Open Chain Knee Extension after ACL-R

The anterior cruciate ligament (ACL) is the primary restraint for anterior translation of the tibia on the femur. There is on-going debate whether non-weightbearing AKA open chain knee extension is appropriate following ACL surgery due to the strain put on the graft. During open chain knee extension, the quadriceps produce an anterior directed force on the tibia via the patellar tendon without co-contraction of the hamstring to balance out the forces on the ACL.  The biggest challenge in exercise selection post-ACL reconstruction is the limited knowledge of the optimal amount of stress that should be applied to the ACL graft as it goes through the incorporation and maturation process. The question regarding open chain knee extension is whether or not the isolated anterior shear force is too much for the healing graft.


Knee Biomechanics

Understanding knee biomechanics and how the ACL is loaded through different movements will aid in exercise selection.  Contraction of the quadriceps between 0 and 60 degrees of knee flexion exerts an anteriorly directed force on the proximal tibia loading the ACL. Contraction of the quadriceps at angles greater than 60 degrees of knee flexion exerts a posteriorly directed force on tibia unloading the ACL. Contraction of the hamstrings throughout the entire range of knee flexion exerts a posterior force on the tibia unloading the ACL. Weight bearing or closed chain exercises have been more favorable due to the co-contraction of quadriceps and hamstrings reducing the net force applied to the ACL.


What does the research say?

A healthy intact ACL can withstand up to 2000 N of tensile force with an ultimate strain threshold of 15-19%. Several studies have looked at ACL strain (defined as change in ACL length with respect to original length expressed as a percentage) measured directly in vivo and tensile force (estimated through mathematical biomechanical models) on the ACL during both weight-bearing and non-weight bearing exercises.

The literature shows that ACL strain during open chain extension is greatest between 10 and 30 degrees of knee flexion. Percent strain gradually decreases between 30 and 60 degrees and is absent at angles greater than 60 degrees.

Peak ACL strain measuring between 3.2% and 4.4% occurs between 10 and 30 degrees of flexion. Peak ACL tensile force measuring 150-350 N also occurs between 10 and 30 degrees of flexion. No loading of the ACL occurs during open chain extension at angles greater than 60 degrees of flexion. ACL strain at 0 degrees of flexion is minimal due to the stable closed packed position. This makes the straight leg raise a low-risk exercise and is typically added to the program within the first 2 weeks once good volitional quad contraction is achieved.  

During a standard squat, there is no strain on the ACL. This does not change when up to 30 pounds is added to the standard squat. However, ACL strain and tensile force can increase or decrease by manipulating trunk position and how far the knees go over toes.

ACL tensile force during level-walking can reach 300-355 N when the knee is between 15-20 degrees of flexion during mid-stance. This is consistent with tensile forces that occur when performing open chain knee extension between 10 and 30 degrees of flexion. Patients can begin walking without crutches or brace around week 3. If the graft is exposed to this level of strain regularly while walking, why should a controlled exercise with similar tensile forces be withheld?  


Is open chain knee extension appropriate?

This is not a yes or no answer. The exercise can be done safely depending on time frame, range of motion, and application of external resistance.

According to the literature, we can begin to implement open chain extension between 90 and 40 degrees of flexion with minimal strain to the ACL at 2 weeks post op. External resistance can be progressively added through this range to optimize quadricep strengthening. Extension from 90 to 0 degrees can be safely implemented at 6 weeks. At this time, the patient has been independently walking for 3 weeks routinely exposing the ACL graft to forces similar to those between 30 and 10 degrees of open chain extension. Progressive resistance loading through full range of knee flexion can begin around week 10-12.


Why do we advocate for open chain knee extension?  

Early-stage quadriceps dysfunction following ACL surgery has implications for long term strength and performance deficits. A recent study showed a significantly strong correlation between quadriceps strength at 12-weeks post op with ultimate quadriceps strength at the time of return to sport in individuals following ACL surgery. This finding emphasizes the importance of early restoration of quadriceps strength during ACL rehabilitation. Open chain knee extension is the best way to isolate the quad. With appropriate timing and range of motion, open chain knee extension can be safely included in ACL-R rehabilitation.

At our clinic, we begin open chain knee extension around week 2 between 90 and 40 degrees of flexion. We add progressive resistance with ankle weights ranging from 2 to 10 pounds through this range of motion. During this resistance progression, we begin unweighted active knee extension from 90 to 0 degrees. The movement is then transitioned to loaded extension through full range of motion based on patient presentation. Our physical therapists frequently utilize blood flow restriction training with open chain extension to optimize quad strengthening while minimizing strain on ACL. Look out for our next blog post “Benefits of BFR following ACL-R” to learn more about this training system.

We do include a variety of closed chain or weight bearing exercises to assist in quad strengthening; however, these exercises do not isolate the quadricep as well as open chain extension. Many patients are really good at compensating, and most of these patients don’t even know they are compensating. It is difficult for therapists to identify how much a patient is utilizing other leg muscles to perform weight bearing exercises without using EMG. Open chain knee extension is a sure way to activate the quad and only the quad.

Our rehabilitation programs are safely designed with respect to current evidence based practice, individual surgeon protocols, and clinical experience. Let us know if you have questions regarding ACL rehab!


Written by Hannah Sweitzer, DPT, OCS



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2. Escemalla RF, Macleod TD, Wilk KE, Paulos L. Anterior Cruciate Ligament Strain and Tensile Forces for Weight-Bearing and Non-Wight-Bearing Exercises: A Guide to Exercise Selection. Journal of Orthopaedic and Sports Physical Therapy. 2012;42(3):208-22

3. Hannon JP, Wang-Price S, Goto S, et al. Twelve-Week Quadriceps Strength as A Predictor of Quadriceps Strength At Time Of Return To Sport Testing Following Bone-Patellar Tendon-Bone Autograft Anterior Cruciate Ligament Reconstruction. IJSPT. 2021;16(3):681-688. doi:10.26603/001c.23421

4. Marieswaran M, Jain I, Garg B, Sharma V, Kalyanasundaram D. A Review on Biomechanics of Anterior Cruciate Ligament and Materials for Reconstruction. Appl Bionics Biomech. 2018;2018:4657824. Published 2018 May 13. doi:10.1155/2018/4657824

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