With the trunk balanced over the hip, the planted leg's hamstring needs essentially zero postural effort; with the trunk forward, it carries the trunk
Chapter 5 of the Spinal Fitness book reports Dr. John Scherger's walking and running analysis: with good posture and the trunk stacked over the hip, the stance leg's hamstring may require essentially zero effort for postural maintenance. Chapter 6 puts the cost of the alternative in pounds: with the upper trunk mass only 4 inches too far forward, the hamstring works at 843 pounds.
Chapter 5 defines effective mobility as the ability to stand upright and move through space on 2 legs, and it ties that ability to 1 condition: the upper trunk balanced over the hip joints. When the trunk is stacked correctly over the 2 hips, the stance-phase leg can remain straight, the stride can extend fully, and the hamstrings can focus on propulsion instead of postural support.
When the S-shaped spine is lost and the upper trunk shifts forward of the hips, the book states, the hamstrings inherit a second job: they must help keep the trunk from falling forward. That muscle effort does not create stride; it is spent on compensation. Chapter 5 lists 4 results: the runner loses stride, the worker loses endurance, the athlete loses explosiveness, and the aging adult loses confidence in balance. Chapter 5 contrasts apes, which can briefly walk upright but often do so with bent knees and constant hamstring muscle effort.
Chapter 6 extends Dr. Scherger's mathematical proofs from the spine into the hip and knee. The 2 cases differ by 1 variable, the position of the upper body trunk mass:
| Condition | Hip socket shear | Compression | Hamstring force |
|---|---|---|---|
| Ideal S-shaped spine, hamstrings relaxed | About 50 lb | About 87 lb, extending into the knee | Relaxed |
| Upper trunk mass 4 inches too far forward | 146 lb | 924 lb | 843 lb |
On those figures, a 4-inch shift of trunk mass raises hip shear about 3-fold and compression more than 10-fold. The book calls this the mechanical cost of posture collapse, playing out in every step. Chapter 6 states that the force effects of poor posture do not stop at the spine; they extend down the chain through the hip into the knee.
Chapter 5 states that chronic hamstring tightness is often misunderstood: the hamstrings are not always short because they need stretching, and they may be overloaded because the spine has lost the structure that should be keeping the trunk balanced. Chapter 2 makes the same point for workers who arrive on the job already in compensation: their hamstrings may be chronically tight not because of simple inflexibility, but because the body is using them to keep the trunk from falling forward.
The book gives 3 more angles on the same reading. Chapter 2 states that a person's hamstrings may feel tight even when stretching does not solve the problem, because the muscle may be responding to a structural disadvantage. Chapter 3 lists "Stretch what feels tight" among the 5 common back-care instructions and states that it does not explain why the tissue tightened in the first place. Chapter 3 also states that a tucked pelvis changes hamstring tension and reduces the ability of the hips to extend fully, and Chapter 6 lists a flattened lumbar curve as the posture change that makes the hamstrings and posterior chain tonic.
Chapter 1 lists 5 reflex-level compensations a worker adopts when the spine feels unstable: increased hip flexor grip, increased lumbar extension, increased shoulder elevation, increased head-forward posture and increased hamstring tension. Chapter 1 states that these 5 are not choices. Chapter 3 places the hamstrings in the same group as the spinal extensors when the trunk mass shifts forward of the hips: both must work to keep the body from tipping farther forward.
The book's Trainer Application in Chapter 2 reads constant hamstring gripping as a sign of increased cost per movement, alongside forward head posture, rib flare and quadratus lumborum dominance. Its coaching message is that better alignment is energy management rather than a cosmetic change.
The book's answer is structural. Chapter 3 states that the solution is not simply to stretch what feels tight, but to restore the curves and teach the body to hold them under real-life doses of gravity. Chapter 9 makes the same argument for the posterior chain as a whole: the goal is to restore structure so the posterior muscles can return to their role as postural stabilizers, and the Sit-Up, the Pelvic Tilt and Neck Flexion restore the curves that let those muscles rest.
All 5 steps of the system work from the anterior direction. Step 1 is Back Trac rolling to restore segmental motion and reduce guarding; Step 2 is the Pelvic Tilt over a fulcrum for lower-back stability and corrective alignment; Step 3 is the Sit-Up over the PowerCushion to restore the lumbar curve during trunk motion against gravity; Step 4 is Neck Flexion; and Step 5 is 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 supports the lumbar curve for the Pelvic Tilt and the Sit-Up, and the Back Trac roller is Step 1.
Practitioner Eileen Durfee, NBS (Nutritional Balancing Science), NASM-CPT - wellness educator, inventor, author, and a Nationally Certified Spinal Fitness Specialist.
Regular exercises that induce curvature by strengthening the postural muscles are critical. Those are the exercises in my book, Steps 2, 3 and 4, each loading the spine from the front over a fulcrum or against gentle resistance.
Our Facebook group is free to join for questions about any of the 5 steps in the Spinal Fitness system.
The book states that the hamstrings may be overloaded because the spine has lost the structure that keeps the trunk balanced over the hips, so they work to keep the trunk from falling forward.
In Dr. Scherger's figures in Chapter 6, upper trunk mass 4 inches too far forward has the hamstring working at 843 pounds.
Hip socket shear rises from about 50 to 146 pounds and compression from about 87 to 924 pounds in the same comparison.
No. It states that stretching alone does not explain why the tissue tightened, and its solution is to restore the curves.
The book's term for the ability to stand upright and move efficiently on 2 legs, which depends on balancing the upper trunk over the hip joints.
Published October 23, 2026 - Updated October 23, 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
