Back to “Visually Memorable Systemic Anatomy”

3. Muscular system

 

 

 


 

Fig. 3-1.

 

There are three kinds of muscles: skeletal muscle, cardiac muscle, and smooth muscle (Fig. 16-30).

 

 

Fig. 3-2.

 

Once the origin and insertion of a (skeletal) muscle are known, the action of the muscle can be anticipated. In the simplified figure of muscle, the insertion is represented as arrowhead (Fig. 3-13).

 

Fig. 3-3.

 

Tendons connect the muscle to the bone, while ligaments connect the bone to another bone (Fig. 2-28). Both the tendon and ligament are dense regular connective tissue (Fig. 16-26).

 

Fig. 3-4.

 

Aponeurosis is a kind of tendon which is flat (Fig. 15-7). Sometimes, the muscle which is not flat becomes the aponeurosis: The palmaris longus becomes the palmar aponeurosis (Fig. 3-54).

 

< Muscles of head, neck >

 

 

Fig. 3-5.

 

Typical skeletal muscle ends at the bone. However, the facial muscle is exceptional as it ends at the skin (Fig. 15).

 

Fig. 3-6.

 

In four-legged animal, this kind of muscles which reach and pull the skin runs in the subcutaneous tissue (Fig. 15-5).

 

Fig. 3-7.

 

For instance, contraction of the frontalis causes horizontal wrinkles in the forehead (Fig. 15). A facial muscle and the wrinkles formed by the muscle are in the right angle.

 

Fig. 3-8.

 

The surrounding muscle which narrows a passage is called sphincter (Fig. 4-36) (Fig. 6-18) or constrictor (Fig. 4-22). Another term is orbicularis, found around the eye and mouth. The orbicularis oculi (Fig. 14-24) and orbicularis oris make wrinkles that make us look aged (Fig. 3-9).

 

Fig. 3-9. Facial muscles.

 

There are numerous facial muscles which move the mouth. The levator anguli oris plays an opposite role from depressor anguli oris; the levator labii superioris from depressor labii inferioris.

 

Fig. 3-10.

 

A small part of “levator labii superioris” around the nose is “levator labii superioris alaeque nasi.” However, the authors require students to only remember the main muscle, “levator labii superioris.”

 

Fig. 3-11.

 

The zygomaticus minor and major (Fig. 3-9) which lift the angle of mouth are used for happy expression (Fig. 3-5). Dimple that appears when smiling is a variation of the zygomaticus major. The variation is only morphological deviation without functional defect.

 

Fig. 3-12. Platysma.

 

The platysma is also a facial muscle, even though most of the muscle is located in the neck. We can intentionally make out the platysma prominent by depressing the angle of mouth.

 

Fig. 3-13. Masseter, temporal muscle.

 

Among the masticatory muscles, ones that are located at the external side of the mandible are the masseter (Fig. 4-12) and temporal muscle. Their action is to elevate the mandible (Fig. 1-12).

 

Fig. 3-14.

 

When one clenches one’s teeth, one can palpate the masseter and temporal muscle bulging out.

 

Fig. 3-15. Lateral and medial pterygoid muscles.

 

Lateral pterygoid muscle protracts the mandible forward, while medial pterygoid muscle elevates the mandible. We may regard the medial pterygoid muscle as the medial masseter (Fig. 3-13) (Fig. 4-12).

The antagonists of most masticatory muscles are the suprahyoid muscles (Fig. 3-19) (Fig. 3-20) and infrahyoid muscles (Fig. 3-21) which pull the mandible down from below. When masticatory muscles are relaxed, the mandible is also depressed by another invisible antagonist, the gravity. This often takes place when we dose off in class.

 

Fig. 3-16. Sternocleidomastoid muscle.

 

The sternocleidomastoid muscle is named after its two origins (sternum and clavicle) (Fig. 1-20) and one insertion (mastoid process of temporal bone) (Fig. 1-11). The three bone structures are easily to be palpated.

 

Fig. 3-17.

 

If the right sternocleidomastoid muscle contracts, the head is rotated to the left. This happens because the obliquely situated muscle becomes more vertical and shortens in length.

 

Fig. 3-18.

 

Koreans and Americans develop their right sternocleidomastoid muscle, since people waiting for cars need to look on their left side. Do not be serious about this gossip.

 

 

Fig. 3-19. Suprahyoid muscles in lateral view (top) and sagittal plane (bottom).

The four suprahyoid muscles (digastric muscle, stylohyoid muscle, mylohyoid muscle, geniohyoid muscle) have the same function in common: elevating the hyoid bone (Fig. 5-9) (or to depress the mandible).

 

Fig. 3-20. Mylohyoid muscle in lateral view (left) and coronal plane (right).

 

The bilateral mylohyoid muscles form the floor of the oral cavity. If one touches the mylohyoid muscles below the mandible and swallow, one can feel the muscles contracting to elevate the hyoid bone (and larynx) (Fig. 5-10) and narrow the oral cavity (Fig. 4-20).

 

Fig. 3-21. Infrahyoid muscles (dotted arrows: deep muscles).

 

There are four infrahyoid muscles (omohyoid muscle, sternohyoid muscle, sternothyroid muscle, thyrohyoid muscle) on one side, just as there are four suprahyoid muscles on the same side. The infrahyoid muscles pull down the hyoid bone.

 

Fig. 3-22.

 

The sternothyroid muscle is likely to be called the sword-shield muscle.

 

< Muscles of thorax,

abdomen >

 

The thorax is composed of the thoracic wall and thoracic cavity. Equally, the abdomen is composed of the abdominal wall and abdominal cavity (Fig. 3-30); the pelvis is composed of the pelvic wall and pelvic cavity (Fig. 3-64). All three walls consist of bones and muscles.

 

Fig. 3-23. Intercostal muscles.

 

The thoracic wall consists of the thoracic vertebrae (Fig. 1-15), ribs (including costal cartilages), sternum (Fig. 1-18), and intercostal muscles (three layers) that elevate the ribs. The elevation of the ribs results in an enlarged thoracic cavity, which is for inhalation.

 

Fig. 3-24. Movements of thoracic wall for inspiration.

 

The mechanism between the rib elevation and thoracic cavity enlargement is explained by two movements. One is the bucket handle movement along the anteroposterior axis, which increases the mediolateral length of the thoracic cavity. At rest, the lateral part of the ribs is inferior to the medial part; the bucket handle movement elevates and abducts the lateral part.

Another is the pump handle movement along the mediolateral axis, which increases the anteroposterior length of the thoracic cavity. At rest, the anterior part of the ribs is inferior to the posterior part; the pump handle movement elevates and protracts the anterior part.

The mediolateral and anteroposterior elongations of thoracic cavity can be palpated with one’s own body during the deep inhalation.

 

Fig. 3-26. Diaphragm (sagittal plane).

 

The other movement to increase the superoinferior length is achieved by the diaphragm. When this dome-shaped muscle contracts, the muscle lowers itself (Fig. 5-2). This movement to increase superoinferior length of the thoracic cavity plays a bigger role than the previous two movements of the ribs (Fig. 3-24).

The diaphragm has three big foramina: the caval opening for inferior vena cava (Fig. 10-68), the esophageal hiatus for esophagus (Fig. 4-24), and the aortic hiatus for descending aorta (Fig. 10-29).

 

Fig. 3-27. Transversus thoracis.

 

A thin muscle deep to the sternum and ribs is the transversus thoracis that depresses the ribs, contributing to exhalation.

 

Fig. 3-28. Three abdominal wall muscles.

 

The three abdominal wall muscles can be studied in relation to the thoracic wall muscles. The external oblique muscle matches the external intercostal muscle, the internal oblique muscle matches the internal intercostal muscle (Fig. 3-23), and the transversus abdominis matches the transversus thoracis (Fig. 3-27). Moreover, the muscle direction in each layer is same between the abdominal and thoracic walls.

 

Fig. 3-29. Actions of three abdominal wall muscles (anterior view).

 

The action of the three abdominal wall muscles is rotating the trunk and laterally flexing the trunk. The muscles are attached to the lower ribs to enable this movement (Fig. 3-28).

 

 

Fig. 3-30. Action of three abdominal wall muscles (horizontal plane).

 

Another notable action of the three abdominal wall muscles is compressing the abdominal cavity since the muscles surround the abdominal cavity in total. Bilateral muscles are attached to the linea alba (Fig. 3-33) to enable this movement,.

 

 

Fig. 3-31. Rectus abdominis.

 

Medial to the three muscles, the rectus abdominis occupies the space (Fig. 3-33). The rectus abdominis has its origin at the pubis (Fig. 1-34) and insertion at the xiphoid process and adjacent ribs. As a result, this muscle enable flexion of the trunk (Fig. 2-21).

The bilateral rectus abdominis muscles have a white midsagittal border called the linea alba (meaning while line) (Fig. 3-28). Each rectus abdominis contains intervening aponeuroses (Fig. 3-4).

 

Fig. 3-32.

 

A strenuous exercise of the rectus abdominis induces the muscle hypertrophy; the linea alba and intervening aponeuroses thus become relatively depressed. With the thinning of the overlying subcutaneous tissue (Fig. 15-6), the bilateral muscles appear as eight-packs (or six-packs).

 

Fig. 3-33. Rectus sheath that are aponeuroses of three muscles.

 

The rectus abdominis is encircled by the rectus sheath that functions as the aponeuroses of the three abdominal wall muscles (Fig. 3-28). A deeper structure is the “transversalis” fascia (Fig. 7-4) (Fig. 7-5) which is derived from the deep fascia of “transversus” abdominis.

 

< Muscles of back, muscles of upper limb >

 

Muscles of back are categorized into the deep and superficial back muscles. The deep muscles are innervated by the posterior rami of spinal nerves (Fig. 13-86) (Fig. 13-96).

 

 

Fig. 3-34. Deep back muscles consisting of transversospinalis (arrows), erector spinae (circled X).

Deep part of the deep back muscles (deep of the deep) is the transversospinalis that is composed of the rotatores, multifidus, and semispinalis. The semispinalis exists in upper half (semi) of the vertebral column (spinalis).

 

Fig. 3-35. Transversospinalis excluding semispinalis.

 

Look at the rotatores and multifidus, whose functions are rotating and extending the vertebral column, respectively. The semispinalis, running vertically, also extends the vertebral column (Fig. 2-21).

Conventionally, muscles are named in one of following three ways. First, like the transversospinalis, the muscle’s origin (transverse process) and insertion (spinous process) are used for naming. Second, like the rotatores, muscles can be named after its own action. Third, like the multifidus and semispinalis, muscle’s location and shape are used (Fig. 3-34).

 

Fig. 3-36.

 

Superficial part of the deep back muscles is the erector spinae that consists of iliocostalis, longissimus, and spinalis (from lateral to medial) (Fig. 3-34). The representative origins of the three muscles are the iliac crest (Fig. 1-31), transverse processes, and spinous processes (Fig. 1-13) (Fig. 3-35), respectively.

 


Fig. 3-37. Trapezius, latissimus dorsi.

 

The superficial back muscles (trapezius, latissimus dorsi) move the clavicle, scapula, and humerus that belong to the bones of upper limb (Fig. 1-20). Therefore, the muscles also belong to the muscles of upper limb.

Main action of the trapezius is adduction of the scapula. When the superior part of trapezius contracts, the scapula is elevated (shrugged shoulder); when the inferior part contracts, the scapula is depressed. A muscle’s broad origin may result in various actions.

When bilateral scapulae are fixed by other muscles (Fig. 3-40) (Fig. 3-42), the contraction of bilateral trapezius leads to the hyperextension of the head (Fig. 2-21). When a muscle’s insertion is fixed, its origin may move instead.

Among origins of the trapezius, the external occipital protuberance and superior nuchal line of the occipital bone are easily palpable. CV7 is to be recognized after head flexion, because the spinous process of CV7 is more prominent than those of CV1–CV6 (Fig. 1-15) (Fig. 3-38) (Fig. 13-85).

 

Fig. 3-38. Superior nuchal line.

 

In the midsagittal plane above, the nuchal ligament between the external occipital protuberance and the spinous process of CV7 is expressed. The ligament is an origin of the trapezius (Fig. 3-37).

 

Fig. 3-39.

 

The “latissimus dorsi” (meaning the “dorsal largest” muscle) (Fig. 3-37) performs adduction, medial rotation, and extension of the humerus (Fig. 3-43).

 

Fig. 3-40. Deltoid muscle.

 

The lateral 1/2 of clavicle and the spine of scapula (Fig. 1-21) are insertion of the trapezius (Fig. 3-37) as well as origin of the deltoid muscle. Action of the deltoid muscle (insertion: deltoid tuberosity) (Fig. 1-22) is abduction of the humerus.

 

Fig. 3-41.

 

Large origin of the deltoid muscle also induces flexion of the humerus (origin: lateral 1/2 of clavicle) (Fig. 2-22) and extension of the humerus (origin: spine of scapula).

 

Fig. 3-42. Scapular region muscles.

 

Among the other muscles originating from the scapula, three are named after their origins: the supraspinatus from the supraspinous fossa, the infraspinatus from the infraspinous fossa, and the subscapularis from the subscapular fossa (Fig. 1-21). The remaining two are the teres minor and teres major.

Their insertions are the humerus structures: greater and lesser tubercles, intertubercular groove (Fig. 1-22).

 

Fig. 3-43. Directions of scapular region muscles.

 

In the shoulder joint (ball and socket joint), X, Y, Z-axes pass the center of the head of humerus (Fig. 1-22). The supraspinatus causes abduction of the humerus (Z-axis), the infraspinatus and teres minor give rise to its lateral rotation (Y-axis), and the subscapularis induces its medial rotation (Y-axis). The teres major performs the same action (adduction, medial rotation, and extension of the humerus) as the latissimus dorsi does (Fig. 3-39).

 

Fig. 3-44.

 

The supraspinatus, infraspinatus, teres minor, subscapularis make up the “rotator” cuff that “rotates” the humerus. The supraspinatus which abducts the humerus is an exception (Fig. 3-43).

 

Fig. 3-45

 

Major structures are placed inferior to minor structures by accident.

 

Fig. 3-46. Pectoralis major.

 

Origins of the pectoralis major are R2R6 (level of the breast in female) (Fig. 15-20) (Fig. 15-21), sternum, and medial 1/2 of clavicle.

The lateral 1/2 of clavicle is occupied by the deltoid muscle (Fig. 3-40). In the surface anatomy, we can see a slight depression between the “deltoid” muscle and “pectoralis” major, which is called the “deltopectoral” triangle (Fig. 10-70a). In the deltopectoral triangle, the coracoid process is palpable (Fig. 1-21).

 

Fig. 3-47. Arm muscles.

 

Arm muscles, enclosed by the brachial fascia, are divided into the anterior and posterior arm muscles by the humerus and two intermuscular septa.

 

Fig. 3-48. Biceps brachii.

 

The prominent one of the anterior arm muscles is biceps brachii. The long head of biceps brachii starts from the supraglenoid tubercle and descends through the articular cavity of shoulder joint. The short head starts from the coracoid process (Fig. 1-21). The two heads unite and eventually form a tendon. The tendon is directed deep to end at the radius, which allows for supination and flexion of the forearm.

 

Fig. 3-49.

 

In a restaurant, the two actions of the biceps brachii are use for picking up a piece of food and for taking out a cork from a wine bottle.

 

 

Fig. 3-50.

 

Even though the biceps brachii passes the shoulder and elbow joints (Fig. 10-42), the muscle hardly flexes the shoulder joint.

 

Fig. 3-51. Triceps brachii.

 

The only posterior arm muscle is triceps brachii. In the humerus, the groove for radial nerve is nearly vertical (Fig. 3-47) (Fig. 13-91). Therefore, two heads of triceps brachii that originate from each side of the groove are named the medial and lateral heads.

Another head, the long head originates from the higher infraglenoid tubercle (Fig. 1-21). The long head covers the medial head entirely.

 

Fig. 3-52.

 

It is natural that a long muscle covers and hides a short muscle.

Basketball players have the outstanding triceps brachii that allows strong and elaborate extension of the forearm. When one does push-ups, the triceps brachii extends the forearm, and the pectoralis major adducts the arm (Fig. 3-46).

 

Fig. 3-53. Forearm muscles.

 

Forearm muscles, encircled by the antebrachial fascia, are categorized into the anterior and posterior forearm muscles. Borders are the radius, ulna, interosseous membrane (Fig. 2-5), and narrow intermuscular septum.

In the horizontal plane, the anterior forearm muscles occupy not only the anterior part but also the medial part of the forearm. This is because the superficial anterior forearm muscles originate from the medial epicondyle of humerus (Fig. 1-22) (Fig. 3-54) (Fig. 3-55) (Fig. 3-56). When one’s wrist joint is flexed intensively, the wrist joint tends to be adducted for this same reason.

 

 

Fig. 3-54. Palmaris longus.

 

An anterior forearm muscle with its origin at the medial epicondyle is the palmaris longus. The wrist flexion (along with opposition of the thumbs and little finger) makes the palmaris longus bulge out more (Fig. 2-22). It is because the palmaris longus is the only muscle superficial to the flexor retinaculum (Fig. 3-61).

 

Fig. 3-55. Flexor carpi radialis.

 

The carpometacarpal joints are hardly mobile, so the muscles that ends at the metacarpal bones actually move the “carpal” bones. That is why the muscle in the above figure is named flexor “carpi” radialis.

 

Fig. 3-56. Flexor carpi ulnaris.

 

The flexor carpi ulnaris approaches the pisiform then the 5th metacarpal bone. The pisiform is a kind of sesamoid bone that is strung to the tendon. Attempt to move the pisiform right and left (Fig. 3-68).

 

Fig. 3-57. Extensor carpi muscles.

 

If one extends one’s wrist vigorously, the wrist joint tends to be abducted because of the muscles’ origin (lateral epicondyle of humerus) (Fig. 1-22).

While there is only one extensor carpi ulnaris, there are two extensor carpi radialis (brevis and longus).

 

Fig. 3-58. Five muscles moving wrist joint.

 

This figure shows courses of the five muscles that move the wrist joint. The two axes of the wrist joint (ellipsoid joint) (Fig. 2-47) pass the carpal bones (Fig. 1-27). Two flexors are anterior to the axis of flexion, extension, while three extensors are posterior to the axis. Three abductors are lateral to the axis of abduction, adduction, while two adductors are medial to the axis.

 

Fig. 3-59. Flexor digitorum superficialis (left) and profundus (right).

 

Deep anterior forearm muscles (flexor digitorum superficialis, flexor digitorum profundus, etc.) originate from the ulna, radius, and interosseous membrane (Fig. 2-5). The muscle’s role is to flex the middle and distal phalanges of the 2nd–5th fingers.

 

  

Fig. 3-60.

 

If one flexes one’s fingers, the wrist joint can be flexed together because the flexor digitorum superficialis and profundus pass the wrist joint (Fig. 3-59). To prevent the undesired flexion of the wrist joint, the extensor carpi ulnaris and extensor carpi radialis contact at the same time (Fig. 3-57).

 

Fig. 3-61. Two retinacula of wrist, carpal tunnel.

 

The carpal bones (Fig. 1-26) form a carpal tunnel with the flexor retinaculum. Numerous tendons of anterior forearm muscles (e.g., flexor digitorum superficialis and profundus) (Fig. 3-59) pass the narrow carpal tunnel. Therefore, the tendons are surrounded by the synovial sheath, which is a balloon-like structure including lubricant, synovial fluid. This is somewhat similar to the synovial membrane producing synovial fluid (Fig. 2-32) (Fig. 2-33).

 

Fig. 3-62. Extensor digitorum.

 

Unlike the two flexor digitorum (superficialis and profundus), there is only one extensor digitorum since extension of the fingers doesn’t have to be as powerful as their flexion. Consider the frequent activity of grasping objects with a hand (Fig. 3-73).

Surely, the tendons of the extensor digitorum pass under the extensor retinaculum. These tendons are also surrounded by the synovial sheath (Fig. 3-61).

The “extensor” muscle tendons become “expanded“ to be the “extensor expansion“ at the dorsum of proximal, middle, distal phalanges of the 2nd–5th fingers (Fig. 1-27). The muscles to move the thumb (Fig. 2-25) are not mentioned in this book.

 


< Muscles of pelvis, muscles of lower limb >

 

Fig. 3-63. Piriformis, obturator internus (lateral view).

 

In the above figure, the sacrospinous and sacrotuberous ligaments in company with the hip bone (Fig. 1-31) form the greater and lesser sciatic foramina. The two foramina are exits of the pelvis cavity and perineum, respectively (Fig. 1-35) (Fig. 3-64).

Two notable muscles contributing to the pelvic wall are the piriformis and obturator internus (Fig. 3-64). The former is superior to pass the greater sciatic foramen, whereas the latter is inferior to pass the lesser sciatic foramen. Both the piriformis and obturator internus end at the greater trochanter of femur (Fig. 1-41), which makes the two muscles to perform the same action, lateral rotation of the femur (Fig. 2-23).

 

Fig. 3-64. Pelvis, perineum (coronal plane of anal triangle).

 

The obturator internus also serves as the origin of the pelvic diaphragm. The pelvic diaphragm is in contact with the boundary of the rectum and anal canal as the insertion. Because this muscle lies obliquely, its contraction induces an “elevation” of the “anal” canal (Fig. 4-36), hence the major part of the pelvic diaphragm is called “levator ani.”

The pelvic diaphragm is border between the pelvic cavity and the perineum (Fig. 1-35). Consequently, the sigmoid colon and rectum belong to the pelvic cavity; the anal canal belongs to the perineum (Fig. 4-33).

 

Fig. 3-65. Thigh muscles.

 

Thigh muscles are covered by the fascia lata, which is homologous to the brachial fascia covering arm muscles (Fig. 3-47). The lateral thickened part of the fascia lata is called the iliotibial tract (Fig. 3-71).

Three intermuscular septa result in the anterior, medial, and posterior thigh muscles. Gluteal muscles (Fig. 3-71) lie superior to the posterior thigh muscles.

 

Fig. 3-66. Iliacus, psoas major.

 

The anterior thigh muscles include the iliacus and psoas major that flex the femur (Fig. 2-27). The iliacus from the iliac fossa (Fig. 1-35) and the psoas major from the lumbar vertebrae (Fig. 1-15) slide down and reach the lesser trochanter (Fig. 1-41).

 

Fig. 3-67. Quadriceps femoris.

 

Another anterior thigh muscle, the quadriceps femoris corresponds to the triceps brachii in upper limb. As a head, the rectus femoris passes two joints like the long head of triceps brachii passes two joints (Fig. 3-51). Consequently, the quadriceps femoris moves two joints when kicking a ball (flexion of hip joint and extension of knee joint) (Fig. 2-27). Principal action is extension of the knee joint (Fig. 3-73).

The vastus lateralis, vastus medialis, and vastus intermedius originate from the lateral lip of linea aspera, medial lip of linea aspera (Fig. 1-41), and anterior surface of femur.

 

 

Fig. 3-68.

 

The tendon of the quadriceps femoris has a sesamoid bone called patella. The tendon distal to the patella (patellar ligament) is attached to the tibial tuberosity (Fig. 1-43) (Fig. 1-43a) (Fig. 13-59). Without the patella, the tendon would be easily worn out by friction with the underlying femur.

 

Fig. 3-69. Adductor muscles.

 

As the medial thigh muscles, adductor longus, brevis, and magnus are arranged from anterior to posterior. This order can be remembered by German-like words, Adductor Longus, Brevis, und Magnus (ALBuM). The adductor longus and brevis originate from the pubis, but the adductor magnus originates from the ischium. This is so because the ischium is located posterior to the pubis (Fig. 1-31). The adductor magnus has a foramen, adductor hiatus (Fig. 10-56).

 

Fig. 3-70.

 

These medial thigh muscles prevent a rider from falling off from a horse.

 

Fig. 3-71. Gluteus maximus.

 

Among the gluteal muscles, the most superficial and biggest one is gluteus maximus. It originates from the posterior part of ilium, the sacrum, and the sacrotuberous ligament (Fig. 3-63). Due to its vast origin, the gluteus maximus covers most of the gluteal region including the ischial tuberosity (Fig. 1-33).

 

Fig. 3-72.

 

The deltoid muscle in the upper limb and the gluteus maximus are equivalent. The two big muscles have similar insertions: the deltoid tuberosity (Fig. 1-22) (Fig. 3-40) and the gluteal tuberosity (Fig. 1-41). Another insertion of the gluteus maximus is the iliotibial tract (Fig. 3-65) (Fig. 3-71).

Let us think about action of the gluteus maximus. Three axes of the hip joint (a ball and socket joint) pass the center of head of femur (Fig. 2-52). Regarding the mediolateral axis of the hip joint, action of the gluteus maximus is extension of the femur (Fig. 2-27); regarding the superoinferior axis, its action is lateral rotation of the femur (Fig. 2-23).

 

 

Fig. 3-73.

 

In the lower limb, the extensor muscles are more developed. The representative extensor muscles are the gluteus maximus (hip joint) (Fig. 3-71) and quadriceps femoris (knee joint) (Fig. 3-67). If two joints are extended, they allow us to stand up and jump (Fig. 2-27).

 

Fig. 3-74. Semitendinosus, semimembranosus.

 

Let us now discuss the posterior thigh muscles (hamstring muscles) (Fig. 3-65). In etymology, the semitendinosus has a distal tendon which is half the length of the muscle, and the semimembranosus is relatively thin like a membrane. Two muscles share a common origin (ischial tuberosity) (Fig. 1-31).

 

Fig. 3-75. Biceps femoris.

 

The biceps femoris is homologous to the biceps brachii. The short head of the biceps femoris passes only one joint unlike the short head of the biceps brachii which passes two joints (Fig. 3-48).

Since the posterior thigh muscles pass hip and knee joints, the muscles extend the hip and flex the knee. The main action, nonetheless, is flexion of the knee joint (Fig. 3-50). Regard the muscles as the antagonists of the quadriceps femoris (Fig. 3-67).

 

Fig. 3-76. Leg muscles.

 

One interosseous membrane and two intermuscular septa as well as the tibia and fibula (Fig. 3-83) partition the leg muscles into the posterior, lateral, and anterior leg muscles. All the leg muscles are encircled by the crural fascia.

 

Fig. 3-77. Three retinacula of ankle.

 

Whereas the wrist has two retinacula (Fig. 3-61), the ankle has three: the flexor, fibular, and extensor retinacula for tendons of the posterior, lateral, and anterior leg muscles (Fig. 3-76). Like those in the wrist, the tendons in the ankle are surrounded by the synovial sheaths.

 

Fig. 3-78. Superficial posterior leg muscles.

 

The posterior leg muscles include the gastrocnemius and soleus. The gastrocnemius, combination of gastro (belly) and cnemius (leg), can be interpreted as a muscle belly in leg.

 

Fig. 3-79. Popliteal fossa.

 

Two superior borders of the popliteal fossa are the posterior thigh muscles (Fig. 3-74) (Fig. 3-75), while its two inferior borders are the gastrocnemius (Fig. 3-78).

 

Fig. 3-80.

 

The soleus, which is not a muscle in the sole (foot), is named after its shape resembling a fish sole. The muscle originates from the soleal line of tibia (Fig. 1-43) (Fig. 3-78).

 

Fig. 3-81.

 

The gastrocnemius and soleus have a common tendon (“calcaneal” tendon) which is attached to the “calcaneus” (Fig. 1-49) (Fig. 3-78).

 

 

Fig. 3-82.

 

Action of the gastrocnemius and soleus is plantar flexion (Fig. 2-29). When this calcaneal tendon is injured, one will be unable to raise one’s heel (no plantar flexion).

 

Fig. 3-83. Tibialis posterior.

 

Throughout this chapter, the foot is often 90 degrees plantar flexed (tiptoe like a ballerino/ballerina) (Fig. 1-51) (Fig. 2-29) (Fig. 8-27), in order to represent the leg muscles efficiently. These drawings are also helpful for comparing the leg muscles with the forearm muscles (Fig. 3-55).

Be mindful of the insertion of each muscle, as insertions greatly influence the muscle action. The tibialis posterior (a posterior leg muscle) which inserts at the inferior side of 3rd metatarsal bone induces the plantar flexion and inversion (Fig. 3-87).

 

Fig. 3-84. Fibularis longus and brevis.

 

The lateral leg muscles (fibularis longus, fibularis brevis), passing posterior to the lateral malleolus (Fig. 1-43), induce plantar flexion and eversion (Fig. 3-87).

 

Fig. 3-85. Fibularis tertius.

 

As the small anterior leg muscle, the fibularis tertius’ action is dorsiflexion and eversion (Fig. 3-87).

 

Fig. 3-86. Tibialis anterior.

 

As the large anterior leg muscle, the tibialis anterior gives rise to dorsiflexion and inversion (Fig. 3-87).

All the above five muscles have the insertion of the metatarsal bones; the tarsometatarsal joint hardly moves, just like the carpometacarpal joint (Fig. 3-55). Therefore, the muscles are ankle joint movers (Fig. 2-29) (Fig. 2-30).

 

Fig. 3-87. Five muscles moving ankle joint.

 

This schematic figure demonstrates the courses of the five muscles. They are equivalent to the five muscles that move the wrist joint (Fig. 3-58).

With regard to the axis of the medial and lateral malleoli, the posterior leg muscle (tibialis posterior) and the lateral leg muscles (fibularis longus and brevis) induce plantar flexion while the anterior leg muscles (fibularis tertius, tibialis anterior) induce dorsiflexion (Fig. 2-29). Considering the axis of the 2nd metatarsal bone, the tibialis muscles make inversion, whereas the fibularis muscles contribute to eversion (Fig. 1-48) (Fig. 2-30).

 

Fig. 3-88. Flexor digitorum longus.

 

The flexor digitorum longus (posterior leg muscle) is equivalent to the flexor digitorum profundus (anterior forearm muscle), because both muscles end at the distal phalanges (Fig. 3-59). An intrinsic muscle of foot (flexor digitorum brevis), equivalent to the flexor digitorum superficialis, is not drawn.

 

Fig. 3-89. Extensor digitorum longus.

 

The extensor digitorum longus (anterior leg muscle) matches the extensor digitorum (posterior forearm muscle) because both muscles become the extensor expansions (Fig. 3-62). The intrinsic muscle of foot (extensor digitorum brevis) is not illustrated. To understand the matching, read the development of limbs (Fig. 8-27).

 

Fig. 3-90.

 

Review the prominent muscles of whole body in the cartoon above.


 

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