Femoral Nerve Tension Test (Prone Knee Bending Test)

The Femoral Nerve Tension Test (FNST), also known as the Prone Knee Bending Test, is used to assess irritation or compression of the L2–L4 nerve roots, often caused by nerve root impingement or tension in posterior thigh soft tissues. The test is particularly useful for identifying upper lumbar radiculopathy, which is less common than lower lumbar involvement[1][2].

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Clinical Anatomy

The femoral nerve (L2–L4) is part of the lumbar plexus, passing through the femoral triangle alongside the femoral artery and vein. It innervates the quadriceps femoris muscle and provides sensation to the anterior thigh and medial leg[4][5].

Dysfunction of the femoral nerve may present with:

• Pain in the groin, anterior thigh, or medial leg

• Weakness of the quadriceps and impaired knee extension

• Sensory disturbances in the femoral distribution

Test Procedure

Patient position

• The patient lies prone on the examination table.

Execution

1. Pelvic stabilization – examiner prevents anterior pelvic tilt with one hand.

2. Passive knee flexion – examiner flexes the knee maximally, attempting to bring the heel toward the buttock.

3. If no pain is reported, combine with hip extension while maintaining knee flexion[6].

Interpretation of Results

• Positive test:

  • Pain in lumbar region, buttock, or anterior thigh between 80–100° knee flexion → L2–L4 radiculopathy (disc herniation, foraminal stenosis)[8]

  • Pain before 80° → more likely quadriceps tightness than nerve root irritation

  • Pain in posterior thigh → possible L4-level radiculopathy

  • Unilateral low back pain → potential lumbar spondylosis

• Negative test:

  • No pain or abnormal findings

Test Variations

• Lateral Femoral Cutaneous Nerve Bias: hip extension + adduction with knee flexion

• Saphenous Nerve Bias: hip extension, abduction, and external rotation with knee flexion, ankle dorsiflexion and eversion

Diagnostic Validity and Evidence

Parameter	Sensitivity	Specificity
FNST for L2–L4 radiculopathy	88–100%	Variable
FNST for lumbar spondylosis	85.96%	64.38%

• FNST has shown very high sensitivity (up to 100%) for detecting upper lumbar radiculopathy[13].

• False positives may occur due to quadriceps tightness or hip pathology (e.g., arthritis, labral tears)[12].

• Combining FNST with the Thomas Test helps differentiate between femoral nerve tension and hip flexor tightness[14].

Clinical Relevance

• FNST is among the most reliable tests for identifying L2–L4 radiculopathy.

• Helps differentiate between nerve root compression and muscle-related restriction (quadriceps vs iliopsoas).

• Should always be interpreted in combination with other physical tests and imaging when needed.

Conclusion

The Femoral Nerve Tension Test is a sensitive and clinically valuable neurodynamic test for assessing upper lumbar radiculopathy and femoral nerve dysfunction. While highly sensitive, it may produce false positives if quadriceps tightness or hip pathology is present. For best diagnostic accuracy, FNST should be combined with complementary tests and imaging when indicated.

Sources

1.  Orthopaedic Manual Physical Therapy From Art to Evidence, Chapter 19: The Theory and Practice of Neural Dynamics and Mobilisation. Christopher H Wise.

2. Magee DJ. Orthopedic physical assessment. Elsevier Health Sciences; 2014 Mar 25.

3.  Shacklock M. Clinical neurodynamics: a new system of musculoskeletal treatment. Elsevier Health Sciences; 2005.

4.  Schünke M, Schulte E, Schumacher U, Voll M, Wesker K. Prometheus anatomy. Houten: Bohn Stafleu van Loghum, 2005.

5. Picture found on http://karate.butsu.net/anatomy/lumbosacral.html

6.  Dutton, M. (2008). Orthopaedic: Examination, evaluation, and intervention (2nd ed.). New York: The McGraw-Hill Companies, Inc.

7. ADVERSE NEURAL TENSION. Manual Therapy Institute Accessed on 4/9/2020 from https://mtitx.com/wp-content/uploads/2016/05/Adverse-neural-tension-workbook.pdf

8. Orthopaedic Physical Therapy 3rd Edition, Robert Donatelli, Michael Wooden, Chapter 19: Evaluation and Treatment of Dysfunction in the Lumbar-Pelvic-Hip Complex

9. John GibbonsHow to test the Femoral Nerve (Lumbar Plexus L2,3,4) or reverse Lasegue's. Available fromhttps://www.youtube.com/watch?v=cN0uou-nZH8

10.  Antonios N. Christodoulides. Ipsilateral Sciatica on femoral nerve stretch test is pathognomonic of an L4/5 protrusion. The Journal of Bone and Joint Surgery 1989; 71-B: 88-89. http://www.ncbi.nlm.nih.gov/pubmed/2915013 Level of evidence: C

11. http://www.radiculopathy.net/

12.  Scott F. Nadler, DO, Gerard A. Malanga, MD, Todd P. Stitik, MD, Rohit Keswani, MD, Patrick M. Foye, MD. The Crossed Femoral Nerve Stretch Test to Improve Diagnostic Sensitivity for the High Lumbar Radiculopathy: 2 Case Reports. Arch Phys Med Rehabil 2001; 82: 522-523. http://www.ncbi.nlm.nih.gov/pubmed/11295015 Level of evidence: B

13. Pradeep Suri, MD, James Rainville, MD, Jeffrey N. Katz, MD, MS, Cristin Jouve, MD, Carol Hartigan, MD, Janet Limke, MD, Enrique Pena, MD, Ling Li, MPH, Bryan Swaim, MS, and David J. Hunter, MBBS, PhD. The Accuracy of the Physical Examination for the Diagnosis of Midlumbar and Low Lumbar Nerve Root Impingement. SPINE 2010, Lippincott Williams & Wilkins. (Published online ahead of print) http://www.ncbi.nlm.nih.gov/pubmed/20543768 Level of evidence: B

14.  Anloague PA, Chorny WS, Childs KE, Frankovich M, Graham C. The Relationship between Femoral Nerve Tension and Hip Flexor Muscle Length. J Nov Physiother. 2015;5(244):2.

15.  Nadler SF, Malanga GA, Stitik TP, Keswani R, Foye PM. The crossed femoral nerve stretch test to improve diagnostic sensitivity for the high lumbar radiculopathy: 2 case reports. Archives of physical medicine and rehabilitation. 2001 Apr 1;82(4):522-3.

16. Uhunmwangho CU, Taiwo FO, Dung-Gwom PS. Femoral nerve stretch test predicts radiological features of lumbar spondylosis in Nigerians with low back pain. Highland Medical Research Journal. 2021;21(2):36-41.