Muscle effort and resistance combine into 1 force, and the spine's position sets whether that force compresses or shears
Chapter 13 of Eileen Durfee's Spinal Fitness book states that when the abdominal muscles contract against resistance, 2 force vectors act at once, the muscle effort and the resistance, and the direction they combine into is the resultant force. The resultant, not the muscle force alone, determines what happens at the lumbar joints.
Chapter 13 credits this section to Dr. John Scherger's unpublished manuscript, Understanding the Abdominal Musculoskeletal Complex. When the lumbar spine keeps its natural curve, the resultant force travels into the spinal joints in a stabilizing direction. The lumbar vertebrae approximate, which the book defines as joint surfaces pressed together rather than pulled apart. The facet joints support one another, the disc is loaded in compression, and the spine behaves as a unified structure capable of transferring force.
When the lumbar curve is flattened or reversed, the same muscular effort produces a different resultant. The combined force shears across the joints instead of traveling into them, the facets are forced apart, and the disc absorbs load it was not designed to carry. The body may respond with co-contraction, opposing muscle groups tightening at the same time, which the book states increases compression and fatigue without restoring stability. Over time, the chapter says, that movement is more likely to irritate discs, facets, ligaments and surrounding soft tissue.
| Lumbar position | Direction of the resultant | Facets | Disc | Muscle response |
|---|---|---|---|---|
| Natural curve | Into the joints | Support one another | Loaded in compression | Effort becomes usable movement strength |
| Flattened or reversed | Across the joints | Forced apart | Absorbs load it was not designed for | Co-contraction: more compression and fatigue |
The book states that whenever effort and resistance interact at a spinal joint, a shear component is always created, and the difference between a strong system and a failing one is whether that shear is controlled. In a stabilized system the abdominal complex provides an opposing force that redirects shear into compression, so the joints approximate. In an unstable system no such control exists and the shear continues in the opposite direction, pulling the joint surfaces apart. The Chapter 13 figure Force Interaction in the Abdominal Musculoskeletal Complex draws both: in the stable, curved spine (A) shear is directed down and back and locks the facet joints together; in the unstable, straight spine (B) shear changes direction and forces the facet joints apart.
Chapter 13 states that strengthening alone is insufficient: true stability depends on the ability to control shear, not simply to generate force.
The Chapter 13 hip joint force comparison: with the pelvis in anterior tilt, the hip carries 146 lb of shear and 924 lb of compression and needs 843 lb of hamstring effort to keep from collapsing. In neutral alignment the shear drops to 50 lb, the compression to 87 lb, and the hamstrings do zero bracing.
Chapter 13 defines effective momentum as the body moving forward or performing effort while the spine is positioned to receive resistance without collapsing. It lists 6 actions where it applies: a sit-up, a lift, a push, a pull, a throw and bracing during contact. In each, the abdominal muscles generate force while the upper body or an external object supplies resistance, and when the lumbar spine is in a favorable position the 2 combine into a resultant that travels through the lumbar joints in a stabilizing direction. Strength, in Chapter 13, comes from the relationship between muscle contraction, spinal alignment, joint position and the direction of resistance, not from contraction alone.
In a strong position, the book lists 6 jobs for the abdominal complex: restore the relationship between the pelvis and the lumbar spine, direct force through the lower back instead of across it, support facet approximation, reduce uncontrolled shear, improve trunk stiffness without excessive bracing, and convert muscular effort into usable movement strength.
The training goal in Chapter 13 is to generate force in a position where the resultant stabilizes the lumbar segments rather than shearing through them. That is the reason the PowerCushion pelvic tilt in Step 2 is designed around position, not just contraction: the cushion gives the pelvis a fixed surface to work against, so abdominal effort produces a resultant that guides the lower lumbar vertebrae toward alignment rather than flattening the back. The book's 2 sentences on it: "Strength follows from correct direction. Direction requires correct position."
Chapter 13 works the principle through Step 2 in 5 stages. 1: the PowerCushion holds the lumbar spine from L4 upward in a fixed position while the sacrum and hips hang free in the cutout. 2: approximately 70 lb of resistance is applied vertically downward through the anterior superior iliac spines (ASIS), while the rectus abdominis pulls the pubic bone upward toward the xiphoid at roughly 10 degrees above horizontal. 3: with the lumbar spine as the fulcrum, the pelvis rotates; the front tilts up and the sacrum is driven downward and posteriorly. 4: the sacrum pulls L5 down relative to L4, and the same relationship propagates upward; Dr. Scherger's calculation puts the shear at 149 lb at S-L5, declining to 7 lb at L1-T12. 5: at each level the superior facet of the lower vertebra is driven into the inferior facet of the vertebra above, so bone stops the movement with no added muscular effort, beginning at the sacrum.
The muscle list for the pelvic tilt names the rectus abdominis as the primary mover, supported by the transversus abdominis creating circumferential tension, the internal and external obliques resisting rotation and lateral drift, the quadratus lumborum and deep segmental stabilizers keeping the pelvis from drifting, and the pelvic floor and diaphragm regulating pressure when breathing is coordinated. The supine version of the same principle is the Sit-Up: in Chapter 14, a 100 lb upper trunk in proper posture takes 170 lb of effort, a 196 lb resultant at each joint, 1,176 lb of total compression across 6 joints and zero shear. In improper posture the multifidus must be recruited at every joint: 388 lb of effort and 1,639 lb of compression in the best model, 741 lb and 2,501 lb in the worst. The analysis is worked through in the Sit-Up over the PowerCushion article, and the role of the lumbar curve itself is covered in the lower back curve article.
Under the Spinal Fitness book's rule, muscular strain is acceptable and pain is not. Pain, numbness, tingling and dizziness are stop signs in the Spinal Fitness book: stop, get evaluated, and proceed with a therapeutic exercise program designed by a Spinal Fitness expert.
For the pelvic tilt, Chapter 13 adds a form check: if the pelvis is being yanked with the legs, or the ribs flare and control is lost, the iliopsoas and rectus femoris have taken over; scale back resistance and reset form.
The PowerCushion is the fixed surface the book uses for Steps 2 and 3, the pelvic tilt and the sit-up; the Creatrix PowerCushion page reads "Coming soon from Creatrix Solutions".
Practitioner Eileen Durfee, NBS (Nutritional Balancing Science), NASM-CPT — inventor, author, and a Nationally Certified Spinal Fitness Specialist.
Dr. John S. Scherger was my chiropractor, my mentor, and the scientific foundation behind the spinal principles in my book. I met him at 17, after I was run over by a car in a store parking lot, with trauma to my neck and upper trunk. I received adjustments and laid over specially rolled towels to support global curvature and to release tight guarding muscles.
The direction that muscle effort and resistance combine into when they act at the same time. The book states it is the resultant, not the muscle force alone, that determines what happens at the lumbar joints.
Approximately 70 lb applied vertically downward through the anterior superior iliac spines.
At the sacrum. Chapter 13 states the sacrum is the first structure set in motion by the rotating pelvis and the first to have its downward displacement stopped by the vertebra above it.
In Chapter 14, 388 to 741 lb of effort against 170 lb in proper posture, with the multifidus recruited at every joint.
The cushion gives the pelvis a fixed surface, which the book states lets abdominal effort produce a resultant that guides the lower lumbar segments toward alignment.
Published November 1, 2026 • Updated November 1, 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
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