Knee Extension Lag
Original Editor - Ajay Upadhyay
The term ‘muscle lag’ or ‘extensor lag’ or ‘quadriceps lag’ is a clinical sign with often profound functional relevance for patients during knee rehabilitation.
Muscle lag is an inability to actively move a joint to its passive limit. In more detail, the passive limit should be the passive limit that can be achieved without producing significant discomfort, and without exerting more than mild force against resistance from joint stiffness or other soft tissue tightness. The active limit should be determined with the patient positioned so that the moving segment is resisted by gravity but no other external load.
Active Lag Test
An active lag is determined by the inability of the erectly seated subject to actively extend the involved knee in maximal dorsiflexion of the ankle to the same level as the normal knee held in maximal extension and ankle in maximal dorsiflexion, as seen by the levels of the toes.
Passive Lag Test
A passive lag is determined by placing the subject prone with the knees just past the edge of the table and determining the high position of the heel in a fully resting extension position compared to the heel on the normal side.
It appears there is a widespread belief that lag is always abnormal.
Physiological Quadriceps Lag
A study done on physiological quadriceps lag has shown that most healthy young adults, when assessed in the manner described, manifest a quadriceps lag.  At the instant of maximum voluntary active extension, 48% of the 64 subjects had a quadriceps lag of between 2.0 and 10.5 degrees. Since these subjects had no pain, and further passive extension was possible without substantial passive resistance, this lag at 0 sec can only be readily explained as a manifestation of normal quadriceps femoris active insufficiency, perhaps augmented in some subjects by a less than maximum voluntary effort. The apparent absence of any reference in the literature to quadriceps lag in normal knees is most likely because quadriceps lag does not appear to have been examined systematically in healthy subjects.
For the sitting active lag test, the inter-rater reliability was 'good' (Kappa 0.792, SE of kappa 0.115, 95% confidence interval). For the prone passive lag test, the inter-rater reliability was 'good' (Kappa 0.636, SE of kappa 0.136, 95% confidence interval). 
- Sebastian D, Chovvath R, Malladi R. The sitting active and prone passive lag test: an inter-rater reliability study.J Bodyw Mov Ther. 2014 Apr;18(2):204-9.
- Tim Richardson. Physical Therapy Diagnosis: Supine Quadriceps lag. Available from: http://www.youtube.com/watch?v=CZK-j5VXo-I [last accessed 21/09/14]