9.1: Background Information

Integrated Human Anatomy and Physiology Lab Manual I: Fundamentals of Structure and Movement · LibreTexts

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    From LibreTexts, under CC BY-NC 4.0, as the source states it; its equations are written in TeX, which the PDF shows typeset. This page at LibreTexts.

    Overview

    The best-known function of skeletal muscle is its ability to contract and cause movement. Skeletal muscles act not only to produce movement but also to stop movement, such as resisting gravity to maintain posture. Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation. Joints can become misaligned or dislocated entirely by pulling on the associated bones; muscles work to keep joints stable. Skeletal muscles are located throughout the body at the openings of internal tracts to control the movement of various substances. These muscles allow functions, such as swallowing, urination, and defecation, to be under voluntary control. Skeletal muscles also protect internal organs (particularly abdominal and pelvic organs) by acting as an external barrier or shield to external trauma and by supporting the weight of the organs.

    Skeletal muscles contribute to the maintenance of homeostasis in the body by generating heat. Muscle contraction requires energy, and when ATP is broken down, heat is produced. This heat is very noticeable during exercise, when sustained muscle movement causes body temperature to rise, and in cases of extreme cold, when shivering produces random skeletal muscle contractions to generate heat.

    • Skeletal muscles exhibit several properties that enable them to perform their functions effectively.
    • Excitability - The ability of muscle fibers to respond to electrical or chemical stimuli.
    • Contractility - The capability of muscle fibers to shorten and generate force.
    • Extensibility - The ability of muscle fibers to be stretched without tearing.
    • Elasticity - The tendency of muscle fibers to return to their original length after being stretched.

    Skeletal Muscle Organization

    Skeletal muscle is an organ composed of various integrated tissues, including muscle fibers, blood vessels, nerve fibers, and connective tissue. The connective tissue layers are:

    • Epimysium: Surrounds the entire muscle.
    • Perimysium: Encases fascicles (bundles of muscle fibers).
    • Endomysium: Surrounds individual muscle fibers.

    These layers organize muscle fibers, provide structural support, and allow blood vessels and nerves to pass through. At the muscle ends, the epimysium extends to form tendons that attach muscles to bones, refer to Figure \(\PageIndex{1}\). Skeletal muscle cells, known as muscle fibers, can be quite large, with diameters up to 100 μm and lengths reaching 30 cm in the Sartorius muscle of the upper leg. During development, myoblasts fuse to create multinucleated muscle fibers, allowing for increased protein and enzyme production essential for muscle contraction.

    Muscle structure diagram with tendon, fascicle, myofiber, and connective tissue layers labeled.
    Figure \(\PageIndex{1}\): Skeletal Muscle Organ Structure. Each level of the organization of a skeletal muscle is wrapped in a connective tissue layer: the cell (myofiber) is surrounded by the endomysium, the bundle of cells (muscle fascicle) is surrounded by the perimysium, and the organ (muscle) is surrounded by the epimysium. (CC BY-NC-SA 4.0 ; Justin Greene via Skeletal Muscle Organ Structure).

    Figure \(\PageIndex{1}\) showcases the intricate organization of skeletal muscle, highlighting its structure from the outer connective tissue to the individual muscle fibers. At the outermost layer, the tendon connects muscle to bone, providing stability and force transfer. The muscle body is enveloped by the epimysium, a connective tissue layer that protects and supports. Within the muscle body, bundles known as fascicles are wrapped in the perimysium. Each fascicle consists of individual muscle fibers called myofibers, encased in a fine layer called the endomysium. These myofibers contain nuclei, essential for muscle function and repair, illustrating the complex yet efficient organization of skeletal muscle tissue.

    Another specialization of the skeletal muscle is the site where a motor neuron’s terminal meets the muscle fiber—called the neuromuscular junction (NMJ). This is where the muscle fiber first responds to signaling by the motor neuron. Every skeletal muscle fiber in every skeletal muscle is innervated by a motor neuron at the NMJ. Excitation signals from the neuron are the only way to functionally activate the fiber to contract. The changes that takes place in the neuromuscular junctions (NMJ) are detailed in Figure \(\PageIndex{2}\).

    Muscle structure diagram showing tendon, fascicle, myofiber, and connective tissue layers.
    Figure \(\PageIndex{2}\): Motor End-Plate and Innervation. At the NMJ, the axon terminal releases ACh. The motor end-plate is the location of the ACh-receptors in the muscle fiber sarcolemma. When ACh molecules are released, they diffuse across a tiny space called the synaptic cleft and bind to the receptors. (CC BY-NC-SA 4.0 ; Motor Endplate and Innervation" by Jennifer Lange and Justin Greene via Skeletal Muscle Organ Structure).

    Figure \(\PageIndex{2}\) explains how a motor neuron communicates with a muscle fiber at the neuromuscular junction. The diagram is arranged top to bottom to show the sequence of events.

    Motor neuron - key structures are labeled:

    • A motor neuron is shown with myelin around the axon.
    • The neuron ends at the axon terminal (synaptic terminal) near the muscle fiber.
    • An arrow indicates the direction of the nerve impulse traveling toward the terminal.
    • A muscle fiber is shown below the neuron.

    Neuromuscular junction - key structures are labeled:

    • Synaptic terminal (end of the neuron)
    • Synaptic vesicles containing acetylcholine (ACh) inside the terminal
    • Synaptic cleft (gap between neuron and muscle)
    • Sarcolemma (muscle cell membrane)
    • Motor end plate (specialized region of the sarcolemma beneath the terminal)

    Chemical signaling - key structures are labeled:

    • Synaptic vesicles release acetylcholine (ACh) into the synaptic cleft.
    • ACh moves across the cleft and binds to ACh receptors on the motor end plate.
    • Receptor activation allows sodium ions (Na+) to enter the muscle cell.
    • This ion movement initiates a muscle electrical response that leads to contraction.

    Muscle Shortening and Lengthening

    When muscles contract, they tighten and bulge in the middle due to shortening; this is called a concentric contraction, where the muscle actively pulls to generate movement, like during a bicep curl. While concentric contractions are generally weaker, they help build strength through a range of motion. In contrast, eccentric contractions occur when a muscle lengthens while resisting a force, often controlling movement; this type is usually stronger and builds resilient muscle tissue. Examples of concentric movements include pushing up in a bench press and lifting a barbell in a dead lift.

    The isometric contraction represents a middle ground, as muscles neither lengthen nor shorten; these movements involve holding a position without movement, improving strength in specific areas. Examples include handstand holds and the top position of a pull-up.

    Structural changes occur at all levels of the muscle during contractions: as the muscle shortens, fascicles, myofibers, and myofibrils also shorten, while the myofilaments remain the same length.

    The sarcomere, the functional unit of muscle, contracts by bringing Z-lines closer together through the interaction of myosin and actin, increasing their overlap. During stretching, Z-lines move apart, decreasing overlap. The I band shortens during contraction and lengthens when stretched as illustrated in Figure \(\PageIndex{3}\).

    Three sarcomere views: stretched, resting, and contracted with labeled bands and lines.
    Figure \(\PageIndex{3}\): Sarcomere length showing the position of I band and Z lines in resting, stretched and contracted states. The change in muscle length results from the cumulative change in the length of the sarcomeres. (CC BY-NC-SA 4.0 ; “Sarcomere Length" by Mark Schaeffer via Skeletal Muscle Organ Structure).

    Figure \(\PageIndex{3}\) compares a sarcomere (the basic contractile unit of skeletal muscle) in three conditions: stretched, resting, and contracted.

    • Stretched: The Z-lines are far apart. The region labeled the I band is wide, indicating less overlap between thin and thick filaments.
    • Resting: The Z-lines are closer together than in the stretched state. The I band is smaller, showing increased overlap.
    • Contracted: The Z-lines are closest together. The I band is the smallest, showing the greatest overlap of filaments.

    Naming Skeletal Muscles

    Learning the names of the muscles in the body can seem daunting, but understanding their etymology can make it easier. Etymology is the study of the origin of words and how their meanings have evolved. Anatomists name skeletal muscles based on various criteria that describe their characteristics. Key Points About Muscle Naming Conventions:

    • Location: For example, "Temporalis" refers to a muscle near the temple, while "sternocleidomastoid" attaches to the sternum, clavicle, and mastoid process.
    • Shape: Muscles like "Deltoid" (triangular) and "trapezius" (trapezoid) are named for their shapes, while "orbicularis" indicates a circular form.
    • Size: Terms like "Maximus" (largest), "Medius" (medium), and "Minimus" (smallest) describe the size of muscles.
    • Fiber Direction: "Rectus" means straight, and "oblique" means angled, indicating the direction of muscle fibers.
    • Number of Origins: Muscles may be named based on their origins, such as "Biceps" (two origins) and "Triceps" (three origins).

    Example Muscle Names and Their Meanings:

    • Biceps Brachii: "Bi" (two) + "ceps" (head) + "brachii" (arm).
    • Gluteus Maximus: "Gluteus" (buttocks) + "maximus" (largest).
    • Rectus Abdominis: "Rectus" (straight) + "abdominis" (abdomen).
    • Serratus Anterior: "Serratus" (saw-like) + "anterior" (front).
    • Orbicularis Oculi: "Orbicularis" (circular) + "oculi" (eye).

    Taking the time to learn these roots is crucial for mastering the vocabulary of anatomy and physiology. While pronunciation may take practice, understanding the basic information will make it easier to accurately name and pronounce muscular terms.

    Skeletal muscles are located throughout the body, including the head, neck, trunk, upper limbs, and lower limbs. Here are some examples of skeletal muscles by location are represented in the Table \(\PageIndex{1}\).

    Table \(\PageIndex{1}\): Understanding a Muscle Name from the Latin roots
    Example Word Latin Roots Meaning Translation
    abductor digitiminimi abductor ab = away from; duct = to move a muscle that moves away from A muscle that moves the little
    finger or toe away
    digiti digitus = digit a finger or toe
    minimi minimus = mini, tiny little
    adductor digitiminimi adductor ad = to, toward; duct = to move a muscle that moves towards A muscle that moves the little
    finger or toe toward
    digiti digitus = digit a finger or toe
    minimi minimus = mini, tiny little

    Interactions of Skeletal Muscles in the Body

    To pull on a bone , that is, to change the angle at its synovial joint by moving the skeleton, a skeletal muscle must also be attached to a fixed part of the skeleton. The end of the muscle attached to a fixed (stabilized) bone is called the origin and the moveable end of the muscle that attaches to the bone being pulled is called the muscle’s insertion. The muscle's line of pull indicates the direction the body part will move when the insertion point is brought closer to the origin point Figure \(\PageIndex{4}\).

    Upper arm muscle diagram. Details in text.
    Figure \(\PageIndex{4}\): Origin and Insertion Points of skeletal muscle. When a muscle contracts and shortens, the bone at the point of insertion will move toward the bone at the point of origin (CC BY-NC-SA 4.0 ; “Origin and Insertion" by Justin Greene and Jennifer Lange via LibreTexts).

    Figure \(\PageIndex{4}\) illustrates the origin and insertion points of a skeletal muscle, viewed from the right lateral side. The origin is situated near the shoulder, serving as the stable anchor point, while the insertion is located near the elbow, representing the mobile end. A dashed arrow along the muscle indicates the direction of muscle pull from the mobile insertion point to the stable origin. As the muscle contracts and shortens, it causes the bone at the insertion to move toward the bone at the origin. This movement is fundamental to understanding how muscles facilitate joint motion and leverage.

    Understanding Muscle Roles in Body Movement: Prime Movers, Synergists, and Antagonists

    Although a number of muscles may be involved in an action, the principal muscle involved is called the prime mover, or agonist. The muscle that acts as the prime mover (or agonist) during one action may become the antagonist during the opposite movement. This dynamic interplay between muscles is essential for coordinated and efficient movement. In the context of arm movement, for example, when flexing the elbow to lift a cup, the biceps brachii serves as the prime mover. Meanwhile, the brachialis and brachioradialis act as synergists, assisting in the action. If you were to extend the elbow, the triceps brachii would take on the role of the agonist, while the biceps would become the antagonist. A synergist can function as a fixator, stabilizing the bone that serves as the attachment point for the prime mover’s origin Figure \(\PageIndex{5}\)

    Arm diagrams showing muscles for elbow flexion. Details in text.
    Figure \(\PageIndex{5}\): Role playing of muscles in elbow movements. Elbow Flexion (left) uses biceps brachii (agonist) and brachialis (synergist), while triceps brachii relaxes. For elbow Extension (right), triceps brachii leads, with biceps brachii and brachialis relaxing (antagonists). (CC BY-NC-SA 4.0; Agonist and Antagonist" by Justin Greene and Jennifer Lange via LibreTexts).

    Figure \(\PageIndex{5}\) presents two diagrams of the human arm, illustrating the mechanics of elbow flexion and extension.

    • Left demonstrates elbow flexion: the biceps brachii acts as the agonist, driving the movement, while the brachialis serves as the synergist, aiding in the action. The triceps brachii functions as the antagonist, relaxing to allow the bending of the elbow.
    • Right, depicts elbow extension: the triceps brachii takes on the role of the agonist, facilitating the straightening of the arm. Biceps brachii and brachialis act as antagonists, lengthening to permit this motion.

    To extend the knee, the quadriceps femoris group in the anterior thigh acts as the agonists. Conversely, to flex the knee, the hamstrings are activated as the antagonistic muscle group. These roles can be reversed depending on the action. For instance, if the action involves knee bending, the hamstrings become the agonists while the quadriceps femoris serve as the antagonists.

    Refer to Table \(\PageIndex{2}\) for summary of the roles of agonist and antagonist muscles during movements of the elbow and knee joints, highlighting how these muscle groups work together to facilitate movement and stability.

    Table \(\PageIndex{2}\): Agonist and Antagonist Skeletal Muscle Pairs for movements of the elbow and knee joints
    Agonist Antagonist Movement/Action
    Biceps brachii - in the anterior compartment of the arm. Triceps brachii - in the posterior compartment of the
    arm.    		 The biceps brachii flexes the forearm, whereas the triceps brachii extends it.
    Hamstrings: group of three muscles in the posterior
    compartment of the thigh.    		 Quadriceps femoris: group of four muscles in the anterior compartment of the thigh. The hamstrings flex the leg, whereas the quadriceps femoris extend it.

    Exercise and Stretching

    When it comes to exercise, the significance of warming up and stretching cannot be overstated! Warming up prepares your muscles for the strain they’re about to encounter by increasing blood flow, which enhances flexibility and reduces the risk of injury. Stretching, in particular, helps elongate the muscle fibers, making them more pliable and less prone to strains. Without a proper warm-up, muscle fibers can be damaged, or tendons might be pulled, leading to painful injuries characterized by swelling and limited mobility. In severe cases, these injuries can sideline you for an extended period, hindering your fitness goals.

    Understanding Key Skeletal Muscles for Laboratory Studies

    To effectively study the human body in a laboratory setting, it’s essential to have a solid grasp of the major skeletal muscles, as they play crucial roles in movement, stability, and overall function. Below is a structured overview of the selected muscle groups:

    1. Muscles that move the lower jaw (Mastication), these muscles are responsible for chewing and include:
      • Masseter: Elevates the mandible (jaw).
      • Temporalis: Assists in closing the jaw and retracting it.
    2. Muscles that move and stabilize the head, these muscles help maintain head posture and facilitate movement:
      • Sternocleidomastoid: Rotates and flexes the neck.
      • Trapezius: Elevates, retracts, and rotates the scapula, also stabilizing the head.
    3. Muscles that position the pectoral girdle (stabilize and move the shoulders):
      • Pectoralis Major: Helps in flexing and adducting the arm.
      • Serratus Anterior: Protracts and stabilizes the scapula.
    4. Muscles of the Back (Key for posture and movement)
      • Latissimus Dorsi: Extends, adducts, and rotates the arm.
      • Rhomboids: Retracts the scapula, helping with shoulder stability.
    5. Abdominal Wall Muscles (support for the trunk)
      • Rectus Abdominis: Flexes the vertebral column.
      • External and Internal Obliques: Rotate and flex the torso.
    6. Muscles involved in breathing (ventilation)
      • Diaphragm
      • External intercostals
      • External intercostals
    7. Muscles That Move the Humerus (vital for arm movement)
      • Deltoid: Abducts the arm.
      • Rotator Cuff Muscles: Stabilize the shoulder joint and allow for a wide range of motion.
    8. Muscles of the Forearm (movements of the wrist and fingers)
      • Flexor Group: Flexes the wrist and fingers.
      • Extensor Group: Extends the wrist and fingers.
    9. Gluteal Region Muscles That Move the Thigh (essential for hip movement):
      • Gluteus Maximus: Extends and laterally rotates the thigh.
      • Gluteus Medius and Minimus: Abduct the thigh and stabilize the pelvis.
      • Quadriceps group: Extends leg at the knee joint.
      • Hamstring group: Flexes leg at the knee joint.
    10. Muscles of the Lower Leg (facilitate movement of the ankle and foot):
      • Gastrocnemius: Plantar flexes the foot and flexes the knee.
      • Soleus: Plantar flexes foot.
      • Tibialis Anterior: Dorsiflexes the foot.

    Understanding these muscle groups will enhance your comprehension of human anatomy and physiology, crucial for effectively conducting laboratory work.

    Anterior and lateral views of facial muscles. Details in text.
    Figure \(\PageIndex{6}\): Muscles of the head and face involved in facial expression and chewing in anterior and lateral view. Highlighted parts include muscle like - Buccinator, orbicularis oris, orbicularis oculi, occipitofrontalis, and epicranial aponeurosis. (CC BY-NC-SA 4.0; Gillian Backus; Heidi W. Wangerin; and Paula Rodgers via Muscles Lab).

    Figure \(\PageIndex{6}\) presents facial muscles in two views: an anterior (front) view and a lateral (side) view. Muscles and what they do:

    • Epicranial aponeurosis: Connects and stabilizes the scalp.
    • Occipitofrontalis: Raises the eyebrows.
    • Corrugator supercilii: Draws the eyebrows together to produce frowning.
    • Orbicularis oculi: Closes the eyelids (blinking and squinting).
    • Orbicularis oris: Shapes and closes the mouth (puckering and lip movement).
    • Buccinator: Keeps the cheeks taut to support chewing and helps prevent the cheeks from sagging inward.
    Lateral and posterior views of neck and back muscles. Details in text.
    Figure \(\PageIndex{7}\): Muscles of the neck and back in lateral and posterior view. The skeletal muscles highlighted are sternocleidomastoid, trapezius, levator scapuli, scalane, splenius and rhomboid major and minor. (CC BY-NC-SA 4.0 ; Gillian Backus; Heidi W. Wangerin; and Paula Rodgers via Pressbooks).

    Figure \(\PageIndex{7}\) shows two labeled anatomical diagrams of the neck and upper back muscles. The left panel shows a lateral (side) view of the neck, with the clavicle provided as a landmark. The following muscles are identified:

    • Sternocleidomastoid - a long band of muscle running diagonally from just below and behind the ear to the collarbone
    • Splenius - a broad muscle that pulls on the base of the skull and lies under the trapezius
    • Trapezius - this is described below; the side view shows how the trapezius connects at the base of the skull and the to the clavicle.
    • Levator scapulae - a slender muscle located at the back of the neck, running from the first few vertebrae to the shoulder blade.
    • Anterior scalene and medial scalene - these slender muscles are located deep within the neck.

    The right panel shows a posterior (back) view of the neck and upper back, identifying the following muscles:

    • Trapezius - a large superficial muscle in a triangular shape, oriented along the spine, with one corner attached to an area at the base of the skull, another attached to an area near a shoulder, and the other lower down on the back (not shown).
    • Splenius capitis - located below the trapezius in the back of the neck, and running from the upper spine to the base of the skull.
    • Splenius cervicis - located deep in the neck, along the spine.
    • Rhomboid minor - a small, cylindrical muscle in the upper back, connecting the scapula to the spinal column, just under the trapezius.
    • Rhomboid major - a wide band of muscle just below the Rhomboid minor, also connecting the scapula and the spinal column.
    Thoracic anatomy showing rib cage muscles and cross-section of intercostal muscle layers.
    Figure \(\PageIndex{8}\): Skeletal muscles involved in breathing (ventilation) in anterior view. Muscles highlighted are pectoralis major, pectoralis minor, internal intercostals, external intercostals, serratus anterior. The image also shows the clavicle, sternum and ribs. (CC BY-NC-SA 4.0 ; Gillian Backus; Heidi W. Wangerin; and Paula Rodgers via Pressbooks).

    Figure \(\PageIndex{8}\) focuses on the thoracic region, emphasizing on the anterior thoracic muscles. Chest wall muscles and intercostal layers (front/side view and cross-section)

    Left panel: anterior and lateral view of the chest

    • Bony landmarks are labeled: clavicle, ribs, and sternum.
    • Pectoralis major is shown partially removed to reveal deeper structures.
    • Pectoralis minor is shown beneath pectoralis major.
    • Intercostal muscles are shown between the ribs in layers:
      • External intercostals (outer layer; fibers run diagonally downward)
      • Internal intercostals (beneath external; fibers run in the opposite direction)
    • Serratus anterior is shown along the side of the rib cage with multiple attachments to the ribs.

    Right panel: cross-section close-up between two ribs. This close-up emphasizes the three intercostal layers between adjacent ribs:

    • External intercostal (outermost)
    • Internal intercostal (middle)
    • Innermost intercostal (deepest)
    Superficial and deep abdominal muscles from anterior lateral view. Details in text.
    Figure \(\PageIndex{9}\): Superficial and deep muscles of the abdominal wall in anterior and lateral view. Highlighted muscles include pectoralis major, latissimus dorsi, rectus abdominis,transversus abdominis, external oblique, anterior serratus. (CC BY-NC-SA 4.0 ; Gillian Backus; Heidi W. Wangerin; and Paula Rodgers via Pressbooks).

    Figure \(\PageIndex{9}\) illustrates the abdominal wall from an anterior-lateral (front/side) view. A main view highlights more superficial structures, and an inset view shows deeper layers.

    Abdominal wall muscles (anterior-lateral view): superficial vs. deep layers:

    • Pectoralis major at the upper chest
    • Latissimus dorsi extending from the back toward the side of the torso
    • Serratus anterior along the ribs
    • External oblique covering the lateral abdomen
    • Rectus abdominis within the rectus sheath, showing:
      • Tendinous intersections
      • The linea alba running vertically at the midline (part of the rectus sheath)

    The inset highlights deeper structures beneath the superficial layer, including:

    • Internal oblique
    • Transversus abdominis
    • Aponeurosis of the internal oblique, contributing to the rectus sheath around the rectus abdominis
    Superficial and deep abdominal muscles. Details in text.
    Figure \(\PageIndex{10}\): Superficial muscles of the shoulder (a) left anterior lateral view showing the pectoralis major and latissimus dorsii, (b) posterior view highlighting deltois and latissimus dorsii. (CC BY-NC-SA 4.0 ; Gillian Backus; Heidi W. Wangerin; and Paula Rodgers via Pressbooks).

    Figure \(\PageIndex{10}\) presents an anterior-lateral (front/side) view of the abdominal wall showing superficial and deep muscle layers. Muscles are labeled and grouped by region.

    Labeled muscles and primary actions

    • Pectoralis major: contributes to upper chest and shoulder movement.
    • Latissimus dorsi: extends from the back and assists with arm movement.
    • Serratus anterior: supports the rib cage and helps stabilize the shoulder region.
    • External oblique: lateral abdominal muscle that assists with trunk rotation and side-bending (lateral flexion).
    • Internal oblique: deeper lateral abdominal muscle that assists with trunk rotation and side-bending.
    • Transversus abdominis: deep abdominal muscle that supports core stability and abdominal compression.
    • Rectus abdominis: midline abdominal muscle shown with tendinous intersections; enclosed in the rectus sheath; supports trunk flexion and posture.
    Deep muscles of the left shoulder. Details in text.
    Figure \(\PageIndex{11}\): Deep muscles of the left shoulder (c) anterior lateral view highlighting deltoid,subscapularis, teres major, pectoralis major, serratus anterior (d) and posterior view highlighting teres minor, supraspinatus, infraspinatus, teres major, teres minor, latissimus dorsii and triceps brachii. (CC BY-NC-SA 4.0 ; Gillian Backus; Heidi W. Wangerin; and Paula Rodgers via Pressbooks).

    Figure \(\PageIndex{11}\) presents labeled diagrams of the left shoulder's deep muscles, providing both an anterior lateral view on the left and a posterior view on the right.

    • Anterior lateral view
    • Deltoid - along the top of the shoulder, plays a crucial role in arm movements such as abduction, flexion, internal and external rotation
    • Pectoralis major - large chest muscle, shown here along the sternum. Important for adduction, medial rotation of the arm at the shoulder joint.
    • Subscapularis - located on the scapula, between the scapula and the rib cage. It is crucial for shoulder stability and internal rotation.
    • Teres major - large, flat muscle that extends from the scapula to the humerus. It assists in arm adduction and medial rotation.
    • Serratus anterior - large, fan-shaped muscle that spans the upper ribs to the scapula. It supports scapular movement.
    • Posterior view
    • Supraspinatus extends along the top of the scapula and is part of the rotator cuff
    • Infraspinatus extends along the posterior of the scapula, below its spine, and forms part of the rotator cuff
    • Teres major. This posterior view shows where it connects to the base of the scapula.
    • Latissimus dorsi. A portion of this large, flat muscle is shown near its origin just below the teres major. It is responsible for arm extension and adduction.
    • Triceps brachii extends along the humerus, from the shoulder to the elbow. From the elbow it branches into two sections: long head (medial) and lateral head. This muscle is vital for elbow extension and shoulder stabilization.
    Left upper arm and forearm muscles. Details in text.
    Figure \(\PageIndex{12}\): Muscles of the left upper arm in anterior lateral highlighting the biceps brachii short and long head and biceps brachii, and the posterior view indicating triceps brachii long head and lateral head). The left forearm superficial muscles palmar view (bottom image) showing the biceps brachii, brachialis, pronator teres, flexor carpi radialis, flexor carpi ulnaris and flexor digitorum superficialis and dorsal view highlighting triceps brachii, brachialis, extensor carpi radialis longus and extensor digitorum. (CC BY-NC-SA 4.0 ; Gillian Backus; Heidi W. Wangerin; and Paula Rodgers via Pressbooks).

    Figure \(\PageIndex{12}\) presents an in-depth look at the muscles of the left upper arm and forearm.

    • Top-left (anterior lateral view of the upper arm) showcases two muscles essential for flexing the elbow and facilitating arm movement.
      • The biceps brachii extends along the humerus, from the shoulder to just above the elbow. Similar to the triceps branchii, the muscle branches into two sections before it meets the shoulder. The short head attaches to the scapula, and the long head to just above the shoulder joint.
      • The brachialis runs along the humerus just below the biceps brachii.
    • Top-right (posterior view of the upper arm) featuring the triceps brachii (lateral and long heads), crucial for elbow extension and arm stabilization.
    • Bottom-left diagram: Palmar view of the left forearm superficial muscles. These muscles are vital for wrist flexion, finger movement, and forearm rotation.
      • The brachioradialis extends along lateral side of the forearm, above the biceps branchii.
      • Four shorter and thinner muscles are shown along the medial side of the forearm, extending diagonally from the medial of the arm and moving down toward the wrist. From top to bottom, we see the pronator teres, flexor carpi ulnaris, palmaris longus, and flexor carpi radialis.
      • The flexor digitorum superficialis lies just below those muscles, extending in the same direction.
    • Bottom-right diagram: Dorsal view of the left forearm superficial muscles, which are essential for thumb movement, wrist extension, and stabilizing the forearm.
      • The extensor carpi radialis longus runs parallel to the brachioradialis and just below it, originating at the lateral epicondyle of the humerus.
      • The anconeus extends from lateral epicondyle of the humerus to the upper ulna.
      • The extensor carpi radialis brevis runs parallel to and just below the extensor carpi radialis longus.
      • The extensor digitorum extends from the lateral epicondyle of the humerus down through the wrist, splitting into four tendons within the digits.
      • Along the medial of the arm, just above the wrist area, three muscles are indicated, but the specific locations are not identified: abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus.
      • Along the lateral of the arm, just above the wrist area, the extensor carpi ulnaris and extensor digiti minimi are indicated (but specific locations are not identified).
      • The following muscles are highlighted as being particularly important: triceps branchii, brachiocradialis, extensor carpi radialis longus, and extensor digitorum.
    Deep pelvic and thigh muscles of the right leg. Details in text.
    Figure \(\PageIndex{13}\): Muscles of the hip and thigh showing the superficial anterior (top) showing the adductor longus, gracilis, adductor magnus, sartorius, vastus medialis, vastus lateralis and rectus femoris; deep pelvic and thigh muscles of right leg in anterior view (bottom, left) highlighting the adductor longus and adductor magnus, and posterior (bottom, right) highlighting gluteus maximus, gluteus medius, gracilis, semimembranosus, semitendinosus and biceps femoris. (CC BY-NC-SA 4.0 ; Gillian Backus; Heidi W. Wangerin; and Paula Rodgers via Pressbooks).

    Figure \(\PageIndex{13}\) provides set of three images of the hip and thigh muscles. The top diagram identifies the superficial anterior muscles that are crucial for hip adduction, leg flexion, and knee extension. To orient the viewer, the quadratus lumborum (an abdominal muscle) is shown at the top, just above the crest of ilium (top of the hip bone), with the iliacus muscle within the pelvis, and then the sacrum.

    • The psoas major appears deep in the thigh, running vertically from an area just above the crest of ilium and alongside the iliacus until it is obscured by the sartorius.
    • The pectineus is shown just below the sacrum. Running vertically along the pectin pectineusus, we see the adductor longus, gracilis, and adductor magnus.
    • The sartorius appears as a long band originating at the crest of ilium and wrapping horizontally to just below and over the adductor magnus.
    • The vastus medialis is a broad muscle that appears directly below and under the sartorius, on the medial side.
    • Just below the vastus medialis the quadriceps tendon (or patellar tendon) and patellar ligament are called out to orient the viewer.
    • Along the lateral side of the leg, extending from the crest of ilium we see the tensor facia latae.
    • The vastus lateralis appears directly below the tensor facia latae and continues to the knee.
    • The largest muscle shown is the rectus femoris, a superficial muscle located in the middle of the thigh. It originates at the pelvis and runs down to the patella.
    • These muscles are highlighted as being particularly important: iliacus, psoas major, adductor longus, gracilis, adductor magnus, sartorius, rectus femoris, vastus lateralis and vastus medialis.

    The bottom left diagram focuses on the deep pelvic and thigh muscles that are crucial for stabilizing the pelvis and facilitating thigh movement. The right leg is shown. To orient the viewer, the Iliac crest (or crest of iliac, the top of the hip bone), pectineus, and pubis are shown at the top, with the femur along the right side of the diagram.

    • The obturator externus muscle covers the pelvis, directly below the pubis.
    • The adductor magnus lies deep within the thigh and extends from the pelvis to the knee.
    • The adductor brevis is a short muscle shown wrapping over the adductor magnus.
    • The adductor longus runs diagonally over the adductor brevis, connecting the pubis to the femur.
    • The adductor longus and adductor magnus are highlighted as being particularly important.

    The bottom-right diagram presents a posterior view highlighting pelvic and thigh muscles that play a significant role in activities such as walking and running. The right leg is shown. To orient the viewer, the crest of ilium (top of the hip bone) and the sacrum are identified at the top of the diagram.

    • A portion of the gluteus maximus is shown as a thick band wrapping over the hip bone and sacrum.
    • On the right side of the diagram, a portion of the gluteus medius is shown on the surface of the ilium, underneath the gluteus maximus.
    • Just below the gluteus medius, a set of muscles are indicated, but not highlighted as being of interest. From top to bottom: gluteus minimus, piriformis, superior gemellus,
    • inferior gemellus, obturator externus, quadratus femoris, and adductor group. A small section of the gluteus maximus is visible near the adductor group.
    • In the middle of the thigh, the biceps femoris is shown as a thick vertical band originating at the pelvis and moving horizontally past the lateral side of the knee.
    • Three thinner bands of muscle are shown parallel to the biceps femoris, but moving past the medial side of the knee. From left to right: Gracilis, semimembranosus, and semitendinosus.
    • The following muscles are highlighted: gluteus medius, gluteus maximus, gracilis, semimembranosus, semitendinosus, and biceps femoris.
    Anatomical diagrams of right lower leg. Details in text.
    Figure \(\PageIndex{14}\): Muscles of the right lower leg. Superficial muscles, anterior view (left) highlighting tibialis anterior, fibularis longus, and extensor digitorum longus; superficial muscles, posterior view(middle) highlighting the gastrocnemius, soleus and calcaneal tendon; and deep muscles, posterior view (right) highlighting the soleus and fibularis longus. (CC BY-NC-SA 4.0 ; Gillian Backus; Heidi W. Wangerin; and Paula Rodgers via Pressbooks).

    Figure \(\PageIndex{14}\) presents a comprehensive view of the right lower leg, highlighting both superficial and deep muscles from different perspectives.

    The left diagram presents an anterior view of the superficial muscles of the right lower leg.The most prominent muscle, the tibialis anterior, originates from the upper tibia and runs along the shin, terminating in the foot above the hallux (great toe). Directly to the left in the diagram, the extensor digitorum longus runs parallel to the tibialis anterior, extending to the ankle and branching into four tendons that reach into each toe.

    • The fibularis longus runs vertically along the lateral side of the leg; the diagram does not show where it terminates.
    • Just below the fibularis longus, the fibularis brevis and fibularis tertius are indicated, both appear to continue vertically down the leg.
    • The extensor hallucis longus is shown underneath the tibilaris anterior.
    • The superior and inferior extensor retinacula are shown underneath the tibilaris anterior and extensor digitorum longus. The superior extensor retinaculum appears just above the ankle joint, while the inferior extensor retinaculum is a band across the foot, just below the ankle joint.

    The middle diagram presents a posterior view of superficial muscles of the right lower leg that are essential for plantar flexion, providing power for movements like jumping and running.

    • Just behind the knee we see the gastrocnemius, which has a medial head and a lateral head.
    • The soleus, or calf muscle, originates just below the knee and under the gastrocnemius, joining the calcaneal (Achilles) tendon and terminating at the calcaneus (heel).

    The right diagram shows a posterior view of the deep muscles of the right lower leg that are crucial for stabilizing the leg and foot, as well as enabling intricate movements like toe flexion and inversion.

    • The popliteus is shown as a band of muscle directly behind the knee joint.
    • A portion of the soleus appears in this deeper view as an inverted Y just below the popliteus at the top of the calf area.
    • The fibularis longus is visible as a wide band running along the lateral side of the calf area.
    • The tibialis posterior is a band of muscle deep in the middle of the lower leg, originating below the popliteus. Its lower end lies underneath the flexor digitorum longus and flexor hallucis longus.
    • The flexor digitorum longus and flexor hallucis longus are shown as two vertical bands of muscle that originate from the medial (flexor digitorum longus) and lateral (flexor hallucis longus) sides of the leg and both appear to terminate at the ankle.
    • The tibilaris brevis is shown as a thin band running along the lateral side of the lower leg, parallel to the flexor hallucis longus.

    This page titled 9.1: Background Information was last modified on Tue, 01 Sep 2026 01:45:34 GMT and is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Lakshmikanta Sengupta and Susmita Sengupta.