Ask a PT: I had knee surgery, shouldn’t I be doing leg extensions to strengthen my quad?

“Ask a PT” will be a weekly feature on Fridays in our blog, where we answer questions we get from our clients or posters on this site or our Facebook page.  So if you have a question about some sort of injury or problem you are having whether pain or fitness related drop us a line in the comments or Facebook message, or you can email us at

This weeks question is one that is asked frequently by patients who have recently had knee surgery. The question is “Shouldn’t I be using that machine where I straighten out my leg to strengthen my quads?”

It depends, but in general, this is actually not a great exercise and in the case of a person who has recently undergone ACL surgery, could potentially be harmful.  Leg extensions are an example of an open-chain exercise.  There are two types of exercise: open chain and closed chain. Open chain exercise is defined as moving a weight (or a limb) freely without being in contact with a surface. One example of this is a bicep curl; the biceps muscle is contracting in order to move the forearm with or without a weight in space. A closed chain exercise is defined as moving the body away or towards a surface by contracting a muscle.  To use the biceps muscle, an example of this would be a chin-up. The biceps is contracting to move the body towards the hand, which is fixed on the pull-up bar.

The leg extension exercise is an open-chain exercise. Most strengthening programs call for a combination of open- and closed-chain exercise.  Open-chain exercise typically isolates a single muscle group, whereas closed-chain exercise recruits multiple muscle groups in order to stabilize the segment. Closed-chain exercise is typically a more functional exercise, particularly for lower extremity muscles. Usually, open-chain exercises are utilized first and then once the target muscle group gets stronger, closed-chain exercises are utilized.

In the case of post-ACL reconstruction rehab and for patients with anterior knee pain, this is not exactly the case. For patients who have recently undergone ACL reconstruction (less than 8 weeks), open-chain knee extension, especially with the knee bent 30 degrees or less, may put the ACL graft at higher risk. This is due to the increase sheer forces and anterior tibial displacement that occurs with open-chain knee extension. In a study conducted on patients with ACL-deficient knees (Sadegh et al, 2013), it was found that open-chain exercises lead to a significantly greater anterior translation of the tibia on the femur, when compared with closed-chain exercise. However, a study by Perry et al (2005) shows after a patient has been in rehab for 8 weeks, the leg extension exercise is not harmful in that it does not increase joint laxity or impact function. The question is, however, is how beneficial is this exercise? Once a patient has gained quadriceps strength through closed-chain exercises (such as a squat or partial squat), it may not be of additional benefit to strengthen using open-chain exercises.

Open Chain Knee Extension, No Resistance

Partial Squat, with support

For patients who are experiencing anterior knee pain (or PFPS- see previous post to learn more about this-, there is a debate about whether or not open-chain exercises cause increased stress at the patellofemoral joint.  Research supports that there may not actually be a significant increase in stress at the patellofemoral joint, so doing open-chain knee extension with low resistance should not be harmful (Zohara et al, 2001).  However,  it is also shown that patients with PFPS who undergo a closed-chain exercise program versus an open-chain exercise program have better strength gains and improved movement.

So to answer the question- if you are post-surgery from ACL reconstruction and are in your first 8 weeks of rehab, the leg extension exercise should be avoided (especially with resistance applied). Otherwise, this exercise is only good if you are unable to tolerate a squat but want to strengthen the quads, have not recently had surgery, and do not experience increased pain during or after this exercise.  Remember, in general, if you are trying to get back to a sport or just regular function, closed-chain exercises will give you the best bang for your buck.

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3 Responses to Ask a PT: I had knee surgery, shouldn’t I be doing leg extensions to strengthen my quad?

  1. Very informative 🙂 Thanks for sharing! I’ve been doing some T25 lately and it’s definitely more intense but some of those poses reminded me of some the stuff we do in T25.

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