The sacrum hangs free, L4 is held, and the pelvis rotates: how one setup moves L5 backward on L4
Step 2 of Eileen Durfee's Spinal Fitness system is the Pelvic Tilt over the PowerCushion. The book's analysis, attributed to Dr. John Scherger, follows the force from about 70 pounds on the front of the pelvis to a downward shear of 149 pounds at S-L5, which the book's figure labels a posterior shear of L5 on L4, arrested by the facet joints.
The book divides spinal training into 2 levels. Global training addresses the larger spinal structure, particularly the lumbar lordotic curve and the overall S-shaped posture, and Back Trac rolling is its primary exercise. Segmental training addresses the joint-by-joint relationships between individual vertebrae, which the book calls especially important in the lower lumbar spine, and the pelvic tilt is the primary segmental exercise, targeting the L4-S1 relationship specifically. The correction loop runs from global decompression in Phase 1 to load correction in Phase 2, where the pelvic tilt begins to change the way force travels through the lower spine and pelvis.
The direction of load is part of the exercise design. All 5 steps work from the anterior direction, because the book states that the spine has excellent mechanical advantage against anterior forces and poor advantage against posterior ones; its adaptation principle is that the body adapts to the effects it is consistently subjected to. Dr. Scherger's analysis of the opposite direction puts compression at the L5-S1 joint above 1,200 pounds, and total effort across the lumbar joints above 1,700 pounds, when a 100-pound upper body is stabilized against a posterior force, in proper and improper posture alike. The PowerCushion and the other equipment for the 5 steps are in the Creatrix spine alignment stretches and exercises collection.
The book states in Chapter 13 that the PowerCushion is not simply a pad. When the pelvis is placed on it, the hips are cradled in the cutout and the lower lumbar spinous processes are supported in the spinal groove, which gives the pelvis and lower back a fixed surface to work against. The sacrum and hips hang free in the cutout while the lumbar spine from L4 upward rests on the supported surface, so the lumbar vertebrae from L4 up are held in a fixed reference position and the sacrum can move relative to them. In the book's words, that freedom is what makes the exercise work.
The book contrasts this with the floor: on a flat surface, many people perform a pelvic tilt by flattening the lumbar spine and bracing globally. Without the fixed support surface, the exerciser may substitute gluteal contraction or general trunk flexion, which the book states reduces the targeted shear effect; with the pelvis stabilized, abdominal effort is directed more specifically into the lower lumbar segments, training segmental alignment rather than simply global abdominal strength.
Chapter 13 also carries a hip-joint figure that shows why pelvic orientation matters below the spine: in anterior pelvic tilt the hip carries 146 lbs of shear and 924 lbs of compression and needs 843 lbs of hamstring effort to keep from collapsing, while neutral alignment reduces hip shear to 50 lbs and compression to 87 lbs with zero hamstring bracing.
In Dr. Scherger's mathematical analysis, 2 forces act on the pelvis and sacrum from opposite directions. The first is the anterior resistance: approximately 70 pounds applied vertically downward through the anterior superior iliac spines (ASIS), the bony prominences at the front of the pelvis. The second is the rectus abdominis, which originates on the pubic symphysis and pulls the pubic bone upward and anteriorly toward the xiphoid process at roughly 10 degrees above horizontal. In the pelvic tilt position the trunk is anchored, so the xiphoid is the fixed end and the pubic bone is the moving end, and the contraction of this muscle lifts the front of the pelvis and draws it toward the trunk.
The cushion adds a 3rd contact: the book states that the fixed cushion applies an anteriorly directed force into the iliac bones of the pelvis, opposing the pull of the rectus muscle. With the lumbar vertebrae supported by the firm fulcrum, the analysis gives a resultant force of approximately 201 pounds acting through the lower lumbar spine, directed so that the shear it creates is stabilized by fixed bony facet contact.
The book resolves the apparent conflict between the downward load and the upward muscle pull through lever geometry. The fulcrum is the lumbar spine supported on the cushion and the sacrum hangs below that support point, so the pelvis functions as a lever with the lumbar spine as its fulcrum. When the rectus muscle pulls the pubic bone up and the load pushes the ASIS down, the pelvis rotates around the lumbar fulcrum: the front tilts up, and the sacrum is driven downward and posteriorly.
The sacrum is attached to L5 through the sacroiliac joint and the disc at S-L5. As the rotating pelvis drives the sacrum down, it pulls the body of L5 with it, and the pull of L5 relative to the fixed L4 above creates shear at the L5-L4 joint; the same relationship propagates upward, L4 relative to L3 and L3 relative to L2. Dr. Scherger's calculation puts this shear at 149 pounds at S-L5, declining progressively through each joint to 7 pounds at L1-T12. The book's illustration of the exercise labels the result a posterior shear of L5 on L4, and its Chapter 13 caption describes a controlled posterior shear of L5 relative to L4 that guides the lower lumbar segments toward alignment.
At each lumbar joint, the book states, the superior facet of the lower vertebra projects upward. When the lower vertebra tends to displace, its superior facet is driven into the inferior facet of the vertebra above, and bone meets bone. The facet joints arrest the movement passively, without additional muscular effort, and the sequence begins at the sacrum, where the sacral facets engage the inferior facets of L5, then propagates upward through each lumbar level.
The book frames the 2 possible outcomes of any effort-resistance pair. In a stabilized system, the abdominal complex provides an opposing force that redirects shear into compression and the joints approximate; in an unstable system, shear continues in the opposite direction and pulls the joint surfaces apart. The Chapter 13 summary describes the Step 2 pelvic tilt as a lower-back strength and alignment drill, not a general core exercise.
The book's setup for the exercise has 5 points:
The primary mover in the exercise is the rectus abdominis. The support system the book lists is the transversus abdominis for circumferential tension, the internal and external obliques against rotation and lateral drift, the quadratus lumborum and deep segmental stabilizers to keep the pelvis from drifting, and the pelvic floor and diaphragm to regulate pressure when breathing is coordinated. The book's caution is that hip flexor muscles should be minimized: 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, and the instruction is to scale back resistance and reset form. Bending the knees, it states, shifts work to the hip flexor muscles and defeats the purpose.
The book's one rep count for the pelvic tilt is in the Warehouse Standard: 10 reps over the fulcrum in the 5 to 8 minute pre-shift routine, alongside 2 minutes of Back Trac rolling, 6 to 10 Neck Flexion reps and 6 to 10 Sit-Up reps at light density. Track B, for high-exposure roles, includes the Pelvic Tilt with resistance inside 10 to 20 minutes of daily practice. The 70 pounds is the load in the mathematical analysis; the execution instruction is a steady vertical load over the ASIS, and the book gives no starting load in pounds and no hold time. For density, the book's Step 3 setup says to start with the softer cushion for stiff, painful, injured or highly guarded spines, and the Step 2 chapter gives no separate density rule.
The book's screen sets readiness. With high muscle tone, swelling or severe guarding, it holds off the Pelvic Tilt until irritability has reduced to moderate or lower and re-screens after 2 to 3 sessions; with mostly Green results, good segmental motion and low muscle tone, it may begin directly with Step 2. Its 3 universal rules apply to every rep: strain is acceptable and pain is not, every rep is controlled, and neutral alignment comes before effort.
The book gives this list in its Back Trac chapter: stop immediately and seek medical evaluation for progressive weakness, changes in bowel or bladder control, saddle numbness or severe radiating symptoms. Do not begin without clinician clearance with a known spinal fracture, advanced osteoporosis, unstable spondylolisthesis, active infection or recent surgery.
The Power Cushion is listed as coming soon at Creatrix in soft (blue) and medium (orange) densities, with a neck support attachment that fits either density.
Practitioner Eileen Durfee, NBS (Nutritional Balancing Science), NASM-CPT — wellness educator, inventor, author, and a Nationally Certified Spinal Fitness Specialist.
Dr. John S. Scherger (1950-2016) was my chiropractor, my mentor, and the scientific foundation behind the principles presented in my book. 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, and the force figures in this article are from his mathematical analysis.
The Chapter 13 photo caption: "Eileen Durfee demonstrating the Pelvic Tilt over the PowerCushion with spinal model - the posterior support geometry creates the fulcrum that directs corrective force to the lumbar curve." The chapter's clinical note: "stability is built, not forced."
Our Facebook group is free to join for questions about any of the 5 steps in the Spinal Fitness system.
The cushion holds the lumbar spine from L4 upward in a fixed position. A free sacrum can move relative to those vertebrae, and the book states that this freedom is what makes the exercise work.
Vertically over the anterior superior iliac spines at the front of the pelvis, applied steadily and never pushed or released quickly.
The book states that bending the knees shifts work to the hip flexors and defeats the purpose of the exercise.
The facet joints. The superior facet of each lower vertebra meets the inferior facet of the vertebra above, so the motion is arrested passively by bone.
10 reps over the fulcrum in the Warehouse Standard pre-shift routine. The book gives no hold time and no starting load in pounds.
Published October 12, 2026 • Updated October 12, 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.
