Muscles of the back, thigh and arm, with a sarcomere diagram

Surface muscles

• Trapezius

  Moves the shoulder blade

  Elevates the shoulders (shrugging), pulls the shoulder blades together and tilts the head backward.

• Rhomboid major and minor

  Moves the shoulder blade

  Move the scapula medially, toward the spine, which can pull the shoulder and upper limb backward.

• Latissimus dorsi

  Moves the humerus

  Moves the humerus inferiorly (down). It is an axial muscle that starts on the middle and lower back.

The erector spinae

• Iliocostalis

  Erector spinae, lateral column

  Has cervicis, thoracis and lumborum parts.

• Longissimus

  Erector spinae, intermediate column

  Has capitis, cervicis and thoracis parts.

• Spinalis

  Erector spinae, medial column

  Has capitis, cervicis and thoracis parts.

Together the three columns are the erector spinae, the main extensor of the vertebral column. The group controls flexion, lateral flexion and rotation of the spine and helps maintain the lumbar curve.

Deeper muscles

• Splenius capitis and cervicis

  Run from the midline laterally and upward

  Can extend the head, laterally flex it and rotate it.

• Semispinalis

  Transversospinales group (transverse to spinous processes)

  Has capitis, cervicis and thoracis parts.

• Multifidus

  Transversospinales group, lumbar region

  Helps extend and laterally flex the vertebral column.

• Interspinales and intertransversarii

  Segmental group

  Connect the spinous or transverse processes of neighboring vertebrae and stabilize the spine.

Gluteal muscles

• Gluteus maximus

  Gluteal group, the largest of the three

  Extends the thigh, moving it back, and rotates it outward.

• Gluteus medius

  Gluteal group

  Abducts the thigh (moves it out to the side, as in a split) and rotates it inward.

• Gluteus minimus

  Gluteal group, the smallest of the three

  Abducts and medially rotates the thigh and helps stabilize the pelvis, like the gluteus medius.

• Tensor fasciae latae

  Hip

  Acts as a synergist of the gluteus medius and iliopsoas in flexing and abducting the thigh, and helps stabilize the knee by pulling on the iliotibial tract.

Quadriceps muscles

• Rectus femoris

  Quadriceps femoris, anterior thigh

  One of the four muscles that extend and stabilize the knee.

• Vastus lateralis

  Quadriceps femoris, lateral thigh

  One of the four muscles that extend and stabilize the knee.

• Vastus medialis

  Quadriceps femoris, medial thigh

  One of the four muscles that extend and stabilize the knee.

• Vastus intermedius

  Quadriceps femoris, deep to the rectus femoris

  Lies between the vastus lateralis and the vastus medialis.

All four share the quadriceps (patellar) tendon, which inserts into the patella. The patellar ligament continues below the patella and attaches to the tibial tuberosity.

Hamstrings and other thigh muscles

• Biceps femoris, semitendinosus and semimembranosus

  The hamstrings

  The posterior compartment of the thigh: as a group they flex the knee and extend the thigh.

• Sartorius

  Thigh

  Flexes the leg at the knee and flexes, abducts and laterally rotates the leg at the hip, which allows us to sit cross-legged.

• Adductor longus, brevis and magnus

  Adductors

  Medial compartment muscles that adduct the thigh (bring it toward the midline). The adductor longus also flexes the thigh and the adductor magnus extends it.

Shoulder

• Deltoid

  Shoulder

  The major abductor of the arm; it also assists with flexion, medial rotation, extension and lateral rotation.

• Supraspinatus

  Rotator cuff

  One of four muscles whose tendons connect the scapula to the humerus around the shoulder joint.

• Infraspinatus

  Rotator cuff

  One of four muscles whose tendons connect the scapula to the humerus around the shoulder joint.

• Teres minor

  Rotator cuff

  One of four muscles whose tendons connect the scapula to the humerus around the shoulder joint.

• Subscapularis

  Rotator cuff

  One of four muscles whose tendons connect the scapula to the humerus around the shoulder joint.

• Pectoralis major

  Moves the humerus

  An axial muscle that moves the humerus forward (anteriorly) from the sternum, covering the upper chest.

Muscles that move the forearm at the elbow

• Biceps brachii

  Front of the upper arm

  Flexes the forearm, supinates the forearm and flexes the arm at the shoulder.

• Brachialis

  Deep to the biceps

  Adds power to forearm flexion.

• Brachioradialis

  Moves the forearm

  Flexes the forearm quickly or helps lift a load slowly.

• Triceps brachii

  Back of the upper arm

  Extends the forearm at the elbow; it is the antagonist of the biceps brachii.

• Coracobrachialis

  Moves the humerus

  Flexes and adducts the arm.

Origin

The end of a muscle attached to a fixed, stabilized bone.

Insertion

The moveable end of a muscle, attached to the bone that gets pulled.

Agonist (prime mover)

The principal muscle in an action. The biceps brachii is the prime mover when lifting a cup.

Antagonist

A muscle with the opposite action of the prime mover. The triceps brachii extends the forearm while the biceps brachii flexes it.

Synergist

A muscle that assists the prime mover. The brachialis is a synergist of the biceps brachii in flexing the forearm.

Fixator

A synergist that stabilizes the bone where the prime mover’s origin attaches.

Sarcomere diagram

When a sarcomere shortens, the Z discs move closer together: the I band gets smaller and the A band stays the same width.

Parts of the fiber

Sarcolemma

The plasma membrane of a muscle fiber.

Sarcoplasm

The cytoplasm of the muscle fiber.

T-tubules

Periodic invaginations in the sarcolemma that carry the action potential deep into the fiber.

Sarcoplasmic reticulum

Specialized smooth endoplasmic reticulum that stores, releases and reclaims calcium ions.

Myofibril

A cylinder-shaped bundle that runs the length of the fiber and contains actin and myosin.

Sarcomere

The functional unit of the muscle fiber: the region between two Z discs.

Z disc

The border of each sarcomere; the actin myofilaments are anchored to it.

Thin filament

Actin with its regulatory proteins, troponin and tropomyosin. It reaches from the Z disc toward the center of the sarcomere.

Thick filament

Myosin, with multiple heads. It sits at the center of the sarcomere.

Connective tissue layers

Endomysium

A thin connective tissue layer of collagen and reticular fibers around each muscle fiber.

Perimysium

The middle layer, wrapped around each fascicle (a bundle of muscle fibers).

Epimysium

The outer sheath of dense, irregular connective tissue around the whole muscle.

Sliding filament steps

1. 1.A motor neuron releases acetylcholine, which depolarizes the muscle fiber. The action potential spreads along the sarcolemma and down the T-tubules, and calcium is released from the sarcoplasmic reticulum.
2. 2.Calcium binds troponin, which moves tropomyosin off the binding sites on actin.
3. 3.Myosin heads attach to the exposed sites and form cross-bridges.
4. 4.The thin filaments are pulled toward the center of the sarcomere and slide past the thick filaments.

The cross-bridge cycle

Attach

The myosin head binds actin while ADP and inorganic phosphate are still attached to it.

Power stroke

The phosphate is released and the head pivots toward the center of the sarcomere, pulling the actin about 10 nm. Then ADP is released.

Detach

A new ATP binds the myosin head, and the head lets go of actin.

Re-cock

ATP is split into ADP and phosphate, which returns the head to its cocked position.

Relaxation starts when the motor neuron stops releasing acetylcholine. ATP-driven pumps move calcium back into the sarcoplasmic reticulum, which covers the binding sites on actin again. After death ATP is no longer made, so the myosin heads cannot detach and the cross-bridges stay in place: that is the stiffness called rigor mortis.