Geometry over anatomy: how the adult spine's curves direct force into compression instead of shear
Eileen Durfee's Spinal Fitness book states that one of the most precise mathematical models of the adult S-shaped spine describes a series of 30-60-90 triangles. Its argument is that the curves are a load-bearing arrangement, and it backs that with Dr. John Scherger's lever figures: 77 lb of effort in proper structure against 1,166 lb in improper structure.
In Chapter 2 the book ranks the 30-60-90 model among the most precise mathematical descriptions of the adult S-shaped spine: a series of 30-60-90 triangles, a geometry that appears repeatedly in structures designed to carry load, distribute force and create strength with efficiency. The book's figure for this section, titled The 30-60-90 Triangle: Nature's Strongest Structure, draws 3 triangles of 30, 60 and 90 degrees against the spine, one at the cervical lordosis, one at the thoracic kyphosis and one at the lumbar lordosis.
The book names 3 engineered examples of the same logic: a Roman arch can hold weight for thousands of years because its shape directs force through the structure, hydroelectric dams use angled geometry to resist water pressure, and the annular fibers of the intervertebral disc are arranged in angled layers that help the disc resist force from multiple directions. The figure labels the disc layers as outer annulus fibers at plus or minus 30 degrees and inner annulus fibers at plus or minus 60 degrees, alternating, to resist compression, tension and twisting. The book does not state which vertebrae form the vertices of each triangle or give a measured curve angle for any region.
The book's section on geometry starts from the parts list: most people think of the spine as anatomy first, meaning bones, discs, nerves, muscles and joints, and the book calls that true but incomplete. A vertebra functions because of where it is positioned, how it is angled, how it meets the disc and facets above and below it, and how force travels through it; a disc functions because its fibers are arranged to resist compression, rotation, tension and shear. A straight spine, a rounded spine, a flattened lumbar curve and a proper S-shaped spine do not distribute force the same way, even with the same parts.
From that the book draws its question for any spinal complaint: what shape is that tissue being forced to work in? Its stated principle for the whole system is that shape determines strength, and that when the spine develops its S-shaped curves, the curves organize the vertebrae, discs, facets, muscles and ligaments into a structure that manages gravity and movement more efficiently than a straight or collapsed spine.
The book states that spinal curves create strength because they change how force is distributed, and that a flat beam tends to concentrate stress at its weakest point while an arch distributes force through its curve. The cervical, thoracic and lumbar curves distribute force through the spine instead of concentrating it into one vulnerable segment, and they let the spine manage both compression and motion: a straight column can bear vertical load, but the human spine must also sit, lift, push, walk, twist, brace and recover balance.
The book's description of a curved spine is strong and mobile at the same time: it does not gain strength by becoming rigid, it gains strength by organizing force. When the curves are lost, muscles do more stabilizing work, joints are exposed to more shear, the resistance arm may increase, and more energy is spent holding the body together before movement begins. In the book's words: "Curves reduce the cost of force." The equipment the book uses to build and keep the curves is in the Creatrix spine alignment stretches and exercises collection.
The book's rule on load, in Chapter 3, is that organized compression supports structure and uncontrolled shear drains structure. When the spine is aligned well, more force travels through the joints as compression, which presses the joint surfaces together in a more stable direction; when it is misaligned, more force becomes shear, which pushes or pulls one joint surface across another and must be stabilized by joint surfaces, ligaments, discs or muscles. Walking, standing and lifting all create compression in the joints, and the book's question is whether the force is organized.
The book sets the order in which a spinal joint must meet 3 equilibria: torque first, translation second, shear third. When the S-shaped spine is present and the forces are favorable, the superior and inferior facets can stop anterior shear through bone-on-bone contact; when the curves are not deep enough, the direction of shear can change, the stabilizing contact is lost, and demand shifts onto ligaments, discs and muscles. The book's companion figure for this section, Compression Equals Stability (30-60-90 Curvature), labels the curved alignment as a self-locking structure in which anterior shear locks the facets and compression wedges them together.
The book puts the geometry into Dr. John Scherger's spinal lever analysis. In the broad proper-versus-improper model, proper structure requires 77 lbs of total effort and improper structure requires 1,166 lbs, a 15-to-1 effort advantage for proper structure. The compression comparison also favors proper structure: 1,218 lbs against 2,708 lbs, approximately a 2-to-1 advantage.
The book attaches a rule to these figures under the heading Do Not Mix the Models: the 77 vs 1,166 lb comparison is the broad mechanical-advantage example, kept separate from an anterior-resistance model used for football contact, cart pushing and patient handling. Chapter 5 gives that second model's figures: 282 lbs of total effort for improper posture against 77 lbs for proper, a 3.5-to-1 effort advantage. The book's reading of both models is the same: proper spinal structure lets the body do more with less, while improper structure forces the body to spend more effort just to remain stable.
The 5 steps follow the book's adaptation principle, that the body adapts to the effects it is consistently subjected to, and they all load the spine from the anterior direction. Step 1 is Back Trac rolling, the primary global exercise, which the book doses at 30 to 60 seconds of easy passes or 6 to 10 slow passes once per day, starting on the medium-density roller. Step 2 is the Pelvic Tilt and Step 3 the Sit-Up, both over the PowerCushion; the sit-up's force figures are covered in the Sit-Up over the PowerCushion as a curvature drill. Step 4 is Neck Flexion, 6 reps 6 times per day for a beginner, with the Neck Shaper as Method B.
Step 5 is the Spinal Twist, 25 seated reps, followed by 18 to 20 minutes on the Neck and Back Shaper rolls, no longer. The Neck Shaper rolls are 7 inches long and the Back Shaper rolls 12 inches, in diameters of 2.75 to 3.75 inches with a center groove. The book's clinical insight in Chapter 9 sets their role: lying over a fulcrum passively is a preparation, not an adaptation stimulus, and adaptation requires active function against the appropriate resistance. The universal rule for every exercise is that strain is acceptable and pain is not.
Chapter 16: supports should never create pain, numbness or tingling, dizziness, headache spikes, or a feeling of being jammed. If symptoms increase, reduce diameter or density, reduce load, or remove the roll and return to the recovery protocol. Chapter 9 adds that naturally or surgically fused segments of the spine cannot be reshaped.
The Back Trac comes in medium and hard densities, singly or as a bundle; the Neck and Back Shaper set carries both roll lengths for supported rest.
Practitioner Eileen Durfee, NBS (Nutritional Balancing Science), NASM-CPT — wellness educator, inventor, author, and a Nationally Certified Spinal Fitness Specialist.
From Chapter 3 of my book: "Spinal Fitness is not posture correction. It is force management. Good structure creates more strength with less strain. Poor structure creates more strain with less strength."
From the geometry section of Chapter 2: "Posture is the visible expression of underlying geometry." And from Chapter 3, in 3 words: "Posture is physics."
Our Facebook group is free to join for questions about any of the 5 steps in the Spinal Fitness system.
The book states that one of the most precise mathematical models of the adult S-shaped spine describes a series of 30-60-90 triangles. Its figure draws one triangle at each of the cervical, thoracic and lumbar curves.
No measured curve angle or vertebral landmarks are given for the triangles. The book uses the model to explain how curves distribute force.
The book states that organized compression supports structure while uncontrolled shear drains it, because shear must be stabilized by bone, ligament, disc or muscle.
Total effort in Dr. Scherger's broad proper-versus-improper structure model, a 15-to-1 advantage. The book keeps it separate from its anterior-resistance contact model.
Back Trac rolling, Step 1 and the primary global exercise, at 30 to 60 seconds of easy passes or 6 to 10 slow passes once per day on the medium-density roller.
Published October 11, 2026 • Updated October 11, 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
This article is for educational purposes only. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease. Nuco Enterprise Solutions LLC.
