Knee Extension Lag

Original Editor - Ajay Upadhyay

Top Contributors - Laura Ritchie, Kim Jackson, Ajay Upadhyay, Nehal Shah, Admin, Evan Thomas, WikiSysop, Wanda van Niekerk and Lauren Lopez

Introduction

When a muscle is not able to actively move a joint to its passive limit, it is known as Muscle lag.

Passive and Active limit of a joint

PASSIVE LIMIT - While moving a joint, passive limit should be obtained by exerting mild force against resistance from joint stiffness or soft tissue tightness and that does not produce any discomfort.

ACTIVE LIMIT - Active limit of a joint is obtained when the moving segment of the patient is resisted by gravity. The patient position should be such that there is no external load on the moving segment except gravity.

Quadriceps Lag

Approximately 60% more force needs to be generated by a contracting Quadriceps Femoris muscle to achieve last 15 degrees of active knee extension. Inability of Quadriceps to achieve this final extension range is coined as "Lag"This may give mechanical disadvantage to the Quadriceps.

Precipitating Factors

Factors that may be responsible for Extensor Lag are as follows

(1) an abnormal increase in muscle length (as may occur following suture of rupture muscle, or fracture with loss of bone length).

(2) Disuse atrophy

(3) Myopathy

(4) Neurological

(5) Pain induced or other arthrogenic muscle inhibition

Technique

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.

Evidence

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).