Why is the knee prone to buckling or giving way?

and there can be an extension quad lag. We call this arthrogenic muscle inhibition(AMI). AMI creates an inability to voluntarily activate a muscle without any structural damage to the muscle or motor neuron innervating that muscle. AMI is a natural response to injury to the knee joint, encouraging the individual to rest and avoid further damage to the vulnerable joint. However, the effect of AMI on quad strength is marked, with “knee extensor peak torque decreasing by 80 to 90% one to three days after knee joint surgery. Despite diminishing over time, residual levels of AMI may persist as long as 4 years after initial joint trauma” (1)

What is quad lag and why is it important?

Quad lag or extensor lag describes the lack of full knee extension active range of motion in the presence of full passive range of motion. Below you can see a video showing approximately 10 degrees of quad lag. A retrospective review of total knee replacement patients with an average follow up of 30 months was done which showed that about 1/3 had more than 15 degrees of quad lag. This quad lag was correlated to higher pain levels. (2) In my experience, pain often results in quad lag and quad dysfunction/weakness in general.

In clinical practice, quad lag can be a symptom of significant functional limitation with inability to control descent going down stairs or worse, instability while walking. Or, it can be only a mild deficit that is present only during testing but not at all functionally limiting. It really depends on the individual case. I have seen patients 1 year post total knee replacement with more than 15 degrees of quad lag and significant funcitonal deficits with walking/stairs.

What can we do about it?

We believe that a robust and progressive quad strengthening program is necessary to remedy the “sleepy quad”. Standing terminal extensions, short arc/long arc quads (both isotonic and isometric), leg press and squats are all great ways to load the quads in a variety of ranges. Using electrical muscle stimulation is a great adjunct to traditional strenght training as well.

References