Anterior cruciate ligament (ACL) injury is a common injury among athletes, with an estimated 120,000 ACL injuries per year in the Unites States alone.1 The ACL is the primary restraint to anterior translation of the femur on the tibia, and is integral to stability in the knee, especially in athletes. Approximately 75% of ACL injuries are non-contact, and females are between two and eight times more likely than males to suffer an ACL injury.2 While there is growing evidence and opinion that this difference is likely due to inequality in training, education, and rehabilitation for female athletes rather than gender differences, that is not the focus of this post. 

Rehabilitation after ACL reconstruction (ACLR) has a lot of myths and misinformation surrounding it. Also, one of the most controversial aspects is whether or not it is safe to perform open chain knee extension after ACLR. While there are many rehab professionals who would argue to never perform knee extension after ACLR, research has proven many times that it is not only safe, but necessary to build the strength required for successful rehab. The primary reason rehab professionals will not prescribe open chain exercises, especially in the first few weeks of rehab, is the myth that it puts too much stress on the ACL after reconstruction. To demonstrate how open chain exercises compare to other commonly prescribed exercises, let’s look at the force placed on the ACL during different activities. Patients often do gait training without any weight-bearing restrictions during the first week or two after surgery.

During walking, the peak anterior shear force at the ACL is 355N at 16.8 degrees of knee flexion.3 The peak anterior shear force with seated knee extension is 248N at 14 degrees of knee flexion.4 This shows that the force on the ACL is higher during walking than when performing open chain knee extension, so why are rehab professionals so hesitant to perform these exercises early on, and in many cases at any point during rehab?

Furthermore, several systematic reviews have shown that patients who perform open chain exercises and those who do not show no difference in ACL laxity.5, 6 We should use our knowledge of healing times and strength and conditioning principles to progressively load the quadriceps and lower extremity as a whole, as with any injury or programming. Just as you wouldn’t load up a maximal back squat the first time squatting, you wouldn’t load up a heavy weighted knee extension right after ACLR. 

It’s important to explain why athletes should do knee extension exercises after ACL surgery. Also aving balanced strength in both legs reduces the risk of reinjury. Athletes who do knee extensions, which are open chain exercises, have stronger legs than those who only do closed chain exercises. However, rehab experts don’t teach athletes how to safely return to their sport after surgery. Moreover, athletes who don’t follow the recommended guidelines for returning to their sport are more likely to reinjure themselves. Unfortunately, many athletes don’t meet these guidelines even after they are cleared to return to their sport. We need to change this so that athletes can be better prepared and avoid reinjury.

After ACL surgery, it’s important to wait for the right time before returning to sports. Rushing can be dangerous because it takes about two years for the ligament to fully heal and for the joint to be healthy again. Athletes who return to their sport before nine months after surgery are seven times more likely to get hurt again. Above all, waiting longer can reduce the risk of getting hurt again by 51% each month up to nine months. So, athletes should follow the recommended guidelines for returning to their sport and wait for the right amount of time before playing again.

Many people don’t know enough about how to rehab after ACL surgery. Clinicians need to make sure they know what’s safe and effective, and how to make a plan that works best for their patient. There are many things that can affect how likely it is that a patient will get hurt again after ACL surgery, so it’s important to make a plan that’s right for each patient.

Connor Stockton, DPT, CSCS, Cert-DN


1.     Mall, N.A.,Chalmers, P.N., Moric, M., Tanaka, M.J., Cole, B.J., Bach, B.R., & Paletta, G.A. (2014). Incidence and trends of anterior cruciate ligament reconstruction in the United States. American Journal of Sports Medicine 42(10), 2363-2370.

2.     Boden, B. P., Sheehan, F. T., Torg, J. S., & Hewett, T. E. (2010). Noncontact anterior cruciate ligament injuries: Mechanisms and risk factors. The Journal of the American Academy of Orthopaedic Surgeons18(9), 520–527.

  1. Nagura T, Matsumoto H, Kiriyama Y, Chaudhari A, Andriacchi TP. Tibiofemoral Joint Contact Force in Deep Knee Flexion and Its Consideration in Knee Osteoarthritis and Joint Replacement. Journal of Applied Biomechanics. 2006;22(4):305-313. doi:10.1123/jab.22.4.305
  2. Wilk KE, Andrews JR. The Effects of Pad Placement and Angular Velocity on Tibial Displacement during Isokinetic Exercise. Journal of Orthopaedic & Sports Physical Therapy. 1993;17(1):24-30. doi:10.2519/jospt.1993.17.1.24
  3. Glass R, Waddell J, Hoogenboom B. The Effects of Open versus Closed Kinetic Chain Exercises on Patients with ACL Deficient or Reconstructed Knees: A Systematic Review. N Am J Sports Phys Ther. 2010;5(2):74-84.
  4. Jewiss D, Ostman C, Smart N. Open versus Closed Kinetic Chain Exercises following an Anterior Cruciate Ligament Reconstruction: A Systematic Review and Meta-Analysis. Journal of Sports Medicine. 2017;2017:1-10. doi:10.1155/2017/4721548

7.     Grindem, H., Snyder-Mackler, L., Moksnes, H., Engebretsen, L., & Risberg, M. A. (2016). Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study. British journal of Sports Medicine50(13), 804–808.

  1. Mikkelsen C, Werner S, Eriksson E. Closed kinetic chain alone compared to combined open and closed kinetic chain exercises for quadriceps strengthening after anterior cruciate ligament reconstruction with respect to return to sports: a prospective matched follow-up study. Knee Surgery, Sports Traumatology, Arthroscopy. 2000;8(6):337-342. doi:10.1007/s001670000143
  2. Tagesson S, Oberg B, Good L, Kvist J. A comprehensive rehabilitation program with quadriceps strengthening in closed versus open kinetic chain exercise in patients with anterior cruciate ligament deficiency: a randomized clinical trial evaluating dynamic tibial translation and muscle function. Am J Sports Med. 2008;36(2):298-307.

10.  Kyritsis, P., Bahr, R., Landreau, P., Miladi, R., Witvrouw, E. (2016). Likelihood of ACL graft rupture: Not meeting six clinical discharge criteria before return to sport is associated with a four times greater risk of rupture. British Journal of Sports Medicine, 50(15), 946-951.

11.  Toole, A.R., Ithurburn, M.P., Rauh, M.J., Hewett, T.E., Paterno, M.V., & Schmitt, L.C. (2017). Young athletes cleared for sports participation after anterior cruciate ligament reconstruction: How many actually meet recommended return-to-sport criterion cutoffs? Journal of Orthopaedic & Sports Physical Therapy, 47(11), 825-833.

12.  Nagelli, C.V. & Hewett, T.E. (2017). Should return to sport be delayed until 2 years after anterior cruciate ligament reconstruction? Biological and functional considerations. Sports Medicine 47, 221–232.

13.  Beischer, S., Gustavsson, L., Senorski, E.H., Karlsson, J., Thomee, C., Samualsson, K., & Thomee, R. (2020). Young athletes who return to sport before 9 months after anterior cruciate ligament reconstruction have a rate of new injury 7 times that of those who delay return. Journal of Orthopaedic & Sports Physical Therapy, 50(2), 83-90.

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