Lecture on Muscles of the Torso and Trunk

Erector Spinae Group

• Comprises three muscles:
  • Spinalis: Connects spine to spine
  • Longissimus: Connects transverse process to transverse process
  • Iliocostalis: Connects rib to rib
• Functions:
  • Bilateral contraction: Extends the neck and trunk
  • Unilateral contraction: Laterally bends the trunk and neck to the same side
• Important points:
  • Short moment arm near the bone, requiring large muscle force
  • Primary factor for back injuries, especially in more mobile regions like the cervical (C) and lumbar (L) spine
• Memory aid: "I" for Iliocostalis, "L" for Longissimus, "S" for Spinalis – mnemonic "I love Susy"

Transversospinalis Group

• Function: Mainly rotates the spine
• Structure:
  • Insertion: Spinous process
  • Origin: Transverse process
• Mechanism:
  • Muscle fibers run from spinous to transverse processes
  • When contracting on one side, they turn the trunk in the opposite direction

Abdominal Muscles

Rectus Abdominis

• Location: Center, straight muscle fibers
• Function: Trunk flexion
• Effects of overstretching: Leads to loosening, increased anterior pelvic tilt, and more lumbar lordosis

External Oblique

• Fiber direction: Similar to putting hands in pockets
• Functions:
  • Bilateral contraction: Trunk flexion
  • Unilateral contraction: Lateral bending and rotation to the opposite side

Internal Oblique

• Fiber direction: Opposite to external oblique (crossing forearm to chest)
• Functions:
  • Bilateral contraction: Trunk flexion
  • Unilateral contraction: Lateral bending and rotation to the same side

Transversus Abdominis

• Fiber direction: Horizontal
• Function: Supports internal organs, assists in trunk flexion and lateral flexion

Quadratus Lumborum

• Location: Close to the bone, involved in pelvic section
• Functions:
  • Bilateral contraction: Raises the pelvis
  • Unilateral contraction: Raises one side of the pelvis, assists in trunk lateral flexion

Summary of Muscle Groups

• Erector Spinae: Trunk extension, some flexion, and rotation
• Transversospinalis: Mainly rotation
• Abdominal Muscles: Trunk flexion, some rotation
• Mechanism: Muscles can cancel out rotational forces to focus on flexion/extension

Functional Implications

• Stabilization: Trunk stabilizes the lower and upper extremities
• Examples:
  • Hip Flexion: Rectus abdominis stabilizes pelvis to facilitate hip flexor contraction
  • Upper Extremity Function: Trunk provides a stable base for the upper extremity, compensating for weak or unstable scapula

Final Thoughts

• Understanding the structure and function of trunk muscles helps in grasping their role in movement and stabilization.