A vertical jump needs trunk mass over the hips at takeoff and fast-twitch fibers not already recruited for postural maintenance
Eileen Durfee's Spinal Fitness book names 2 attributes of the great jumper: greatest vertical velocity, which requires structural mass aligned with the axis of gravity at takeoff, and greatest takeoff velocity, which depends on fast-twitch muscle fiber recruitment. Upper-trunk mass displaced forward of the hip joints works against both, and the book's screen measures it with a single vertical jump.
Chapter 5 of the book states that jumping ability is often attributed to 3 things, leg strength, body weight or raw talent, and that mechanically it depends on 2 attributes directly influenced by spinal posture. Attribute 1 is greatest vertical velocity: when the person leaves the ground, the structural mass is aligned with the axis of gravity. Under attribute 1, any structural mass displaced forward of the gravity axis moves the jump horizontally rather than vertically, so the jumper goes forward instead of up.
Attribute 2 is greatest takeoff velocity, a function of fast-twitch fiber recruitment: the more muscle fibers that fire quickly and together, the greater the force generated at takeoff. The book's condition for attribute 2 is that fast-twitch fibers are available only if they have not already been recruited for posture maintenance. Its illustration, shown with this article, contrasts 2 jumps: the good jump, with balanced trunk mass, vertical force and reduced knee demand, and the poor jump, with forward trunk mass, forward drift and increased knee demand. The Creatrix guide on how to jump higher covers the alignment side of jump training.
With poor spinal posture, the upper trunk mass sits forward of the hip joints. On a vertical jump, 2 muscle groups then work against each other: the quadriceps drive the body upward while the hamstrings work to keep the trunk from collapsing forward. The agonist and the antagonist fight each other, and the jump is weak, uncoordinated and misdirected.
In the air, the nervous system works to maintain balance against gravity, and the book gives 2 consequences. Consequence 1: because the trunk axis is forward of the gravity axis, the nervous system involuntarily drives the feet and lower body upward and forward to compensate. Consequence 2: the person is always slightly falling forward, even when trying to jump straight up. The book states that 3 responses, training harder, jumping more or adding leg strength, cannot correct this, because the cause is structural; its solution is restoring the neutral spinal posture so the center of mass is balanced over the hips at takeoff.
In Dr. John Scherger's walking and running analysis, with the trunk balanced over the hip, the planted leg's hamstring muscle may require essentially zero effort for postural maintenance; with the upper trunk shifted forward, the same hamstring must generate substantial effort to keep the body upright. The book's names for the 2 abilities the S-shaped spine provides are effective mobility, moving efficiently on 2 legs, and effective momentum, moving the trunk forward against resistance while becoming more stable.
Chapter 5 states that fast-twitch fibers are recruited for explosive movements, not for slow, steady posture work. When the spine loses its posture and the center of mass moves forward of the hips, 3 muscle groups, the hamstrings, spinal extensors and hip stabilizers, must work continuously to maintain the upright position, which is slow-twitch work: constant, low-level, endurance-type contraction. The book states that over time this continuous demand signals the body to convert fast-twitch muscle fibers to slow-twitch, so the athlete progressively loses the explosive capacity being trained.
The book's formula for preserving fast-twitch recruitment has 2 components. The external environment is explosive training with adequate rest, not continuous endurance-style loading. The internal environment is proper spinal posture, which places the musculature in what the book calls a minimum energy state, where the posterior chain is not constantly recruited for posture and fast-twitch fibers stay available. Its list of 6 performance costs of poor posture adds faster spinal fatigue under repeated loading, less stable joint surfaces at impact, and more disc compression as muscles substitute for missing bony stabilization.
Section 12 of Appendix C defines momentum as mass × velocity and adds 1 requirement: the athlete must deliver the accelerated mass through a stable structure. Chapter 5's football contact model puts numbers on the delivery:
| Contact model | Total force effort | Total resultant force | Anatomy used |
|---|---|---|---|
| Poor posture | About 349 lb | About 479 lb | 4 muscles and 3 joints |
| Good posture | About 191 lb | About 223 lb | 2 muscles and 1 joint |
The force paths the book contrasts are ground, legs, pelvis, spine and point of contact for the good-posture player, and ground, legs, collapsed spine, shear, muscle stabilization and reduced contact force for the poor-posture player. In the book's words, more parts means more places to fail; the poor-posture model loads L3-L4, L4-L5 and L5-S1. The book gives 4 athletic cases: a thrower whose spine collapses leaks rotational force into compensation; a striker whose spine cannot transmit force becomes arm-dominant; a wrestler, lineman or martial artist whose spine becomes the bending point loses structure; and a runner whose posture collapses spends hamstring and spinal stabilizer energy holding the trunk up.
Screen 6 of the book's Spinal Curvature and Capacity Screen, in Chapter 17, asks the person to jump straight up and land, and scores it 3 ways:
The book states that when trunk mass is chronically forward of the hips, the jump cannot be truly vertical, which reduces jump height in sport and creates severe compressive and shear forces in the knee joint on landing; the knee figures are in the book's leg extension and leg curl analysis.
The book's 6 trainer questions for effective momentum: does the person hinge from the hips or collapse through the spine; does the spine stay a structural column or become the bending point; does force travel through the pelvis and legs or leak into the low back; does the person brace before moving; do the hamstrings, neck or low back act as emergency stabilizers; and does the person gain stability moving into force, or lose it.
Chapter 11 sets these stop signs as a universal rule across its 3 standards, Desk, Driver and Warehouse: new numbness or tingling, radiating symptoms, weakness, bowel or bladder changes, or sharp escalating pain. When any of them appears, the instruction is to pause training and escalate to medical evaluation.
The book's solution is restoring the neutral spinal posture. Its first 3 steps are Back Trac rolling, the Pelvic Tilt over a fulcrum, and the Sit-Up over the PowerCushion.
Practitioner Eileen Durfee, NBS (Nutritional Balancing Science), NASM-CPT — wellness educator, inventor, author, and a Nationally Certified Spinal Fitness Specialist.
At an age when many people expect decline, I feel stronger, healthier and more vibrant than I did in my twenties, and I have worked with people in 91 countries through my products, education and one-on-one guidance. The athletic mechanics in Chapter 5 of my book are based on Dr. John Scherger's unpublished manuscript, Secrets of Human Movement.
Our Facebook group is free to join for questions about any of the 5 steps in the Spinal Fitness system.
Greatest vertical velocity, with structural mass aligned with gravity at takeoff, and greatest takeoff velocity, which depends on fast-twitch fiber recruitment.
With trunk mass forward of the hips, the nervous system drives the feet and lower body upward and forward in the air to keep balance, so the jump travels horizontally.
The book states it cannot, because the cause is structural; the correction is a neutral spinal posture with the center of mass over the hips.
In the book, continuous low-level postural work by the hamstrings, spinal extensors and hip stabilizers when the trunk sits forward of the hips.
Screen 6: jump straight up and land. Green lands on the takeoff spot; Red lands noticeably forward of it.
Published October 18, 2026 • Updated October 18, 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.
