A spinal joint is stable only when rotation, sliding and shear are all controlled, and the book sets the order in which the body solves them
Chapter 3 of Eileen Durfee's Spinal Fitness book states that a spinal lever system must satisfy 3 forms of equilibrium: equilibrium of torque, equilibrium of translation and equilibrium of shear. The book fixes the order of the 3 as torque first, translation second and shear third, and adapts the mechanics from Dr. John Scherger's Spinal Fitness Series, Volume 2c, December 2002.
Chapter 3 states its model in 1 sentence: "The spine is a lever system under gravity." In the book's model, bones act as levers, joints act as fulcrums, muscles create effort, and body weight and external loads create resistance. The shape of the spine determines how those 4 elements meet at each joint, which is why the book reads posture as a mechanical arrangement rather than an appearance.
The book then names 2 force outcomes at a joint. When the spine is aligned well, more force travels through the structure as compression, meaning the joint surfaces are pressed together in a more stable direction. When the spine is misaligned, more force becomes shear, meaning one joint surface is pushed or pulled across another. The stabilization for that shear may come from joint surfaces, ligaments, discs or muscles, and when the passive structures cannot control it, the muscles work harder.
Torque, in the book's definition, is turning force around a joint. When the head, the trunk or a carried load moves farther from a joint, torque increases, and the muscles must work harder to keep the body from rotating or collapsing. Translation is sliding force at a joint: instead of rotating around the joint, 1 bone tends to slide across another. Chapter 3 identifies translation as the mechanical problem behind shear at a spinal joint.
For a spinal joint to be stable, Chapter 3 states, the body must control both rotation and sliding. A worker lifting a box, pushing a cart or sitting at a desk performs 1 visible task, while at the joint level the body balances 5 quantities at once: torque, translation, compression, shear and muscle effort.
| Principle | The question the book assigns it | What it governs |
|---|---|---|
| 1. Equilibrium of torque | How much muscular effort is required to stop unwanted rotation around the joint? | The counterforce muscles supply when body weight or a load tends to rotate a vertebra |
| 2. Equilibrium of translation | Where does the resultant force go at the fulcrum, and what structure stops the joint from sliding? | The direction of the combined force at the joint |
| 3. Equilibrium of shear | If shear is present, what stabilizes it? | The bone, ligament, disc or muscle that must reduce shear to zero |
Equilibrium of torque, principle 1, is the force problem most people understand first, the book notes, because it feels like holding yourself up. For equilibrium of translation, principle 2, Chapter 3 states that a joint can satisfy rotational balance and still experience translation, so the body may stop a bone from rotating while still allowing it to slide. That sliding at the joint is what leads directly into shear.
Equilibrium of shear is principle 3. Chapter 3 states that shear does not stabilize the way compression does; shear attempts to dislocate or slide the lever at the fulcrum, and the body must reduce it to zero through some stabilizing mechanism, which may be bone, ligament, disc or muscle. Its 3-part sequence for the body is to manage the muscle effort that controls rotation, then identify where the resultant force goes, and then, if shear remains, stabilize it.
Chapter 3 lists 6 components that share the work of meeting the 3 principles. Each has a different role, and only some of them can stop shear without spending energy.
| Part | Role the book assigns |
|---|---|
| Muscle | Actively creates effort and stabilization |
| Bone | Can act as a fixed stabilizing structure |
| Superior and inferior facets | Can act as bony stops when they are aligned to receive force correctly |
| Ligaments | Passively resist excessive motion, and can strain or tear when overloaded |
| Discs | Absorb load, permit motion and help maintain spacing between vertebrae |
| Joints | The places where the forces meet and must be stabilized |
When the S-shaped spine is present and the forces are favorable, the book states, the superior and inferior facets can stop anterior shear through bone-on-bone contact. When the curves are not deep enough or the posture is poor, the direction of shear can change, the spine can lose that bone-on-bone contact, and the demand shifts onto ligaments, discs and muscles. Chapter 3 calls that outcome a mechanical failure of force direction not just a posture problem.
Chapter 4 applies all 3 principles to 1 joint in the lower neck, the junction of the 6th and 7th cervical vertebrae. 3 forces act there at once: skull load, the force of the posterior cervical stabilizers including the interspinalis muscles, and the orientation of the facet joints.
Torque at the C6-C7 joint: the weight of the head tends to rotate the 6th vertebra forward on the 7th, and the interspinalis and related posterior stabilizers supply the counterforce. When the head moves forward, the resistance arm lengthens and the muscle effort must increase. Chapter 4 sums up the neck case in 2 sentences: "The head did not become heavier. The leverage changed."
Translation at C6-C7: with cervical lordosis intact and the head balanced over the center of gravity, the resultant force at C6-C7 is directed predominantly into joint compression. When the curve is lost, the head shifts forward of the vertical axis, the resultant force develops an anterior component, and C6 tends to slide forward on C7. Chapter 4 states that this is a mechanical consequence of misalignment, and that the same muscle forces that were adequate with the curve may become inadequate without it.
Shear at C6-C7: in a lordotic neck, when C6 attempts to translate anteriorly, its inferior facets are contacted and blocked by the superior facets of C7, so the bony architecture satisfies the equilibrium of shear passively. When the curve is lost, the facets are no longer positioned to intercept the anterior resultant force, the disc may become a primary structure absorbing the translational load, ligaments are progressively strained, and muscles co-contract. Compression on the joint increases while shear may still not be fully resolved.
The book's conclusion from the 3 principles is a cost rule: when shear requires active muscle stabilization, the cost of movement rises sharply. Poor structure often creates shear that cannot be controlled passively by good joint alignment, so the body must recruit more muscles and create more compression to stabilize the same movement.
That rule is the reason Chapter 3 ends with "Restore the curve. Shorten the resistance arm. Reduce shear." and names 3 elements of the Spinal Fitness approach: support, meaning fulcrum positioning and proper setup to guide curvature and protect joints during training; controlled movement that teaches the spine to organize force and reduce damaging shear; and appropriate resistance and progression, enough load to create adaptation without overwhelming the structure. In Chapter 9, the 5 steps carry that out in order, from Back Trac rolling through the Pelvic Tilt, the Sit-Up over the PowerCushion, Neck Flexion and the Spinal Twist with supported rest on the Neck and Back Shaper rolls.
Pain, numbness, tingling and dizziness are the stop signs in the Spinal Fitness book. When one appears, stop the exercise, get evaluated, and then proceed with a therapeutic exercise program designed by a Spinal Fitness expert. The book's training rule for every step is that strain is acceptable and pain is not.
The PowerCushion is the fulcrum for the Pelvic Tilt and the Sit-Up, and the Back Trac roller is Step 1 of the 5-step system.
Practitioner Eileen Durfee, NBS (Nutritional Balancing Science), NASM-CPT - wellness educator, inventor, author, and a Nationally Certified Spinal Fitness Specialist.
The Editor's Note in my book names Dr. John Scherger as my chiropractor and my mentor, and reads: "The concepts of the fulcrum, the sit-up, the pelvic tilt, neck flexion, spinal curvature, and the mathematical case for the S-shaped spine all trace back to his work and teaching." The torque, translation and shear sequence in Chapter 3 is adapted from his Volume 2c, December 2002.
Our Facebook group is free to join for questions about any of the 5 steps in the Spinal Fitness system.
Equilibrium of torque, equilibrium of translation and equilibrium of shear. Chapter 3 of the book states that a spinal lever system must satisfy all 3 to function.
Torque is turning force around a joint; translation is sliding force at a joint. The book identifies translation as the mechanical problem behind shear.
Yes. The book states that a joint can satisfy rotational balance and still experience translation, which leads directly into shear.
Bone, ligament, disc or muscle. With proper curves, the superior and inferior facets can stop anterior shear through bone-on-bone contact.
The book states that when shear requires active muscle stabilization, the cost of the movement rises sharply, because the body recruits more muscles and creates more compression.
Published October 21, 2026 - Updated October 21, 2026
By Eileen Durfee, Nationally Certified Spinal Fitness Specialist, NASM-CPT, and author of the Spinal Fitness book.
More from Eileen Durfee
Eileen's free guide, Is Your Posture Aging You?, on EileenDurfee.com
