Spinal Biomechanics: Structure, Function, and Load Transmission

Posted on Dec 11, 2025 in Human Biology

The spine is a complex mechanical structure that supports the body, protects the spinal cord, and allows controlled mobility. Its biomechanics are governed by the interaction of bones (vertebrae), joints, intervertebral discs, ligaments, and muscles.

1. Structural Features of the Spine

  • Vertebrae: 33 in total (7 cervical, 12 thoracic, 5 lumbar, 5 sacral [fused], 4 coccygeal).
  • Vertebral bodies bear compressive load.
  • Facet (zygapophyseal) joints guide and limit motion.
  • Intervertebral Disc (IVD): Acts as a shock absorber.
    • Consists of nucleus pulposus (gel-like, resists compression).
    • Annulus fibrosus (fibrous rings, resists tension and shear).

Ligaments and Muscles

  • Ligaments: Provide passive stability (e.g., anterior and posterior longitudinal ligaments, ligamentum flavum, interspinous, supraspinous).
  • Muscles: Provide dynamic stability and movement (erector spinae, multifidus, abdominals, etc.).

2. Spinal Curvatures and Their Biomechanical Role

The spine features four main curves:

  • Cervical Lordosis: Concave posteriorly.
  • Thoracic Kyphosis: Convex posteriorly.
  • Lumbar Lordosis: Concave posteriorly.
  • Sacral Kyphosis: Convex posteriorly.

Significance of Spinal Curves

These curvatures act like a spring system, dissipating loads during standing, walking, and jumping. They:

  • Increase flexibility and shock absorption.
  • Help maintain balance with minimal muscle energy.

3. Load Transmission in the Spine

Axial Loading (Body Weight)

  • Approximately 80% is transmitted via vertebral bodies and discs.
  • Approximately 20% is transmitted via facet joints (this percentage increases in extension and decreases in flexion).

Intervertebral Disc Mechanics

  • The nucleus pulposus distributes pressure equally in all directions (hydrostatic mechanism).
  • The annulus fibrosus resists tensile forces and shear stress.

Effect of Posture on Disc Pressure

  • Standing: Moderate disc pressure.
  • Sitting, Forward Bending: Highest disc pressure.
  • Supine Lying: Lowest disc pressure.

4. Biomechanics of Spinal Movements

Each motion segment (two adjacent vertebrae, the disc, and facet joints) contributes to overall movement.

Flexion

  • Anterior disc is compressed; posterior annulus is stretched.
  • Facet joints open (separate).
  • Limited by posterior ligaments (supraspinous, interspinous, ligamentum flavum).

Extension

  • Posterior disc is compressed; anterior annulus is stretched.
  • Facet joints approximate (close).
  • Limited by the anterior longitudinal ligament.

Lateral Flexion

Compression occurs on the ipsilateral side, with tension on the contralateral annulus and ligaments. This movement is coupled with rotation, especially in the cervical spine.

Rotation

Facet orientation determines the range of rotation:

  • Cervical Spine: Greatest rotation.
  • Thoracic Spine: Limited by the ribs.
  • Lumbar Spine: Minimal rotation due to sagittal facet orientation.

5. Regional Biomechanics

  • Cervical Spine: High mobility (flexion, extension, rotation, lateral flexion). The atlas-axis complex (C1–C2) accounts for 50% of cervical rotation.
  • Thoracic Spine: Stable due to the rib cage. Rotation is possible, but flexion and extension are limited.
  • Lumbar Spine: High load-bearing capacity. Good flexion and extension, but limited rotation (which protects the discs).
  • Lumbosacral Junction (L5–S1): Oblique orientation of facets resists anterior shear forces resulting from body weight.

6. Spinal Stability: The Biomechanical Concept

According to Panjabi’s model, spinal stability relies on three interconnected subsystems:

  1. Passive Subsystem: Vertebrae, discs, and ligaments.
  2. Active Subsystem: Muscles and tendons.
  3. Neural Subsystem: Neuromuscular control.

Together, these subsystems maintain stability under loads and during movement.

7. Clinical Biomechanics

  • Scoliosis: Abnormal lateral curvature coupled with vertebral rotation, leading to altered load distribution and risk of progression.
  • Kyphosis/Lordosis Abnormalities: Increased curvature changes load transmission, potentially causing pain and deformity.
  • Disc Degeneration: Loss of nucleus hydration reduces shock absorption, resulting in increased facet joint load.

Orthotic Principles

Braces apply 3-point pressure systems, axial unloading, and motion restriction to restore biomechanical balance.

8. Summary of Spinal Function

The spine functions as a weight-bearing, flexible, and shock-absorbing column. Key takeaways include:

  • Spinal curves distribute loads efficiently.
  • Discs and facets share load and guide motion.
  • Movements (flexion, extension, rotation, lateral flexion) depend on regional anatomy.
  • Stability is maintained by the passive, active, and neural control subsystems.

A thorough biomechanical understanding is crucial in orthotic treatment, physiotherapy, and surgical planning.