A quads lad typically refers to an inability to contract the quadriceps group during terminal knee extension. This may be seen in longsitting or in supine. The different positions will to a larger or lesser extent assess rectus femoris's inolvement vs the other quads.

Quads lag can be the result of any trauma to the thigh or more importantly the knee. Effusion over 15-30ml of fluid will maintain inhibition to the quadriceps and can delay resolution of a lag. Sever fractures to the femur or tibial plateau can also prolong a quads lag, as does the usual join replacement surgery.

Importantly for the neurological patient, a quads lag at the terminal 15 degrees of knee extension can and often leads to hyperextension and damage to the joint. This results from the hamstrings pulling the knee into extension to lock the knee. In these patients bracing is useful until the quads function has been restored. I hope this gives some overview about quads lag. There are many more scenarios.. regards richard