6 The Muscular System.

6 The Muscular System

The Muscular System Muscles are responsible for all types of body movement Three basic muscle types are found in the body Skeletal muscle Cardiac muscle Smooth muscle

Characteristics of Muscles
Skeletal and smooth muscle cells are elongated (muscle cell = muscle fiber) Contraction and shortening of muscles is due to the movement of microfilaments All muscles share some terminology Prefixes myo and mys refer to “muscle” Prefix sarco refers to “flesh”

Table 6.1

Comparison of Skeletal, Cardiac, and Smooth Muscles
Characteristic Skeletal Cardiac Smooth Body location Attached to bone or skin (for some facial muscles) Walls of the heart Mostly in walls of visceral organs (other than the heart) Cell shape and appearance Single, very long, cylindrical, multinucleate cells with very obvious striations Branching chains of cells, uninucleate, striations, intercalated discs Single, fusiform, uninucleate, no striations Connective tissue components Endomysium, perimysium, and epimysium Endomysium

Comparison of Skeletal, Cardiac, and Smooth Muscles
Characteristic Skeletal Cardiac Smooth Regulation of contraction Voluntary Involuntary Speed of contraction Slow to fast Slow Very slow Rhythmic contractions No Yes Yes, in some

Skeletal Muscle Characteristics
Most are attached by tendons to bones Cells are multinucleate Striated—have visible banding Voluntary—subject to conscious control

Connective Tissue Wrappings of Skeletal Muscle
Cells are surrounded and bundled by connective tissue Endomysium—encloses a single muscle fiber Perimysium—wraps around a fascicle (bundle) of muscle fibers Epimysium—covers the entire skeletal muscle Fascia—on the outside of the epimysium

Fascicle (wrapped by perimysium)
Muscle fiber (cell) Blood vessel Perimysium Epimysium (wraps entire muscle) Fascicle (wrapped by perimysium) Endomysium (between fibers) Tendon Bone Figure 6.1

Skeletal Muscle Attachments
Epimysium blends into a connective tissue attachment Tendons—cord-like structures Mostly collagen fibers Often cross a joint due to toughness and small size Aponeuroses—sheet-like structures Attach muscles indirectly to bones, cartilages, or connective tissue coverings

Skeletal Muscle Attachments
Sites of muscle attachment Bones Cartilages Connective tissue coverings

Smooth Muscle Characteristics
Lacks striations Spindle-shaped cells Single nucleus Involuntary—no conscious control Found mainly in the walls of hollow organs

Longitudinal layer of smooth muscle (cross-sectional view of cells)
Circular layer of smooth muscle (longitudinal view of cells) Mucosa Submucosa Longitudinal layer of smooth muscle (cross-sectional view of cells) (a) Figure 6.2a

Cardiac Muscle Characteristics
Striations Usually has a single nucleus Branching cells Joined to another muscle cell at an intercalated disc Involuntary Found only in the walls of the heart

Cardiac muscle bundles
Figure 6.2b

Skeletal Muscle Functions
Produce movement Maintain posture Stabilize joints Generate heat

Microscopic Anatomy of Skeletal Muscle
Sarcolemma—specialized plasma membrane Myofibrils—long organelles inside muscle cell Sarcoplasmic reticulum—specialized smooth endoplasmic reticulum

(a) Segment of a muscle fiber (cell)
Sarcolemma Myofibril Dark (A) band Light (I) band Nucleus (a) Segment of a muscle fiber (cell) Figure 6.3a

Myofibrils are aligned to give distinct bands I band = light band Contains only thin filaments A band = dark band Contains the entire length of the thick filaments

Thick (myosin) filament
Z disc H zone Z disc Thin (actin) filament Thick (myosin) filament (b) Myofibril or fibril (complex organelle composed of bundles of myofilaments) I band A band I band M line Figure 6.3b

Sarcomere—contractile unit of a muscle fiber Organization of the sarcomere Myofilaments Thick filaments = myosin filaments Thin filaments = actin filaments

Thick filaments = myosin filaments Composed of the protein myosin Has ATPase enzymes Myosin filaments have heads (extensions, or cross bridges) Myosin and actin overlap somewhat Thin filaments = actin filaments Composed of the protein actin Anchored to the Z disc

Thick (myosin) filament
Sarcomere M line Z disc Z disc Thin (actin) filament Thick (myosin) filament (c) Sarcomere (segment of a myofibril) Figure 6.3c

At rest, within the A band there is a zone that lacks actin filaments Called either the H zone or bare zone Sarcoplasmic reticulum (SR) Stores and releases calcium Surrounds the myofibril

(d) Myofilament structure (within one sarcomere)
Thick filament Bare zone Thin filament (d) Myofilament structure (within one sarcomere) Figure 6.3d

Stimulation and Contraction of Single Skeletal Muscle Cells
Excitability (also called responsiveness or irritability)—ability to receive and respond to a stimulus Contractility—ability to shorten when an adequate stimulus is received Extensibility—ability of muscle cells to be stretched Elasticity—ability to recoil and resume resting length after stretching

The Nerve Stimulus and Action Potential
Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract Motor unit—one motor neuron and all the skeletal muscle cells stimulated by that neuron

Axon terminals at neuromuscular junctions Spinal cord
Motor unit 1 Motor unit 2 Nerve Axon of motor neuron Motor neuron cell bodies Muscle Muscle fibers (a) Figure 6.4a

Axon terminals at neuromuscular junctions Muscle fibers
Branching axon to motor unit (b) Figure 6.4b

Neuromuscular junction Association site of axon terminal of the motor neuron and muscle PLAY A&P Flix™: Events at the Neuromuscular Junction

Figure 6.5

Synaptic cleft Gap between nerve and muscle Nerve and muscle do not make contact Area between nerve and muscle is filled with interstitial fluid Action potential reaches the axon terminal of the motor neuron Calcium channels open and calcium ions enter the axon terminal

Transmission of Nerve Impulse to Muscle
Calcium ion entry causes some synaptic vesicles to release their contents (acetylcholine, a neurotransmitter) by exocytosis Neurotransmitter—chemical released by nerve upon arrival of nerve impulse in the axon terminal The neurotransmitter for skeletal muscle is acetylcholine (ACh)

Transmission of Nerve Impulse to Muscle
Acetylcholine attaches to receptors on the sarcolemma of the muscle cell In response to the binding of ACh to a receptor, the sarcolemma becomes permeable to sodium (Na+) Sodium rushes into the cell generating an action potential and potassium leaves the cell Once started, muscle contraction cannot be stopped

Figure 6.5, step 1 Synaptic vesicle containing ACh
Action potential reaches axon terminal of motor neuron. 1 Axon terminal of motor neuron Mitochondrion Ca2+ Ca2+ Synaptic cleft Sarcolemma Fusing synaptic vesicle Sarcoplasm of muscle fiber ACh Folds of sarcolemma ACh receptor Figure 6.5, step 1

Action potential reaches axon terminal of motor neuron. 1 Axon terminal of motor neuron Mitochondrion Calcium (Ca2+) channels open and Ca2+ enters the axon terminal. 2 Ca2+ Ca2+ Synaptic cleft Sarcolemma Fusing synaptic vesicle Sarcoplasm of muscle fiber ACh Folds of sarcolemma ACh receptor Figure 6.5, step 2

Action potential reaches axon terminal of motor neuron. 1 Axon terminal of motor neuron Mitochondrion Calcium (Ca2+) channels open and Ca2+ enters the axon terminal. 2 Ca2+ Ca2+ Synaptic cleft Sarcolemma Fusing synaptic vesicle Sarcoplasm of muscle fiber Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine, a neurotransmitter) by exocytosis. 3 ACh Folds of sarcolemma ACh receptor Figure 6.5, step 3

Action potential reaches axon terminal of motor neuron. 1 Axon terminal of motor neuron Mitochondrion Calcium (Ca2+) channels open and Ca2+ enters the axon terminal. 2 Ca2+ Ca2+ Synaptic cleft Sarcolemma Fusing synaptic vesicle Sarcoplasm of muscle fiber Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine, a neurotransmitter) by exocytosis. 3 ACh Folds of sarcolemma ACh receptor Acetylcholine diffuses across the synaptic cleft and binds to receptors in the sarcolemma. 4 Figure 6.5, step 4

Figure 6.5, step 5 Ion channel in sarcolemma opens; ions pass. Na+ K+
ACh binds and channels open that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. More Na+ ions enter than K+ ions leave and this produces a local change in the electrical conditions of the membrane (depolarization), which eventually leads to an action potential. Figure 6.5, step 5

Figure 6.5, step 6 ACh Degraded ACh
Ion channel closed; ions cannot pass. Na+ ACh effects are ended by its breakdown in the synaptic cleft by the enzyme acetylcholinesterase. 6 Acetylcholinesterase K+ Figure 6.5, step 6

Neuromuscular junction
Muscle cell or fiber Small twig Nerve fiber Striations Match flame Na+ diffuses into the cell. 1 Flame ignites the twig. 1 Flame spreads rapidly along the twig. 2 Action potential spreads rapidly along the sarcolemma. 2 (a) (b) Figure 6.6a-b

The Sliding Filament Theory of Muscle Contraction
Activation by nerve causes myosin heads (cross bridges) to attach to binding sites on the thin filament Myosin heads then bind to the next site of the thin filament and pull them toward the center of the sarcomere This continued action causes a sliding of the myosin along the actin The result is that the muscle is shortened (contracted)

Myosin Actin Z H Z I A I (a) Z Z I A I (b) Figure 6.7a–b

Protein complex In a relaxed muscle cell, the regulatory proteins forming part of the actin myofilaments prevent myosin binding (see a). When an action potential (AP) sweeps along its sarcolemma and a muscle cell is excited, calcium ions (Ca2+) are released from intracellular storage areas (the sacs of the sarcoplasmic reticulum). Myosin myofilament Actin myofilament (a) Figure 6.8a

Myosin-binding site Ca2+ The flood of calcium acts as the final trigger for contraction, because as calcium binds to the regulatory proteins on the actin filaments, the proteins undergo a change in both their shape and their position on the thin filaments. This action exposes myosin-binding sites on the actin, to which the myosin heads can attach (see b), and the myosin heads immediately begin seeking out binding sites. Upper part of thick filament only (b) Figure 6.8b

A&P Flix™: The Cross Bridge Cycle
PLAY A&P Flix™: The Cross Bridge Cycle Figure 6.8c

Contraction of Skeletal Muscle
Muscle fiber contraction is “all or none” Within a skeletal muscle, not all fibers may be stimulated during the same interval Different combinations of muscle fiber contractions may give differing responses Graded responses—different degrees of skeletal muscle shortening

Contraction of Skeletal Muscle
Graded responses can be produced by changing: The frequency of muscle stimulation The number of muscle cells being stimulated at one time

Types of Graded Responses
Twitch Single, brief contraction Not a normal muscle function

Figure 6.9a

Summing of contractions One contraction is immediately followed by another The muscle does not completely return to a resting state due to more frequent stimulations The effects are added

Figure 6.9b

Unfused (incomplete) tetanus Some relaxation occurs between contractions but nerve stimuli arrive at an even faster rate than during summing of contractions Unless the muscle contraction is smooth and sustained, it is said to be in unfused tetanus

Figure 6.9c

Fused (complete) tetanus No evidence of relaxation before the following contractions Frequency of stimulations does not allow for relaxation between contractions The result is a smooth and sustained muscle contraction

Figure 6.9d

Muscle Response to Strong Stimuli
Muscle force depends upon the number of fibers stimulated More fibers contracting results in greater muscle tension Muscles can continue to contract unless they run out of energy

Energy for Muscle Contraction
Initially, muscles use stored ATP for energy ATP bonds are broken to release energy Only 4–6 seconds worth of ATP is stored by muscles After this initial time, other pathways must be utilized to produce ATP

Direct phosphorylation of ADP by creatine phosphate (CP) Muscle cells store CP CP is a high-energy molecule After ATP is depleted, ADP is left CP transfers a phosphate group to ADP, to regenerate ATP CP supplies are exhausted in less than 15 seconds About 1 ATP is created per CP molecule

Figure 6.10a

Aerobic respiration Glucose is broken down to carbon dioxide and water, releasing energy (about 32 ATP) A series of metabolic pathways occur in the mitochondria This is a slower reaction that requires continuous oxygen Carbon dioxide and water are produced

Figure 6.10c

Anaerobic glycolysis and lactic acid formation Reaction that breaks down glucose without oxygen Glucose is broken down to pyruvic acid to produce about 2 ATP Pyruvic acid is converted to lactic acid This reaction is not as efficient, but is fast Huge amounts of glucose are needed Lactic acid produces muscle fatigue

Figure 6.10b

Muscle Fatigue and Oxygen Deficit
When a muscle is fatigued, it is unable to contract even with a stimulus Common cause for muscle fatigue is oxygen debt Oxygen must be “repaid” to tissue to remove oxygen deficit Oxygen is required to get rid of accumulated lactic acid Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

Types of Muscle Contractions
Isotonic contractions Myofilaments are able to slide past each other during contractions The muscle shortens and movement occurs Example: bending the knee; rotating the arm Isometric contractions Tension in the muscles increases The muscle is unable to shorten or produce movement Example: push against a wall with bent elbows

Muscle Tone Some fibers are contracted even in a relaxed muscle
Different fibers contract at different times to provide muscle tone and to be constantly ready

Effect of Exercise on Muscles
Exercise increases muscle size, strength, and endurance Aerobic (endurance) exercise (biking, jogging) results in stronger, more flexible muscles with greater resistance to fatigue Makes body metabolism more efficient Improves digestion, coordination Resistance (isometric) exercise (weight lifting) increases muscle size and strength

Figure 6.11a-b

Five Golden Rules of Skeletal Muscle Activity
1. With a few exceptions, all skeletal muscles cross at least one joint. 2. Typically, the bulk of a skeletal muscle lies proximal to the joint crossed. 3. All skeletal muscles have at least two attachments: the origin and the insertion. 4. Skeletal muscles can only pull; they never push. 5. During contraction, a skeletal muscle insertion moves toward the origin.

Muscles and Body Movements
Movement is attained due to a muscle moving an attached bone Muscles are attached to at least two points Origin Attachment to a moveable bone Insertion Attachment to an immovable bone

Muscle contracting Origin Brachialis Tendon Insertion Figure 6.12

Types of Body Movements
Flexion Decreases the angle of the joint Brings two bones closer together Typical of bending hinge joints like knee and elbow or ball-and-socket joints like the hip Extension Opposite of flexion Increases angle between two bones Typical of straightening the elbow or knee Extension beyond 180° is hypertension

Figure 6.13a

Figure 6.13b

Rotation Movement of a bone around its longitudinal axis Common in ball-and-socket joints Example is when you move atlas around the dens of axis (shake your head “no”)

Figure 6.13c

Abduction Movement of a limb away from the midline Adduction Opposite of abduction Movement of a limb toward the midline

Figure 6.13d

Circumduction Combination of flexion, extension, abduction, and adduction Common in ball-and-socket joints

Figure 6.13d

Special Movements Dorsiflexion
Lifting the foot so that the superior surface approaches the shin (toward the dorsum) Plantar flexion Depressing the foot (pointing the toes) “Planting” the foot toward the sole

Figure 6.13e

Special Movements Inversion Turn sole of foot medially Eversion
Turn sole of foot laterally

Figure 6.13f

Special Movements Supination
Forearm rotates laterally so palm faces anteriorly Radius and ulna are parallel Pronation Forearm rotates medially so palm faces posteriorly Radius and ulna cross each other like an X

Figure 6.13g

Special Movements Opposition
Move thumb to touch the tips of other fingers on the same hand

Figure 6.13h

Types of Muscles Prime mover—muscle with the major responsibility for a certain movement Antagonist—muscle that opposes or reverses a prime mover Synergist—muscle that aids a prime mover in a movement and helps prevent rotation Fixator—stabilizes the origin of a prime mover

(a) A muscle that crosses on the anterior side of a joint produces flexion*
Example: Pectoralis major (anterior view) Figure 6.14a

(b) A muscle that crosses on the posterior side of a joint produces extension*
Example: Latissimus dorsi (posterior view) Figure 6.14b

(c) A muscle that crosses on the lateral side of a joint produces abduction
Example: Medial deltoid (anterolateral view) Figure 6.14c

(d) A muscle that crosses on the medial side of a joint produces adduction
Example: Teres major (posterolateral view) Figure 6.14d

Naming Skeletal Muscles
By direction of muscle fibers Example: Rectus (straight) By relative size of the muscle Example: Maximus (largest)

By location of the muscle Example: Temporalis (temporal bone) By number of origins Example: Triceps (three heads)

By location of the muscle’s origin and insertion Example: Sterno (on the sternum) By shape of the muscle Example: Deltoid (triangular) By action of the muscle Example: Flexor and extensor (flexes or extends a bone)

Extensor digitorum longus
Orbicularis oris Pectoralis major Deltoid (d) Circular (a) Convergent (e) Multipennate Biceps brachii (d) Rectus femoris (e) (a) (b) (c) (b) Fusiform (f) Bipennate Sartorius (f) Extensor digitorum longus (g) (c) Parallel (g) Unipennate Figure 6.15

Head and Neck Muscles Facial muscles Frontalis—raises eyebrows
Orbicularis oculi—closes eyes, squints, blinks, winks Orbicularis oris—closes mouth and protrudes the lips Buccinator—flattens the cheek, chews Zygomaticus—raises corners of the mouth Chewing muscles Masseter—closes the jaw and elevates mandible Temporalis—synergist of the masseter, closes jaw

Head and Neck Muscles Neck muscles
Platysma—pulls the corners of the mouth inferiorly Sternocleidomastoid—flexes the neck, rotates the head

Cranial aponeurosis Frontalis Temporalis Orbicularis oculi Occipitalis
Zygomaticus Buccinator Masseter Orbicularis oris Sternocleidomastoid Trapezius Platysma Figure 6.16

Muscles of Trunk, Shoulder, Arm
Anterior muscles Pectoralis major—adducts and flexes the humerus Intercostal muscles External intercostals—raise rib cage during inhalation Internal intercostals—depress the rib cage to move air out of the lungs when you exhale forcibly

Clavicle Deltoid Sternum Pectoralis major Biceps brachii Brachialis
Brachio- radialis (a) Figure 6.17a

Muscles of the abdominal girdle Rectus abdominis—flexes vertebral column and compresses abdominal contents (defecation, childbirth, forced breathing) External oblique—flex vertebral column; rotate trunk and bend it laterally Internal oblique—flex vertebral column; rotate trunk and bend it laterally Transversus abdominis—compresses abdominal contents

Transversus abdominis
Pectoralis major Rectus abdominis Transversus abdominis Internal oblique External oblique Aponeurosis (b) Figure 6.17b

Posterior muscles Trapezius—elevates, depresses, adducts, and stabilizes the scapula Latissimus dorsi—extends and adducts the humerus Erector spinae—back extension Quadratus lumborum—flexes the spine laterally Deltoid—arm abduction

Muscles that arise from the shoulder girdle and cross the shoulder joint to insert into the humerus include: Pectoralis major Latissimus dorsi Deltoid PLAY A&P Flix™: Muscles that act on the shoulder joint and humerus: An overview. PLAY A&P Flix™: Muscles of the pectoral girdle. PLAY A&P Flix™: Muscles that cross the glenohumeral joint. PLAY A&P Flix™: Movement at the glenohumeral joint: An overview.

Occipital bone Sternocleidomastoid Spine of scapula Trapezius
Deltoid (cut) Deltoid Triceps brachii Latissimus dorsi Humerus Olecranon process of ulna (deep to tendon) (a) Figure 6.18a

C7 T1 Erector spinae • Iliocostalis • Longissimus • Spinalis
Quadratus Iumborum (b) Figure 6.18b

Muscles of the Upper Limb
Biceps brachii—supinates forearm, flexes elbow Brachialis—elbow flexion Brachioradialis—weak muscle; elbow flexion Triceps brachii—elbow extension (antagonist to biceps brachii) PLAY A&P Flix™: The elbow joint and forearm: An overview. PLAY A&P Flix™: Muscles of the elbow joint. PLAY A&P Flix™: Movement at the elbow joint.

Clavicle Deltoid Sternum Pectoralis major Biceps brachii Brachialis
Brachio- radialis (a) Figure 6.17a

Occipital bone Sternocleidomastoid Spine of scapula Trapezius
Deltoid (cut) Deltoid Triceps brachii Latissimus dorsi Humerus Olecranon process of ulna (deep to tendon) (a) Figure 6.18a

Muscles of the Upper Limb
Muscles of the forearm, which insert on the hand bones and cause their movement include: Flexor carpi—wrist flexion Flexor digitorum—finger flexion Extensor carpi—wrist extension Extensor digitorum—finger extension PLAY A&P Flix™: Muscles that act on the wrist and fingers: An overview. PLAY A&P Flix™: Movements of the wrist and fingers (a). PLAY A&P Flix™: Movements of the wrist and fingers (b).

Muscles of the Lower Limb
Muscles causing movement at the hip joint include: Gluteus maximus—hip extension Gluteus medius—hip abduction, steadies pelvis when walking Iliopsoas—hip flexion, keeps the upper body from falling backward when standing erect Adductor muscles—adduct the thighs PLAY A&P Flix™: Muscles that act on the hip joint and femur: An overview. PLAY A&P Flix™: Movement at the hip joint: An overview.

Gluteus medius Gluteus maximus Adductor magnus Iliotibial tract Biceps femoris Semitendinosus Hamstring group Semimembranosus Gastrocnemius (a) Figure 6.20a

Posterior superior iliac spine
IIiac crest Safe area in gluteus medius Gluteus maximus Sciatic nerve (b) Figure 6.20b

12th thoracic vertebra 12th rib Iliac crest Psoas major lliopsoas lliacus 5th lumbar vertebra Anterior superior iliac spine Sartorius Adductor group Rectus femoris Vastus lateralis Quadriceps Vastus medialis Patella Patellar ligament (c) Figure 6.20c

Muscles causing movement at the knee joint Hamstring group—thigh extension and knee flexion Biceps femoris Semimembranosus Semitendinosus

Gluteus medius Gluteus maximus Adductor magnus Iliotibial tract Biceps femoris Semitendinosus Hamstring group Semimembranosus Gastrocnemius (a) Figure 6.20a

Muscles causing movement at the knee joint Sartorius—flexes the thigh Quadriceps group—extends the knee Rectus femoris Vastus muscles (three) PLAY A&P Flix™: Muscles that cross the knee joint: An overview.

12th thoracic vertebra 12th rib Iliac crest Psoas major lliopsoas lliacus 5th lumbar vertebra Anterior superior iliac spine Sartorius Adductor group Rectus femoris Vastus lateralis Quadriceps Vastus medialis Patella Patellar ligament (c) Figure 6.20c

Inguinal ligament Adductor muscles Sartorius Vastus lateralis (d)
Figure 6.20d

Muscles causing movement at ankle and foot Tibialis anterior—dorsiflexion, foot inversion Extensor digitorum longus—toe extension and dorsiflexion of the foot Fibularis muscles—plantar flexion, foot eversion Soleus—plantar flexion PLAY A&P Flix™: Muscles that act on the ankle and foot: An overview. PLAY A&P Flix™: Posterior muscles that act on the ankle and foot. PLAY A&P Flix™: Movements of the ankle and foot.

Fibularis longus Tibia Fibularis brevis Soleus Tibialis anterior Extensor digitorum longus Fibularis tertius (a) Figure 6.21a

Gastrocnemius Soleus Calcaneal (Achilles) tendon Medial malleolus Lateral malleolus (b) Figure 6.21b

Figure 6.22 Facial • Frontalis Facial • Orbicularis oculi • Temporalis
• Zygomaticus • Masseter • Orbicularis oris Neck Shoulder • Platysma • Trapezius • Sternocleidomastoid Thorax • Deltoid • Pectoralis minor • Pectoralis major Arm • Serratus anterior • Triceps brachii • Biceps brachii • Intercostals • Brachialis Abdomen • Rectus abdominis Forearm • External oblique • Brachioradialis • Internal oblique • Flexor carpi radialis • Transversus abdominis Pelvis/thigh • lliopsoas Thigh • Sartorius • Adductor muscle Thigh (Quadriceps) • Rectus femoris • Gracilis • Vastus lateralis • Vastus medialis Leg • Fibularis longus • Extensor digitorum longus Leg • Gastrocnemius • Tibialis anterior • Soleus Figure 6.22

Figure 6.23 Neck • Occipitalis • Sternocleidomastoid • Trapezius
Shoulder/Back • Deltoid Arm • Triceps brachii • Brachialis • Latissimus dorsi Forearm • Brachioradialis • Extensor carpi radialis longus • Flexor carpi ulnaris • Extensor carpi ulnaris Hip • Extensor digitorum • Gluteus medius • Gluteus maximus Thigh lliotibial tract • Adductor muscle • Hamstrings: Biceps femoris Semitendinosus Semimembranosus Leg • Gastrocnemius • Soleus • Fibularis longus Calcaneal (Achilles) tendon Figure 6.23

Deltoid muscle Humerus Figure 6.19

Posterior superior iliac spine
IIiac crest Safe area in gluteus medius Gluteus maximus Sciatic nerve (b) Figure 6.20b

Inguinal ligament Adductor muscles Sartorius Vastus lateralis (d)
Figure 6.20d

6 The Muscular System.

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