How do you diagnose quadriceps (quadriceps muscle) extensor paralysis?

How to Check for Quadriceps Extensor Paralysis

To diagnose quadriceps extensor paralysis, perform a straight leg raise test with the patient supine: inability to actively extend the knee against gravity or maintain full extension indicates quadriceps weakness, and complete inability to perform this maneuver confirms paralysis. 1

Primary Clinical Assessment

Active Extension Testing

• Position the patient supine with the knee fully extended and ask them to lift the leg off the examination table while keeping the knee straight 1
• Complete quadriceps paralysis manifests as inability to extend or lock the knee in extension, resulting in giving way of the knee joint 2
• Observe for compensatory movements: patients may attempt hand-to-knee gait, extreme internal or external rotation of the limb to stabilize via collateral ligaments, or pelvic tilting 2

Extension Lag Assessment

• With the patient sitting, ask them to actively extend the knee from a flexed position and measure any gap between achieved extension and full extension 2, 3
• An extension lag (inability to achieve terminal knee extension) is a hallmark finding, with mean extension lags of 30 degrees commonly observed in quadriceps paralysis 3
• This test is particularly sensitive when performed in the sitting position where gravity works against the weakened muscle 3

Motor Strength Grading

Standardized Strength Testing

• Evaluate muscle strength using the Medical Research Council (MRC) grading scale, with grade III or less indicating significant weakness requiring intervention 2
• The American College of Emergency Physicians recommends evaluating muscle strength in both lower extremities to identify patterns suggesting upper motor neuron versus lower motor neuron pathology 1
• Express measurements relative to normal values for age and sex when using dynamometry 4

Quantitative Force Measurement

• Use a dynamometer (isocinetic, handheld, or chair-mounted with ankle tension meter) to measure isometric voluntary maximum force of the quadriceps 4
• Proper patient fixation is essential to avoid compensatory movements during assessment 4

Reflex and Sensory Examination

Deep Tendon Reflexes

• Test the patellar tendon reflex (knee jerk): diminished or absent reflexes suggest lower motor neuron pathology affecting the quadriceps 1
• The Society of Critical Care Medicine recommends testing plantar responses (Babinski sign) bilaterally, as upgoing toes with diminished knee jerks suggests combined upper and lower motor neuron pathology 1

Sensory Testing

• The American Academy of Neurology suggests testing proprioception at the great toes and ankles, as diminished knee jerks with sensory loss suggests peripheral neuropathy or posterior column dysfunction 1
• Assess pain and temperature sensation in a stocking distribution to evaluate for length-dependent neuropathy 1

Anthropometric Measurements

Thigh Circumference

• Measure thigh circumference 1 cm below the gluteal line on the right leg or below the gluteal fold on the left leg to assess muscle atrophy 4
• Thigh circumference correlates strongly with total muscle mass and provides a simple, non-invasive method for tracking muscle wasting 4
• This measurement is particularly valuable as it is not affected by fluid retention and can be performed rapidly at low cost 4

Advanced Diagnostic Modalities

Electromyography (EMG)

• EMG during submaximal voluntary isometric contractions at 0° extension in the supine position can detect arthrogenic muscle inhibition patterns, showing quadriceps inhibition with hamstring hypercontraction 5
• Surface EMG of the vastus medialis and vastus lateralis during isometric contraction provides objective evidence of muscle activation failure 5

Imaging Studies

• Ultrasound can visualize the quadriceps muscle and tendon, allowing assessment of muscle architecture, tears, and atrophy 6
• MRI provides excellent tissue contrast for simultaneous assessment of muscle, joint, and bone, though it remains a second-line study due to cost 6

Critical Red Flags Requiring Urgent Evaluation

• The American College of Emergency Physicians recommends expedited workup for acute onset of quadriceps weakness with bowel/bladder dysfunction, suggesting cauda equina syndrome 1
• Consider Guillain-Barré syndrome in patients with progressive weakness ascending from the legs 1
• Evaluate patients with bilateral absence of all reflexes and altered mental status for critical illness polyneuropathy 1

Laboratory Workup for Underlying Causes

• The American Academy of Neurology suggests ordering fasting glucose/HbA1c, vitamin B12, folate, TSH, and comprehensive metabolic panel to evaluate for peripheral neuropathy as a cause of quadriceps weakness 1
• Consider hypothyroidism as a potential reversible cause of diminished reflexes and muscle weakness 1