Chapter 10: Muscle Function, Lever Systems, and Major Muscles of the Human Body

Ch. 10 The Muscular System

Muscle Roles in Movement

Muscles work together to produce movement, each playing a specific role. Understanding these roles is essential for analyzing body mechanics and muscle coordination.

• Prime Mover (Agonist): The main muscle responsible for producing a specific movement. It contracts to create the desired action. Example: The biceps brachii is the prime mover for elbow flexion.

• Antagonist: A muscle that opposes or reverses the action of the prime mover. It relaxes while the agonist contracts, but can also contract to provide control. Example: The triceps brachii is the antagonist during elbow flexion.

• Synergist: A muscle that assists the prime mover by adding extra force or reducing unnecessary movement. Example: The brachialis assists the biceps brachii in elbow flexion.

• Fixator: A type of synergist that stabilizes the origin of the prime mover, allowing efficient movement. Example: The muscles that stabilize the scapula during arm movements.

Lever Systems in the Human Body

The body uses bones as levers and joints as fulcrums to produce movement. Levers are classified into three types based on the relative positions of the fulcrum, effort, and load.

• First-Class Lever: The fulcrum is located between the effort and the load. Purpose: Can provide mechanical advantage or disadvantage, depending on the location of the fulcrum. Anatomical Example: The atlanto-occipital joint (nodding the head). Diagram: Effort — Fulcrum — Load

• Second-Class Lever: The load is located between the fulcrum and the effort. Purpose: Always provides a mechanical advantage (less effort needed to move a load). Anatomical Example: Standing on tiptoe (ball of foot is fulcrum, body weight is load, calf muscles provide effort). Diagram: Fulcrum — Load — Effort

• Third-Class Lever: The effort is applied between the fulcrum and the load. Purpose: Always at a mechanical disadvantage, but allows for greater speed and range of motion. Anatomical Example: Flexing the forearm at the elbow (elbow is fulcrum, biceps apply effort, hand holds load). Diagram: Fulcrum — Effort — Load

Key Muscles: Actions, Origins, and Insertions

Understanding the actions, origins, and insertions of major muscles is fundamental for analyzing movement and diagnosing injuries.

• Sternocleidomastoid (SCM):

  • Action: Flexes neck; rotates head to opposite side.

  • Origin: Manubrium of sternum and medial portion of clavicle.

  • Insertion: Mastoid process of temporal bone.

• Diaphragm:

  • Action: Prime mover of inspiration; flattens to increase thoracic cavity volume.

• Internal Intercostals:

  • Action: Depress ribs during forced expiration.

• External Intercostals:

  • Action: Elevate ribs during inspiration.

• Orbicularis Oris:

  • Action: Closes and protrudes lips (kissing muscle).

• Buccinator:

  • Action: Compresses cheek (as in blowing or sucking); holds food between teeth during chewing.

• Tibialis Anterior:

  • Action: Dorsiflexes and inverts foot.

• Sartorius:

  • Action: Flexes, abducts, and laterally rotates thigh; flexes knee (tailor's muscle).

• Gastrocnemius:

  • Origin: Medial and lateral condyles of femur.

  • Insertion: Posterior calcaneus via calcaneal (Achilles) tendon.

Major Muscle Groups

Several muscle groups are responsible for major movements of the limbs and joints.

• Quadriceps Femoris Group: Extends the knee.

  • Rectus femoris

  • Vastus lateralis

  • Vastus medialis

  • Vastus intermedius

• Hamstrings Group: Flexes the knee and extends the hip.

  • Biceps femoris

  • Semitendinosus

  • Semimembranosus

• Rotator Cuff Muscles: Stabilize the shoulder joint.

  • Supraspinatus

  • Infraspinatus

  • Teres minor

  • Subscapularis

Most Powerful Muscle in the Body

The gluteus maximus is considered the most powerful muscle in the human body. It is responsible for the extension and outward rotation of the hip, playing a crucial role in climbing, running, and rising from a sitting position.

Summary Table: Major Muscles and Their Functions

Muscle	Action	Origin	Insertion
Sternocleidomastoid	Flexes neck, rotates head	Sternum, clavicle	Mastoid process
Diaphragm	Inspiration (breathing in)	Inferior, internal surface of rib cage and sternum	Central tendon
Gastrocnemius	Plantar flexes foot, flexes knee	Femur (condyles)	Calcaneus (via Achilles tendon)
Tibialis Anterior	Dorsiflexes, inverts foot	Lateral condyle, upper tibia	Medial cuneiform, first metatarsal
Sartorius	Flexes, abducts, laterally rotates thigh; flexes knee	Anterior superior iliac spine	Medial aspect of proximal tibia

Additional info: The gluteus maximus is often cited as the most powerful muscle due to its size and role in powerful lower limb movements. The lever system classification is based on the arrangement of bones and muscles around joints, which determines the efficiency and type of movement produced.