A 'quadriceps lag' or 'extension lag' is present when a patient cannot fully 'actively' extend the knee (straighten the leg using muscle contraction), but it can be fully 'passively' extended by the clinician.

Page updated July 2023 by Dr Sheila Strover (Clinical Editor)

Weak quads muscle

If the leg can be fully passively extended then that means that there is nothing mechanically blocking extension, so any failure to achieve this actively suggests that the quads muscle is weak.

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Extension lag
Extensor lag

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Dr Sheila Strover (Editor)
BSc (Hons), MB BCh, MBA

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