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. This implies that there is nothing obstructing the joint.
Page updated June 2024 by Dr Sheila Strover (Clinical Editor)
What are the common causes of quadriceps lag?
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 or inhibited.
An effusion (fluid in the joint cavity) seems to be a trigger of inhibition.
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Quote from peer-reviewed paper:
[Muscle inhibition]...."may prevent effective muscle strengthening, leading to long-term quadriceps muscle atrophy and weakness that is difficult to reverse...."
Citation: Rice DA, McNair PJ, Lewis GN, Dalbeth N. Quadriceps arthrogenic muscle inhibition: the effects of experimental knee joint effusion on motor cortex excitability. Arthritis Res Ther. 2014 Dec 10;16(6):502. doi: 10.1186/s13075-014-0502-4. PMID: 25497133; PMCID: PMC4271337.
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Quote from peer-reviewed paper:
"....Failure to regain full extension by 3 weeks after ACL reconstruction is an important predictive factor for subsequent cyclops syndrome...."
Citation: Deepak S, Priti G; DScPT; Zishu T, Chetan P. The sitting active and prone passive lag test: a validity study in a symptomatic knee population. J Phys Ther Sci. 2023 May;35(5):312-319. doi: 10.1589/jpts.35.312. Epub 2023 May 1. PMID: 37131358; PMCID: PMC10149295.
What is quadriceps inhibition?
When we talk of muscle inhibition we refer to a lack of 'muscle firing' - it's not that the muscle is necessarily weak, but just that the patient simply cannot get the muscle to recognise their instruction to contract!
This seems to be some kind of reflex...
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Quotes from peer-reviewed papers:
"...findings provide no evidence for a cortical [ie brain] contribution to quadriceps [arthrogenic muscle inhibition] and suggest that ongoing spinal reflex inhibition may be sufficient to explain the marked neural activation deficit that occurs following acute knee injury, knee surgery and in chronic knee joint pathologies."
Citation: Rice DA, McNair PJ, Lewis GN, Dalbeth N. Quadriceps arthrogenic muscle inhibition: the effects of experimental knee joint effusion on motor cortex excitability. Arthritis Res Ther. 2014 Dec 10;16(6):502. doi: 10.1186/s13075-014-0502-4. PMID: 25497133; PMCID: PMC4271337.
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"Loss of the ability to fully straighten the knee is a potential complication of ACL rupture and treatment, known to occur following surgical reconstruction....[but]....loss of extension (LOE) has also been reported in patients electing to undergo nonoperative treatment."
Citation: Ektas N, Scholes C, Kulaga S, Kirwan G, Lee B, Bell C. Recovery of knee extension and incidence of extension deficits following anterior cruciate ligament injury and treatment: a systematic review protocol. J Orthop Surg Res. 2019 Mar 28;14(1):88. doi: 10.1186/s13018-019-1127-8. PMID: 30922410; PMCID: PMC6437951.
Forum discussions
- Re: Ruptured Quadriceps Tendon
Patients defining terms so that they can compare symptoms.
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