A force's effect depends on its perpendicular distance from the fulcrum, and that distance changes at every joint angle
Appendix C of Eileen Durfee's Spinal Fitness book states that mechanical advantage equals the effective effort arm divided by the effective resistance arm, where each effective arm is the perpendicular distance from a force's line of action to the fulcrum. The book credits Dr. John Scherger with applying this correction to musculoskeletal biomechanics and dates its first statement to Benedetti in 1599.
Appendix C traces the law of the lever to Archimedes, around 200 BC, summarized as magnitudes in equilibrium at distances reciprocally proportional to their weights: resistance force × resistance arm = effort force × effort arm. The appendix, which opens by noting that levers have been handled as settled science for over 2,000 years, states that the formula is sound and that the error lies in how the arm has been read: as the fixed linear length from the fulcrum to where force is applied.
In the book's account, Benedetti clarified in 1599 that a force's effectiveness depends on its perpendicular distance to the fulcrum, not merely the distance along the lever. Borelli applied mechanics to the human body in the 17th century and showed that the internal forces the muscles and joints generate and stabilize can be far greater than the external load being carried. Chapter 3 adds Leonardo da Vinci, who studied the body as a mechanical system and recognized the S-shaped spinal posture, and places Dr. Scherger as the one who applied torque, translation, shear, compression and effective lever arms to the spine as a living multi-joint system. The PowerCushion and Back Trac, which the appendix calls part of the mechanical system, are in the Creatrix spine alignment and exercise collection.
The appendix defines 2 arms:
The appendix gives 2 consequences of the difference: a force applied at a long fixed distance can produce little torque if its line of action passes close to the fulcrum, and a force at a shorter fixed distance can produce more torque if its perpendicular distance is larger.
| Version | Equilibrium of torque |
|---|---|
| Classical, as commonly taught | Force R × FIXED resistance arm = Force E × FIXED effort arm |
| Corrected, dated by the book to 1599 | Force R × EFFECTIVE resistance arm = Force E × EFFECTIVE effort arm |
Muscle force is the reason the correction matters in the body. In the book's Figure 4, resistance keeps acting in the direction of gravity while effort changes direction, because muscle force follows the line from insertion to origin as the bone moves. Section 4 of the appendix states that as a joint moves, the fixed arm, the attachment and the bone length stay the same while the line of force moves, so the effective arm and the mechanical advantage change continuously. In section 3 of the appendix, Dr. Scherger's statement reads: "What matters is the distance perpendicular to the direction of force back to the fulcrum."
Textbooks commonly state that second-class levers, with the resistance between the fulcrum and the effort, always provide a mechanical advantage greater than 1. The book's Figure 7, shown with this article, starts in that configuration: the effective effort arm is longer than the effective resistance arm. As the lever moves, the muscle's line of action shifts, the effective effort arm shortens, and the effective resistance arm becomes comparatively longer. At that point in Figure 7 the effective effort arm is less than the effective resistance arm, and the effort force required exceeds the resistance force.
The corrected statement in the book: a second-class lever may have mechanical advantage in 1 position and lose it in another. Figure 8 shows the third-class case, in which the effective effort arm stays shorter than the effective resistance arm throughout the movement even though both change length. In section 6 of the appendix, first-class levers remain variable systems, with the reason given as the effective arm relationship at each point in motion. The appendix's takeaway for Figure 7 is that geometry and force direction at a specific moment in movement, not lever class, set the mechanical advantage.
The book states that the knee, the hip and the spinal joints function as wheel-and-axle systems rather than pulleys: the fixed effort arm and the fixed resistance arm share the same rotating axis, and the effective arms change geometry with every degree of motion. Analyzing a movement at 1 position is therefore insufficient, because the same exercise can move from a first-class to a third-class lever relationship partway through its range. The book's leg curl analysis is its worked case. With 100 pounds of resistance, the knee joint is a first-class system at about 5 degrees off horizontal (550 pounds of hamstring force, 37 pounds of posterior shear) and at 45 degrees (450 pounds, 15 pounds posterior), and a third-class system at about 85 degrees from horizontal, where the hamstring needs only 25 pounds while the knee carries 94 pounds of anterior shear toward the ACL. The highest ACL load falls where the effort is lowest.
The book's functional definition of the classes follows the force at the joint. In a first-class system, the true resistance arm and the true effort arm are 180 degrees apart or close to it, the joint receives compression, and any shear is directed toward the line of pull of the effort, where structures such as the facets can intercept it. In a third-class system the 2 arms approach 90 degrees, the force at the joint becomes increasingly shear, and that shear is directed away from the effort, so ligaments, discs, cartilage or active muscle must control it.
In the spinal column, the book states, a single lever system is 2 vertebrae: the superior vertebra is the fixed resistance arm, the inferior vertebra provides the true effort arm, and the joint between them is the fulcrum. Every level is analyzed individually, because a movement that creates a first-class relationship at L2-L3 may create a third-class relationship at L4-L5 from the same starting posture. Proper posture in the Sit-Up over the PowerCushion creates a first-class relationship at each lumbar joint; poor posture creates third-class relationships that require active muscular stabilization at every joint.
Small changes in posture change the effective arms, which is why the book handles spinal position, pelvic tilt, cervical flexion and thoracic alignment as geometry. The spinal muscles attach close to the vertebrae and keep a short effort arm, so the variable a person can change is the resistance arm: the position of the head, trunk, arms and load relative to the spinal fulcrum. The book's 3 cues, keep the load close, hinge at the hips, and restore the curve, are each stated as leverage.
The appendix separates 2 movements: rising from the prone position, face down, and rising from the supine position. In prone rising the shear is directed toward the facets: the inferior facet of the superior vertebra presses against the superior facet of the inferior vertebra, and bone-to-bone contact stabilizes the shear without muscular effort. In supine rising, the multifidus and other posterior stabilizing muscles can produce an opposing shear force if bony stabilization is insufficient. In prone rising no active muscle is positioned to oppose the shear, so with degenerated, injured, inadequately formed or overloaded facets the book states there is no muscular backup.
Chapter 9 names 5 patterns to approach with caution: back extensions, bent-over rows, standing bicep curls, repetitive forward bending under load, and any exercise where the posterior spinal muscles are the primary movers. The book states that the 5-item list is not a prohibition on all posterior-chain activity; deadlifts and Olympic lifts performed under expert coaching have their place. The lever reason: the posterior spine has severely poor mechanical advantage at every position, and stabilizing a 100-pound upper body against a posterior force puts over 1,200 pounds of compression on L5-S1 and over 1,700 pounds of effort across the lumbar joints, with proper and improper posture alike.
Section 11 of Appendix C names 2 fixed structural supports, the PowerCushion and the Back Trac, that let muscular effort be directed into alignment rather than lost to shear.
Practitioner Eileen Durfee, NBS (Nutritional Balancing Science), NASM-CPT — wellness educator, inventor, author, and a Nationally Certified Spinal Fitness Specialist.
Appendix C of my book is a modern, independently written explanation of Dr. John Scherger's central insight: a correction to how lever systems have been taught in 4 fields, physics, biomechanics, physiology and kinesiology. I included it to give Dr. Scherger proper credit for identifying and emphasizing this misconception, and its diagrams, Figures 1 to 8 among them, were redrawn for clarity, consistency and print quality.
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The perpendicular distance from a force's line of action to the fulcrum. The book states it, not the distance along the bone, belongs in every torque calculation.
The book's Figure 7 shows one that starts with an advantage and loses it as the lever moves, because the effective effort arm shortens.
The effort and resistance arms share one rotating axis and change geometry with every degree of motion, so a joint's lever class can change within one exercise.
2. The superior vertebra is the fixed resistance arm, the inferior vertebra provides the true effort arm, and the joint between them is the fulcrum.
The book dates it to Benedetti in 1599 and credits Dr. John S. Scherger with applying it to musculoskeletal biomechanics.
Published October 16, 2026 • Updated October 16, 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.
