What are the layers of the spinal and paraspinal muscles?

Spinal and Paraspinal Muscles: Layer by Layer

The spinal and paraspinal muscles are organized in distinct anatomical layers, with each layer serving specific biomechanical functions in spinal stability, movement, and protection.

Superficial Layer

• The most superficial layer consists of the skin, followed by subcutaneous tissue (hypodermis) which contains adipose tissue that connects the skin with underlying tissues and organs 1
• Beneath the hypodermis lies the superficial fascia, which envelops the more superficial back muscles 2

Intermediate Layer

• The intermediate layer contains the trapezius and latissimus dorsi muscles, which are broad, flat muscles that cover large portions of the back 2
• These muscles primarily function in shoulder and upper limb movements but contribute to overall back stability 2

Deep Layer - Extrinsic Back Muscles

• This layer includes the rhomboids (major and minor) and the levator scapulae muscles 2
• These muscles connect the vertebral column to the upper limbs and are primarily involved in scapular movements 2

Deepest Layer - Intrinsic Back Muscles (True Paraspinal Muscles)

• The deepest layer contains the true paraspinal muscles that directly attach to the vertebral column 3
• These muscles are arranged in three distinct groups from lateral to medial:

1. Erector Spinae Muscle Group (Lateral)

• The erector spinae is the largest and most lateral muscle group of the deep back muscles 3
• It consists of three vertical columns:
  • Iliocostalis (most lateral): Attaches to the ribs and has significant cross-sectional area (mean ACSA of 9.16 cm²) 3
  • Longissimus (intermediate): Has the greatest cross-sectional area among paraspinal muscles (mean ACSA of 10.42 cm²) 3
  • Spinalis (most medial): Runs close to the spinous processes 2
• These muscles function as powerful extensors of the spine and contribute to lateral flexion when contracting unilaterally 3

2. Transversospinal Muscle Group (Intermediate)

• Located deeper and medial to the erector spinae 2
• Includes:
  • Semispinalis: Runs from transverse processes to spinous processes spanning 4-6 vertebral levels 2
  • Multifidus: A segmental muscle with smaller cross-sectional area (mean ACSA of 0.24 cm²) but better mechanical advantage for stabilization 3
  • Rotatores: Short muscles spanning 1-2 vertebral levels 2
• These muscles provide fine control of spinal movements and are critical for segmental stability 4

3. Deep Segmental Muscles (Medial)

• The deepest layer includes:
  • Interspinales: Connect adjacent spinous processes 2
  • Intertransversarii: Connect adjacent transverse processes 2
• These small muscles provide proprioceptive feedback and fine-tuning of vertebral movements 2

Associated Muscles

• Quadratus lumborum: Located in the posterior abdominal wall but functions as an important stabilizer of the lumbar spine 5
• Psoas major: Though primarily considered a hip flexor, it attaches to the lumbar vertebrae and influences lumbar spine mechanics 5

Biomechanical Considerations

• The paraspinal muscles exhibit a paradoxical anatomy with large muscle bellies but disproportionately thin tendons, particularly in the erector spinae group 3
• The ratio of muscle cross-sectional area to tendon cross-sectional area (rCSA) is extremely small in erector spinae (1/156 to 1/739), suggesting these muscles function differently than traditional "chord-like" muscles 3
• The multifidus and erector spinae muscles significantly influence the maintenance of pelvic alignment and overall spinal stability 4

Clinical Significance

• Atrophy of paraspinal muscles, particularly multifidus, is associated with both acute and chronic low back pain 5
• In patients with adult spinal deformities, the condition of paraspinal muscles (especially multifidus and erector spinae) correlates with pelvic tilt and sacral slope measurements 4
• MRI studies show selective ipsilateral atrophy of paraspinal muscles specific to the symptomatic side in patients with unilateral low back pain 5

Understanding the layered arrangement of spinal and paraspinal muscles is essential for diagnosing and treating various spinal conditions, as well as for planning surgical approaches to the spine 2, 3.