knee extension machine the prehab guys what is the acl

The knee extension machine is present in most gyms across the United States. While many people use this machine, many others do not for fear of causing damage to their knees. Why is it that some people fear the knee extension machine? Is it safe to use? Is it a useful tool? Is it “functional”? This article will teach you why the knee extension machine is safe, why it can be crucial to overcoming knee pain, and how to include it in your exercise or rehabilitation program.


Why are people afraid of this machine?

The year is 1993. Bill Clinton is inaugurated for his first term as President of the United States, the writer of this article is born, the iPhone has not been invented yet, and a research study is published comparing the stresses placed on the kneecap while using the knee extension machine versus the leg press machine. This study shows that the knee extension machine places greater stress on the kneecap (patellofemoral joint) than the leg press from 0 degrees to 48 degrees of knee flexion, while the leg press places greater stress on your patellofemoral joint from 48 degrees to 90 degrees.

In layman’s terms: 0 degrees of knee flexion is a straight leg, and 90 degrees of knee flexion is having your knee bent at a right angle. As both the knee extension and the leg press exercises generally start in a position of 90 degrees of knee flexion, one can simply state that the patellofemoral joint experiences more stress during the first half of the leg press motion, and the second half of the knee extension motion. This is detailed in the graph below which shows the lines intersecting at 48.4 degrees.

Patellofemoral Joint Stress: Leg Press Versus Knee Extension Motion

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Learn What It Takes To Gain and Maintain Knee Extension

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Knee extension is not just a recommendation it is needed! Hypothetically, you could walk on a bent knee but this is going to waste energy and ultimately limit your movement endurance. Instead, let’s get you moving efficiently by achieving and keeping full knee extension.


So….why are people afraid to use the knee extension machine?

The authors of the 1993 study concluded that since most activities of daily living and sports utilize 0 to 45 degrees of knee flexion the leg press is more of a “functional” exercise. This explanation stuck and for many gym-goers, therapists, and doctors the knee extension machine was dubbed “not functional” “too stressful for the kneecap” and “dangerous”! That same year, another research study concluded that knee extension exercises place more stress on the Anterior Cruciate Ligament (ACL) than squats do and thus should be avoided during ACL Reconstruction surgery rehabilitation, placing another nail in the coffin.


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So… in 2020, do I need to fear the knee extension machine?

As you can imagine, in the 27 years and 11 iPhone models that have passed since these initial articles were published, there has been additional research in this area. Some of it, like a 2014 study out of USC, has had similar findings: concluding that stress to the kneecap is greater during higher degrees of knee flexion during a squat and fewer degrees of knee flexion during a knee extension exercise. This is shown in the graph below where the orange line represents the knee extension machine and green line represents a squat.

Knee Cap Stress: Squat Versus Knee Extension Exercise

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Other studies, however, have been much more groundbreaking. For example in 2012 Chiu et al showed that increasing quad strength by utilizing the knee extension machine (along with the leg press) 3x/week for 8 weeks can actually increase patellofemoral joint contact area. This means that the knee extension machine can actually reduce the stress placed on the kneecap by increasing the area that joint forces are spread over. This decreases kneecap pain as was shown in the subjects involved in this study.


What about after ACL surgery?

Quadriceps strength is arguably the most important component in post-operative ACL rehabilitation. There is strong evidence showing that a variety of neurological changes occur after an ACL injury which decreases quadriceps strength.1-3 Furthermore, quadriceps strength has been shown to be essential for achieving normalized functional outcomes such as walking, running, successful return to sport, and self-reported functional success.  As the knee extension machine is the best way to strengthen the quadriceps, we are fortunate that research has shown it to be safe after ACL surgery! In spite of the concerns that the knee extension machine exercise places too much strain on the ACL graft and leads to anterior knee laxity, a 2018 systematic review of 10 randomized controlled trials found absolutely no evidence of this.  Moreover, a study in 2000 comparing a group of patients whose rehab included the knee extension exercise to a group whose rehab did not found that the group who utilized the knee extension machine had significantly greater strength at 6 months, and were more likely to return to their sport at the same or higher level, and were able to return to sport earlier. The Journal of Orthopedic and Sports Physical Therapy even recently published a viewpoint article articulating the critical importance of the use of the knee extension machine during ACL rehab.


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The Knee Extension Machine: So it is safe – but is it “functional”?

Let’s tackle the original criticism that led to all of the fear surrounding the knee extension machine in the first place – is it functional? Well, as we have just highlighted above, the knee extension machine is the best way to strengthen your quadriceps. Quadriceps strength alone is critical for normal function, particularly after ACL reconstruction. Additionally, quadriceps strength had been shown to decrease patellofemoral joint stress thereby decreasing patellofemoral pain. Thus, while the machine itself may not be considered a “functional” exercise, it might be necessary to build the strength necessary to perform functional exercises. Additionally, while it places more stress on the kneecap at lower levels of knee flexion, as the graph from the 2014 study shows, squatting beyond 45 degrees placed much more stress on your kneecap than the knee extension machine does in any range. It is useful to remember that simply getting up from the couch is akin to squatting to 90 degrees and thus places more stress on your kneecap than the knee extension machine does. Just as you would never avoid using your couch for fear of stress to your kneecaps, you shouldn’t avoid the knee extension machine!

Knee Extensions: Safe and Functional!

Knee Extension Strengthening Exercises

Here are some examples of knee extension exercises specific for strengthening that can be implemented into a specific exercise routine!

Isometric Single Leg Knee Extension – Swissball

Sample Knee Extension Mobility Program Exercise

  • HOW: Place a swissball against a wall. While in a seated, upright position, kick into the ball and hold for a prescribed amount of time. As you kick, you shouldn’t be moving at all but you will feel your muscles activating.


  • FEEL: You should feel your quadriceps muscles in your thigh working.


  • COMPENSATION: Your leg shouldn’t move much as you push into the ball. Don’t arch your back.


Long Arc Quad – 45 to 90 degrees

Get set-up seated on the edge of a surface with your feet hanging off at a 90 degree angle. Place a band around your ankles. On the side you want to perform this exercise, kick the leg straight until your thigh and shin make a 45 degree angle, hold for a moment, and then slowly return to the starting position and repeat. Your other leg shouldn’t move to keep tension on the band.


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Long Arc Quad – 0 to 90 degrees

Sample Knee Extension Mobility Program Exercise

Get set-up seated on the edge of a surface with your feet hanging off at a 90 degree angle. Place a band around your ankles. On the side you want to perform this exercise, kick the leg straight until your knee is fully straight, hold for a moment, and then slowly return to the starting position and repeat. Your other leg shouldn’t move to keep tension on the band.


Long Arc Quad – Band

Sample Knee Extension Mobility Program Exercise

Follow the instructions in the video to properly tie the theraband around an anchor, and then to your ankle so that it doesn’t slip. Once set up, try to straighten your knee against the resistance of the band. Think of lifting your heel up while pushing your knee down. Do not use the momentum of your body to push out against the band.


Closing thoughts

The knee extension machine is crucial to building quadriceps strength, which is crucial to a healthy, functional, and resilient knee. Remember, using the machine:

  • Does not loosen an ACL reconstruction


  • Exposes your kneecap to less stress than squatting to parallel does


  • Helps to decrease kneecap stress and pain, over time, by increasing joint contact area.


IF you have pain when performing this exercise, try decreasing the weight to a level that you can perform without pain. If you still have pain, try performing the exercise in a limited range of motion from 90 to 45 degrees of knee flexion until you have built enough strength to perform through a full range. If you are still unable to perform it without pain, utilize the exercises from the knee program to build a baseline before trying again. Just as you would work toward being able to squat without pain, you can work towards being able to do the knee extension machine without pain.


The Ultimate Knee Extension Program At Your Fingertips

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Knee extension is not just a recommendation it is needed! Take control of your knee health today.



  1. Needle AR, Lepley AS, Grooms DR. Central ner­vous system adaptation after ligamentous injury: a summary of theories, evidence, and clinical interpretation. Sports Med. 2017;47:1271-1288.
  1. Noehren B, Andersen A, Hardy P, et al. Cellular and morphological alterations in the vastus lateralis muscle as the result of ACL injury and reconstruction. J Bone Joint Surg Am. 2016;98:1541-1547.
  1. Snyder-Mackler L, De Luca PF, Williams PR, Eastlack ME, Bartolozzi AR, 3rd. Reflex inhibition of the quadriceps femoris muscle after injury or reconstruction of the anterior cruciate ligament. J Bone Joint Surg Am. 1994;76:555-560.


About The Author

Tommy Mandala, PT, DPT, SCS, OCS, CSCS

[P]rehab Writer & Content Creator

tommy mandala the prehab guysTommy Mandala is currently a Doctor of Physical Therapy and Board Certified Sports Clinical Specialist in New York City. Prior to that, he completed a sports residency at the University of Delaware where he had opportunities to work with their Division I baseball team, as well as the Philadelphia 76’ers NBA G-league affiliate, the Delaware Blue Coats. A former high school baseball player, this experience drove his interest in treating the throwing athlete and led him to pursue a rotation at Champion Sports Medicine in Birmingham, Alabama to learn from leading baseball researchers in the American Sports Medicine Institute. While Tommy has a special interest in throwing athletes and ACL rehab, he believes that everyone should train like an athlete. As the son of an FDNY firefighter, he also has a passion for treating the occupational athlete. One of his favorite aspects of his job is teaching patients the proper form to allow them to continue doing the things they love in spite of an injury.

Disclaimer – The content here is designed for information & education purposes only and is not intended for medical advice.

About the author : Tommy Mandala PT, DPT, SCS, OCS, CSCS

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