Study Guide: The Muscular System (HKIN 162 Lab 4)

The Muscular System

Overview of the Muscular System

The muscular system is a complex network of tissues responsible for movement, posture, and heat generation. Skeletal muscles, which attach to bones via tendons or aponeuroses, facilitate voluntary movements by contracting and pulling on the skeleton. Most muscles work in groups to produce coordinated actions, with some acting as agonists (prime movers), antagonists, or synergists.

• Agonist (Prime Mover): The main muscle responsible for a specific movement.

• Antagonist: Muscle that opposes the action of the agonist.

• Synergist: Muscle that assists the agonist by reducing unnecessary movement.

Example: During elbow flexion, the biceps brachii is the agonist, the triceps brachii is the antagonist, and the brachialis acts as a synergist.

Major Muscle Groups of the Body

The body’s muscles are organized into major groups based on their location and function. Understanding these groups is essential for identifying muscle actions and their anatomical relationships.

• Head and Neck Muscles: Responsible for facial expressions, mastication, and head movement (e.g., sternocleidomastoid, masseter, temporalis, orbicularis oculi, orbicularis oris).

• Shoulder and Upper Arm Muscles: Move the arms and stabilize the shoulder (e.g., deltoid, biceps brachii, triceps brachii, pectoralis major, rotator cuff muscles).

• Forearm and Hand Muscles: Control wrist, hand, and finger movements (e.g., flexor carpi radialis, extensor carpi ulnaris, flexor digitorum superficialis, extensor digitorum, brachioradialis).

• Thoracic and Abdominal Muscles: Aid in breathing and core stability (e.g., pectoralis major, rectus abdominis, external and internal obliques, diaphragm).

• Back Muscles: Support posture and allow for trunk movements (e.g., latissimus dorsi, trapezius, erector spinae, rhomboid major, levator scapulae).

• Hip and Thigh Muscles: Essential for locomotion and stability (e.g., gluteus maximus, gluteus medius, quadriceps, hamstrings, adductor longus).

• Leg and Foot Muscles: Facilitate walking, running, and balance (e.g., gastrocnemius, soleus, tibialis anterior, fibularis longus, extensor digitorum longus).

Muscle Fascicle Arrangements

Types of Muscle Fascicle Shapes

Muscle fascicles are bundles of muscle fibers arranged in specific patterns that influence a muscle’s range of motion and power. The main types are:

• Parallel: Fascicles run parallel to the muscle’s long axis (e.g., sartorius, biceps brachii). High range of motion, less power.

• Pennate: Fascicles attach obliquely to a central tendon. Types include:

  • Unipennate: Fascicles insert into one side of the tendon (e.g., extensor digitorum longus).

  • Bipennate: Fascicles insert into both sides of the tendon (e.g., rectus femoris).

  • Multipennate: Fascicles insert into multiple tendons (e.g., deltoid).

• Convergent: Fascicles converge toward a single tendon (e.g., pectoralis major). Versatile movement, strong contraction.

• Circular: Fascicles arranged in concentric rings (e.g., orbicularis oris, orbicularis oculi). Control openings.

• Fusiform: Spindle-shaped with a thick belly (e.g., biceps brachii). Combination of range and power.

Major Muscles: Origins, Insertions, and Actions

Muscles of the Face and Neck

These muscles control facial expressions, mastication, and head movement.

Muscle	Origin	Insertion	Action
Frontalis	Epicranial aponeurosis	Skin superior to supraorbital margin	Draws scalp anteriorly, elevates eyebrows, wrinkles forehead
Masseter	Maxilla and zygomatic bone	Ramus of mandible	Elevates mandible, assists in side-to-side movement, protracts mandible
Temporalis	Temporal bone	Coronoid process and ramus of mandible	Elevates and retracts mandible, assists in side-to-side movement

Muscles of the Abdominal Wall

The abdominal wall consists of four main muscles that flex, rotate, and compress the abdomen. The rectus abdominis is notable for its tendinous intersections, while the aponeuroses of the other muscles form the linea alba.

Muscle	Origin	Insertion	Action
Rectus abdominis	Pubic crest and pubic symphysis	Cartilage of 5th–7th ribs, xiphoid process	Flexion and rotation of spine
External oblique	5th–12th ribs	Iliac crest, linea alba	Compresses abdomen, flexes and rotates spine
Internal oblique	Iliac crest, inguinal ligament, thoracolumbar fascia	Cartilage of 7th–10th ribs, linea alba	Compresses abdomen, flexes and rotates spine
Transversus abdominis	Iliac crest, inguinal ligament, lumbar fascia, cartilage of 5th–10th ribs	Xiphoid process, linea alba, pubis	Compresses abdomen

Muscles that Act on the Scapula

These muscles stabilize and move the scapula, indirectly affecting shoulder movements.

Muscle	Origin	Insertion	Action
Trapezius	Occipital bone, spines of C7 and T1–T12	Clavicle, acromion, spine of scapula	Elevates clavicle, adducts and rotates scapula, elevates/depresses scapula, extends head

Muscles that Act on the Humerus

These muscles move and stabilize the shoulder joint, including the rotator cuff group.

Muscle	Origin	Insertion	Action
Pectoralis major	Clavicle, sternum, ribs 2–6	Greater tubercle, intertubercular sulcus of humerus	Flexes, adducts, rotates arm medially
Latissimus dorsi	Inferior thoracic/lumbar vertebrae, sacrum, ilium, ribs 9–12	Intertubercular sulcus of humerus	Extends, adducts, rotates arm medially
Deltoid	Clavicle, acromion, spine of scapula	Deltoid tuberosity of humerus	Abducts, flexes, extends, rotates arm
Rotator cuff muscles (subscapularis, supraspinatus, infraspinatus, teres minor)	Scapula	Humerus	Stabilize and move shoulder joint

Muscles that Act on the Thigh and Leg

These muscles are responsible for movements at the hip and knee joints, including the quadriceps and hamstrings.

Muscle	Origin	Insertion	Action
Quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius)	Os coxae/femur	Tibial tuberosity via patellar ligament	Extend leg; rectus femoris also flexes thigh
Hamstrings (biceps femoris, semitendinosus, semimembranosus)	Ischial tuberosity/femur	Tibia/fibula	Flex leg, extend thigh
Gluteus maximus	Iliac crest, sacrum, coccyx	Iliotibial tract, gluteal tuberosity	Extends, rotates thigh laterally
Gluteus medius	Ilium	Greater trochanter of femur	Abducts, rotates thigh medially
Adductors (magnus, longus)	Pubis/ischium	Linea aspera of femur	Adduct, flex, extend, rotate thigh
Sartorius	ASIS	Proximal medial tibia	Flexes leg, flexes and laterally rotates thigh
Gracilis	Pubis	Medial tibia	Adducts thigh, flexes leg

Muscles that Act on the Foot and Digits

These muscles control movements such as dorsiflexion and plantar flexion of the foot.

Muscle	Origin	Insertion	Action
Gastrocnemius	Femur (lateral and medial condyles)	Calcaneus via Achilles tendon	Flexes leg, plantar flexes foot
Soleus	Head of fibula, medial tibia	Calcaneus via Achilles tendon	Plantar flexes foot

Muscles in Action: Functional Application

Understanding muscle actions in real movements helps reinforce anatomical knowledge. For example:

• Flexing the elbow: Biceps brachii contracts, triceps brachii relaxes.

• Extending the elbow: Triceps brachii contracts, biceps brachii relaxes.

• Abducting the arm: Deltoid contracts.

• Standing on toes: Gastrocnemius and soleus contract for plantar flexion.

• Dorsiflexing the foot: Tibialis anterior contracts.

Additional info: Practicing muscle identification and movement on oneself or a volunteer reinforces the connection between anatomical structure and function.