These patients had quadriceps activation of 78.6% on the painful knee and 77.7% on the non-painful knee. Quadriceps activation was even worse in patients with a more common and less traumatic injury: anterior knee pain. Quadriceps activation rates for patients after ACL surgery were similar. In 21% of patients with a torn ACL on one knee, quadriceps weakness was present on both sides. Prevalence of quadriceps activation failure was 57.1% on the injured side and 34.2% on the uninjured side. Uninjured controls averaged 95% activation. 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.
WHEN CAN I GET ON MY KNEES AFTER ACL SURGERY PLUS
They classified quadriceps activation failure as voluntary force production less than 95% of the voluntary plus electrically stimulated contraction.
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 Training 1. 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. Muscle activation is estimated from muscle electrical activity as recorded by intra muscular wires or surface electrodes. This reflex may be protective at first but as it persists atrophy sets in and the weakness limits functional recovery. 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 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”). The scenario described above is formally called “quadriceps activation failure” and is common after traumatic knee injuries such surgery or an ACL tear. Quadriceps weakness makes life difficult and running sports nearly impossible.
You have to lock your knee during walking and rely on your other leg to get in and out of a car. 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. Imagine being not only weak but unable to even contract the muscle. Now imagine being so weak in the quadriceps that you are unable to straighten your knee. Straighten your knee and feel the quadriceps on the front of your thigh contract.