25 pounds on a leg extension loads the ACL with 61 pounds of shear, and the leg curl's highest ACL load comes where the lift feels easiest
Eileen Durfee's Spinal Fitness book leaves the leg extension and the leg curl out of its recommended exercises, based on a force analysis attributed to Dr. John Scherger: 25 pounds of load on a leg extension generates 61 pounds of anterior cruciate ligament (ACL) shear at the knee, and a 100-pound leg curl reaches 94 pounds of anterior shear near the top of the movement.
Chapter 9 states that every joint manages 2 forces: compression, which pushes the joint surfaces together along their load-bearing axis, and shear, which slides the surfaces against each other perpendicular to that axis. Of the 2, articular cartilage handles compression efficiently, while ligaments, including the ACL, exist to resist shear. A joint under primarily compressive loading is carrying the forces it was built for; a joint under primarily shear loading relies on its ligaments to stop dislocation.
Chapter 6 states the training consequence under the heading "Strength Is Not the Same as Stability." If the resultant force at a joint travels into compression, bone geometry stabilizes the joint with less muscular effort. If it travels into shear, stabilization must come from ligament, disc, cartilage, facet or muscle, and if those structures are weak, recovering, inflamed or already damaged, the exercise strains the tissue that needs protection. The book's illustration, shown with this article, labels compression joint-stabilizing and shear ligament-demanding. The Creatrix guide on sports injury risk and spinal structure sets out the wider program.
The leg extension is a seated exercise: the person extends the lower leg against a resistance pad near the ankle. Chapter 9's force vector analysis, built on measured human anatomy and wheel-and-axle lever mechanics, shows that as the lower leg rises, the geometry of the knee joint creates a consistent anterior shear that drives the tibia forward relative to the femur. The ACL is the structure that resists that shear, and the analysis finds compression and anterior shear present through the full range of motion, with the ACL absorbing the shear component at every angle.
In the force analysis, 25 pounds of load produces 61 pounds of ACL shear. The book adds that a stronger quadriceps does not redirect shear away from the ACL; it generates more muscle tension, which produces more shear, so greater strength in this exercise increases the load on the ligament. A healthy ACL, the book states, may tolerate this stress in the short term; for an injured, surgically repaired or incompletely recovered ACL, it calls repetitive tensile shear from the machine, particularly in the name of rehabilitation, counterproductive at best and injurious at worst for the knee joint.
The leg curl is a prone exercise, curling the lower leg upward against a resistance pad behind the ankle. The book's analysis with 100 pounds of resistance at 3 positions:
| Position | Hamstring force | Joint shear | Stabilized by | Lever class |
|---|---|---|---|---|
| 5 degrees | 550 lb | 37 lb posterior | Posterior cruciate ligament | 1st |
| 45 degrees | 450 lb | 15 lb posterior | Posterior cruciate ligament | 1st |
| 85 degrees | 25 lb | 94 lb anterior | ACL | 3rd |
At the start of the curl the forces at the knee are primarily compressive, with a modest posterior shear managed by the posterior cruciate ligament, and the book describes the position as a first-class lever system. Near the top, at about 85 degrees from horizontal, the hamstring needs only about 25 pounds of force to hold the leg, while the vector calculation shows about 94 pounds of anterior shear directed toward the ACL; the movement has become a third-class lever system. Chapter 9's summary line: "The exercise feels easiest exactly where it is most dangerous to the anterior cruciate ligament (ACL)." A trainer judging the curl by perceived exertion alone, the book states, has no way to identify that risk without the lever mechanics.
The book lists 3 upgrades attributed to Dr. Scherger. First, total effort includes the force required for shear stabilization, not only the visible muscle contraction, and every 3rd-class lever movement creates shear. Second, training must account for all forces created; the leg curl strengthens the hamstrings while creating anterior shear that strains the ACL. Third, objective training requires total anatomy: the effort muscles, the stabilizing muscles on both sides, and every joint surface the movement recruits.
Chapter 6 applies the 3 upgrades to the "no pain, no gain" instruction. When a young athlete reports knee pain during leg curls and is told to push through, the book states that the pain is the ligament signaling that the stabilization demand has exceeded its capacity, and that continuing accumulates damage in the tissue that keeps the joint stable for the rest of the athlete's career. Its sequencing rule is structure first, strength second: a stronger muscle helps only when the body has the structure to receive, direct and stabilize the force.
Dr. Scherger's hip and knee data in Chapter 6: with an ideal S-shaped spine and relaxed hamstrings, hip socket shear is approximately 50 pounds and compression approximately 87 pounds, extending into the knee. With the center of upper-body trunk mass only 4 inches too far forward, shear rises to 146 pounds and compression to 924 pounds, with the tightened hamstring working at 843 pounds. The screening chapter adds that when trunk mass is chronically forward of the hips, a vertical jump lands forward of the takeoff point and creates severe compressive and shear forces in the knee on landing.
The book's stated reason for the exclusion is that the force environment these 2 machines create is incompatible with joint longevity. Its selection rule follows: strengthening the hamstrings and quadriceps through movement patterns that load the knee joint in compression-dominant, shear-controlled positions is safer and more mechanically appropriate, particularly with a history of ACL injury, knee surgery or cartilage damage. The book names no specific replacement machine or knee exercise; its program builds the S-shaped spine that sets the trunk mass back over the hips.
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.
All 5 steps of the book's system work from the anterior direction. Back Trac rolling is Step 1, and the PowerCushion is the fulcrum for Step 2, the Pelvic Tilt, and Step 3, the Sit-Up, which restores the lumbar curve.
Practitioner Eileen Durfee, NBS (Nutritional Balancing Science), NASM-CPT — wellness educator, inventor, author, and a Nationally Certified Spinal Fitness Specialist.
I have worked in the wellness industry since 2010 and trained as a NASM Certified Personal Trainer. The leg extension and leg curl figures in my book are Dr. John Scherger's, from his work at the Athletic Spinal Fitness Institute, and the section's rule is to load the knee in compression-dominant, shear-controlled positions.
Our Facebook group is free to join for questions about any of the 5 steps in the Spinal Fitness system.
In its force analysis, 25 lb of load produces 61 lb of ACL shear, and anterior shear is present at every angle of the movement.
Near the top, at about 85 degrees, where the hamstring needs only about 25 lb of force but the knee carries about 94 lb of anterior shear.
The book states it does not: more quadriceps tension produces more shear, so added strength increases the load on the ligament.
With trunk mass 4 inches forward, the book gives hip socket shear of 146 lb and compression of 924 lb, against about 50 lb and 87 lb with an ideal S-shaped spine.
Movement patterns that load the knee in compression-dominant, shear-controlled positions. It names no specific replacement exercise.
Published October 17, 2026 • Updated October 17, 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.
