Shear Instability: The Hidden Cause of Chronic Low Back Pain

May 7, 2025
Perth Injury & Pain Clinic

If you’ve been battling persistent low back pain without clear answers, shear instability could be the missing piece of your puzzle.

While disc bulges, muscle strains, and degenerative changes often receive the spotlight, shear instability — subtle, uncontrolled movement between spinal segments — is a major, yet frequently overlooked, driver of chronic pain, as described by renowned spine researcher Professor Stuart McGill.

WHAT IS SHEAR STABILITY?

The spine is structurally designed to withstand heavy compressive forces. However, it is inherently vulnerable to shearing forces — sliding forces where one vertebra translates or shifts relative to another.

In a healthy, resilient spine, passive structures like ligaments, and active control mechanisms provided by spinal muscles, work together to resist shear. When these systems fail — whether through injury, deconditioning, or cumulative overload — the result is micro-movement at the segmental level. Over time, these small uncontrolled movements cause persistent irritation and sensitization of pain-generating tissues like discs, facet joints, and nerves.

This is shear instability: not a visible “tear” or “fracture,” but a hidden loss of mechanical integrity that keeps tissues irritated and pain lingering.

Common Causes of Shear Instability

Injury to Passive Structures: Damage to ligaments, annular fibres of the disc, or endplate structures can weaken the spine’s passive restraints.
Muscle Deconditioning: The deep stabilizing muscles of the spine, especially the multifidus, play a crucial role in segmental control. The multifidus acts as an intersegmental stabilizer, dynamically stiffening and supporting individual vertebrae to maintain alignment during movement. Atrophy or inhibition of the multifidus (common after low back injury) leaves the spine vulnerable to shear forces.
Poor Movement Habits: Repetitive extension, twisting, and poor lifting techniques excessively load spinal tissues, compounding micro-injury over time and accelerating mechanical breakdown.

How to Recognise Shear Instability

Recognizing shear instability requires a careful clinical eye, as it often presents subtly. Key indicators include:

Pain during transitional movements (e.g., getting out of a chair, rolling in bed)
Pain with standing or walking for prolonged periods, often relieved temporarily by sitting
“Catch” sensations or sudden jolts of pain during certain movements
Inconsistent symptoms that vary dramatically depending on posture, load, and movement

If these symptoms sound familiar, shear instability could be an underlying factor.

Why Standard Treatments Often Fail

Conventional rehabilitation frequently falls short because it does not target the underlying mechanical dysfunction. General stretching programs can inadvertently worsen instability, while random strengthening efforts might overload vulnerable structures.

McGill’s approach to rehabilitation emphasizes “spine-sparing” strategies and the creation of a “stiffness shield” — a protective mechanism built through precise movement correction and targeted muscle endurance training.

Management Principles for Shear Instability

Successful management of shear instability follows a structured, evidence-informed path:

Movement Pattern Correction: Patients must learn to move, lift, sit, and bend in ways that minimize shear forces. Movement retraining focuses on maintaining a neutral spine through daily tasks.
Spinal Stability Training: Targeted exercises, such as the McGill Big 3 (modified curl-up, side plank, bird-dog), build the endurance and bracing ability of key stabilizing muscles like the multifidus, erector spinae, and quadratus lumborum.
Load Management: Properly grading and controlling external loads is critical. Patients gradually reintroduce activities while ensuring loads remain within their current “spare capacity” threshold.

A structured, patient-specific approach is essential to break the cycle of recurrent pain and vulnerability.

The Path to a Stronger Spine

If you suspect that shear instability may be driving your persistent back pain, the path forward lies not in endless passive treatments, but in an individualized mechanical assessment and a carefully designed rehabilitation program.

At Perth Injury & Pain CLinic, our team conducts comprehensive mechanical assessments to help identify contributing factors like shear instability. For complex or challenging cases, consultations with one of our McGill Method certified practitioners — the only practitioners in Western Australia holding this advanced certification — are available to guide you through a structured, evidence-based recovery plan.

Book your initial assessment today and start rebuilding the foundation for a pain-free, resilient spine!