act on the
  vertebral column. Its main job is to compress the
  abdomen.

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The Major Muscles of the Shoulder, Back, and Arm

• Looking at the muscles from a posterior point of
  view, we see some of the major muscles of the
  shoulder and upper arm.
• The trapezius muscle extends up the neck to the
  shoulder and down the thoracic vertebrae.
• This muscle extends and laterally flexes the
  head. It is sometimes called the shoulder
  shrugging muscle as it can also elevate,
  depress, and retract the scapula.

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• The latissimus dorsi works opposite the deltoid
  to adduct the arm.
• It brings an arm which is pointing away from the
  trunk back to anatomical position. It also
  medially rotates and extends the arm.
• When you row a boat or use exercise equipment to
  pull powerfully downward, you are working your
  lats.

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• The teres major works with the latissimus dorsi
  to adduct, extend, and medially rotate the arm.
• The rhomboideus major and rhomboideus minor work
  with the levator scapulae, the trapezius, the
  serratus anterior, and the pectoralis minor to
  move the scapula.
• This provides for a wide range of motion, and it
  also helps stabilize the scapula.
• The triceps brachii, as we discussed previously,
  works to extend the forearm at the elbow.

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• The shoulder joint is a ball-and-socket joint.
  To provide maximum range of motion, the socket is
  rather shallow.
• In addition, because ligaments reduce the range
  of motion in a joint, the ball of the humerus
  is held into the socket of the scapula by
  muscle tension during shoulder movements.
• The four muscles that cause this tension are the
  supraspinatus, infraspinatus, teres minor, and
  subscapularis.
• These four muscles provide joint security and
  are often called the rotator cuff muscles. A
  rotator cuff injury (damage to one or more of
  these muscles) is quite common in many sports.

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This picture will be on Chapter 6 Exam!
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• The biceps brachii flexes the forearm at the
  elbow.
• When you flex your forearm to show your muscle,
  the muscle that you see bulging is the biceps
  brachii.
• The brachioradialis and the pronator teres aid
  the biceps brachii in flexing the forearm.
• As its name implies, however, the pronator teres
  is also involved in pronation of the forearm at
  the elbow, which is the palms down movement.
• The pronator quadratus aids the pronator teres in
  this function.

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• The supinator supinates the forearm.
• The biceps brachii, because of its point of
  insertion at the radius, aids the supinator in
  this function.
• The biceps brachii aids the supinator, supination
  it is a more powerful action than pronation.

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• The flexor carpi radialis flexes and abducts the
  wrist, and the flexor carpi ulnaris also flexes
  the wrist.
• Rather than abducting the wrist, however, the
  flexor carpi ulnaris acts as the antagonist of
  the flexor carpi radialis, adducting the wrist.
• If you put your hands in anatomical position, you
  are adducting the wrist when you bring your
  little fingers together. When you cock your thumb
  (like you are hitchhiking), you are abducting
  your wrist. There is an easy way to remember
  this, because hitchhikers get abducted a lot.
• The palmaris longus is also involved in flexing
  the wrist. The extensor carpi radialis longus on
  the posterior forearm is an antagonist to the
  three muscles above, because it extends the
  wrist.

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• The flexor digitorum superficialis flexes both
  the wrist and the four fingers (not the thumb) to
  which it inserts.
• There is a deeper muscle called the flexor
  digitorum profundus which is not shown. It aids
  the flexor digitorum superficialis in its
  function.

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• On the posterior side of the forearm, we find
  mostly the extensor muscles.
• The extensor carpi radialis longus and the
  shorter extensor carpi radialis brevis extend and
  abduct the wrist.
• While the extensor carpi ulnaris extends the
  wrist with the two muscles above, it is also an
  antagonist to those same muscles because it
  adducts the wrist.
• Notice that the wrist adductors are on the medial
  side, while the wrist abductors are on the
  lateral side. This should make sense to you,
  given the motion they create.
• The extensor digitorum of the posterior forearm
  is the antagonist of the flexor digitorum
  superficialis, as it extends the wrists and the
  four fingers (not the thumb).

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This picture will be on Chapter 6 Exam!
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• One important structure illustrated in the figure
  is the extensor retinaculum.
• It is not a muscle, and that's why it is labeled
  in blue. Instead, it is a bracelet of dense
  regular connective tissue which winds around the
  wrist, covering the extensor tendons.
• This holds the tendons down as the muscles work.
  If it weren't for the extensor retinaculum, the
  extensor tendons would bow outwards when the
  extensors contracted. There is a similar
  structure for the flexors, called the flexor
  retinaculum.

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Muscles of the Hand

• The hand is a strong yet delicate instrument.
• It gets its strength from the fact that it is
  cable operated by powerful muscles in the
  forearm. The cables are long, thin tendons that
  run from the muscle in the forearm to the hand.
• You can see some of these tendons by looking at
  the posterior side of your hand and then making a
  cat's claw. The ridges you see rise underneath
  the skin are tendons from the muscles in the
  forearm.
• Because these muscles are in the forearm but make
  motion in the hands, they are called the
  extrinsic hand muscles.

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• Extrinsic hand muscles - Muscles in the forearm
  which create motion in the handsThe term
  extrinsic refers to the fact that the muscles
  control hand motions but are in the forearm.

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• Where do the hands get their ability to move
  delicately enough to play the guitar or thread a
  needle? That delicate motion comes mostly from
  tiny intrinsic hand muscles.Intrinsic hand
  muscles - Muscles within the hand which create
  motion in the hand

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Major Muscles of the Thigh
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• Many of the lateral muscles can be seen in the
  posterior view as well.
• The most prominent, of course, is the gluteus
  maximus. It extends, abducts, and laterally
  rotates the thigh. As a result, it is a very
  important muscle for running.
• The gluteus medius runs deep to the gluteus
  maximus. This is one of the most common parts of
  your body into which a nurse will inject
  medicine.
• The gluteus medius abducts and medially rotates
  the thigh.
• Deep to the gluteus medius is the gluteus
  minimus. It also abducts and medially rotates the
  thigh.

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• Looking now at the posterior view, you can see
  the biceps femoris, the semimembranosus, and the
  semitendinosus.
• These three muscles are known collectively as the
  hamstring muscles. Their main jobs are to flex
  the leg and extend the thigh.

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• The iliacus and the psoas major flex the thigh.
  They have the same insertion and work towards the
  same motion, so they are often considered one
  muscle, the iliopsoas.
• When you do sit-ups, these two muscles do most of
  the work. These muscles are unusual in that they
  originate in the posterior abdominal wall, on the
  lumbar vertebrae and ilium. They insert into a
  process on the proximal end of the femur.
• In people who sit for long periods of time, these
  muscles do little work. As a result, they tend to
  shorten. This causes backaches when the person
  stands. The fencer's position (one lower limb
  forward, the other one extended) stretches the
  iliopsoas passively. This can help relieve such
  backache, if it is done regularly.

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• The rectus femoris, the vastus lateralis, the
  vastus intermedius, and the vastus medialis are
  grouped together and called the quadriceps
  femoris.
• They are grouped together in this way because
  their insertions are all on a process of the
  tibia.
• The quadriceps extends the leg (in human anatomy,
  leg means only the lower leg). In addition,
  the rectus femoris flexes the thigh.
• The sartorius is the longest muscle in the body.
  It flexes and laterally rotates the thigh. It
  also flexes the leg. When you sit Indian style,
  you are using all three actions of your sartorius
  muscle.
• The tensor fasciae latae also flexes the thigh.
  In addition it abducts and medially rotates the
  thigh.

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• The adductor longus and the adductor magnus work
  together to adduct the thigh.
• These are the muscles that will ache after a long
  day of horseback riding. They also work together
  to rotate the thigh laterally. They are
  antagonists when it comes to flexion and
  extension, however.
• The former flexes the thigh while the latter
  extends it.
• The gracilis adducts the thigh and flexes the
  leg.

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Major Muscles of the Leg

• The gastrocnemius and soleus muscles form the
  bulge on the posterior side of the leg which is
  commonly called the calf.
• They both plantar flex the foot (which is the
  motion of standing on your tiptoes) and the
  gastrocnemius also flexes the leg. They insert
  into the calcaneus (heel bone) via the calcaneal
  tendon. This is commonly referred to as the
  Achilles tendon, and is often the site of
  injury in athletes.
• Why is it called the Achilles tendon? This is a
  reference to the Greek legend of Achilles, a hero
  whose mother dipped him into the river Styx to
  make him invincible. However, she had to hold
  onto him when she dipped him into the river, so
  she held onto his heel. Thus, his heel was not
  covered in the water, and that was his only
  vulnerable spot.

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• The tibialis anterior works to dorsiflex (lift
  the foot up) and invert the foot (turn the foot
  inward).
• The peroneus longus and peroneus brevis muscles
  evert the foot against the tibialis anterior.
  These peroneus muscles are especially active when
  you are walking on your toes, because they also
  plantar flex the foot.
• The flexor digitorum longus flexes the four
  lateral toes (not the big toe). You use this
  muscle, for example, when you curl your toes.
• The extensor digitorum longus is the antagonist,
  as it extends the four lateral toes. Notice the
  extensor retinacula (plural of retinaculum) in
  the figure. These structures perform the same
  task as the retinacula in the wrist - they keep
  the tendons from bowing when the muscles are
  contracted.

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• The only two we have not discussed are shown in
  the lower middle drawing. The extensor hallucis
  longus dorsiflexes and inverts the foot as well
  as extends the big toe. The peroneus tertius has
  essentially the same origin as the peroneus
  muscles discussed above, but it is really just a
  part of the extensor digitorum longus.
• The other structures of note in this figure are
  not muscles at all. The patella is a sesamoid
  bone. It is located in the tendon of the knee
  joint, and it acts like a lever, reducing the
  force that the muscles must exert in order to
  move the femur. Extending from the patella is the
  patellar ligament. When testing your reflexes,
  your doctor will strike this ligament with a
  hammer.