Why do people get weak quadriceps after knee surgery?

A Question

Take a look at your leg. Straighten your knee and feel the quadriceps on the front of your thigh contract. Now imagine being so weak in the quadriceps that you are unable to straighten your knee. Imagine being not only weak but unable to even contract the muscle. If you can’t wiggle your ears or splay your toes, the feeling is the same, except when you can’t contract your quadriceps the consequences are much greater. You have to lock your knee during walking and rely on your other leg to get in and out of a car. Quadriceps weakness makes life difficult and running sports nearly impossible.

The scenario described above is formally called “quadriceps activation failure” and is common after traumatic knee injuries such surgery or an ACL tear. This weakness is beyond conscious control and thought to be caused by an ongoing reflex response to joint injury called arthrogenic muscle inhibition (arthro=joint, genic= “from the”). In arthrogenic muscle inhibition , pain signals from the joint feedback through the spinal cord to inhibit muscles that cross that joint and reduce pressure on the joint. This reflex may be protective at first but as it persists atrophy sets in and the weakness limits functional recovery.

Muscle activation is estimated from muscle electrical activity as recorded by intra muscular wires or surface electrodes. To determine what percent of their muscle someone is able to activate, the person is asked to contract fully against a force plate, then the muscle is electrically stimulate and the additional force is recorded.Having a voluntary contraction that is more than 95% of your voluntary plus electrically stimulated contraction is considered fully activated.


Some Data

Two years ago Joseph Hart PhD and his team at the University of Virginia conducted a systematic review on quadriceps activation after knee injury and their work was published in the Journal of Athletic Training1.

They classified quadriceps activation failure as voluntary force production less than 95% of the voluntary plus electrically stimulated contraction. In a total of 352 patients with a torn ACL, overall weighted mean quadriceps activation was 87.3% for the injured side and 89.1% for the uninjured. Uninjured controls averaged 95% activation.

Prevalence of quadriceps activation failure was 57.1% on the injured side and 34.2% on the uninjured side. In 21% of patients with a torn ACL on one knee, quadriceps weakness was present on both sides. Quadriceps activation rates for patients after ACL surgery were similar.

Quadriceps activation was even worse in patients with a more common and less traumatic injury: anterior knee pain. These patients had quadriceps activation of 78.6% on the painful knee and 77.7% on the non-painful knee.


My Application


I must admit I have neglected the quadriceps over the last few years. The gluteal muscles have been the favored son in rehabilitation and performance training as of late and the quadriceps have fallen from the glory they experienced in the 80s and 90s.

Quadriceps activation failure is a painful reminder that life without the quads is very difficult indeed. Unfortunately this systematic review was focused on describing the prevalence and incidence of quadriceps weakness after knee injury and did not systematically investigate any of the treatments for it. Treatments that have reported benefits in the research included joint mobilization, local cooling and electrical stimulation.

I found it very interested that the patients with anterior knee pain (of which a common cause is patella-femoral pain syndrome) had less quadriceps activation on both sides than the ACL injured patients did on their injured side. This is consistent with the risk factors for these two injures. One identified risk factor for ACL tear is weak hamstrings relative to the quadriceps2, while quadriceps weakness is a risk factor for anterior knee pain3. The solution is to have a consistent strengthening, balance and agility program that focuses on functional movements and has been shown to reduce risk of both of these injuries4,5.




1. Hart JM, Pietrosimone B, Hertel J, Ingersoll CD. Quadriceps activation following knee injuries: A systematic review. Journal of athletic training. 2010;45(1):87.

2. Hewett TE, Myer GD, Ford KR, et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes. Am J Sports Med. 2005;33(4):492-501.

3. Witvrouw E, Lysens R, Bellemans J, Cambier D, Vanderstraeten G. Intrinsic risk factors for the development of anterior knee pain in an athletic population. Am J Sports Med. 2000;28(4):480-489.

4. Coppack RJ, Etherington J, Wills AK. The effects of exercise for the prevention of overuse anterior knee pain. Am J Sports Med. 2011;39(5):940-948.

5. Mandelbaum BR, Silvers HJ, Watanabe DS, et al. Effectiveness of a neuromuscular and proprioceptive training program in preventing anterior cruciate ligament injuries in female athletes. Am J Sports Med. 2005;33(7):1003-1010.



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