How should I assess lower extremity deep tendon reflexes, including technique, normal findings, and interpretation?

Assessment of Lower Extremity Deep Tendon Reflexes

Technique for Eliciting Reflexes

Position the patient supine with knees slightly flexed and muscles relaxed for at least 10 minutes before testing to allow hemodynamic stabilization and reduce paratonia, which is particularly important in elderly patients. 1

Patellar Tendon Reflex (Knee Jerk)

• Standard method: Strike the patellar tendon directly below the patella with the reflex hammer while the patient sits with legs dangling freely 2, 3
• Alternative "superior patellar supine" method: With the patient supine, place your hand under the knee to create slight flexion, then strike the superior aspect of the patella—this method is superior when standard technique fails, particularly in elderly patients with paratonia 4
• Apply the hammer tap at approximately 21-50 Newtons of force for normal reflexes, using a controlled tapping angle to ensure reproducible input 3, 5
• The reflex arc involves the L2-L4 nerve roots, primarily L4 1

Achilles Tendon Reflex (Ankle Jerk)

• "Plantar strike" method (preferred initial approach): With the patient supine or sitting, dorsiflex the foot slightly and strike the sole of the foot at the metatarsal heads—this indirect method was best or equal-best in 64% of cases when standard methods failed 4
• "Achilles strike elevated" method: Elevate the foot with slight dorsiflexion and strike the Achilles tendon directly—this was best or equal-best in 71% of cases 4
• The reflex arc involves the S1-S2 nerve roots, primarily S1 1

Normal Findings and Grading

Assess reflex "briskness" by calculating the quotient of knee excursion divided by peak tendon tap force, which provides better discrimination between normal and pathological reflexes than amplitude alone. 3, 6

Grading Scale Interpretation

• 0 (Absent): No visible or palpable muscle contraction—suggests lower motor neuron lesion 1
• 1+ (Diminished): Slight contraction, may require reinforcement maneuver—may indicate lower motor neuron dysfunction 1
• 2+ (Normal): Visible muscle contraction with slight limb movement 2, 3
• 3+ (Brisk): Exaggerated response with full limb movement—may suggest upper motor neuron lesion 1
• 4+ (Hyperactive with clonus): Sustained rhythmic contractions—indicates upper motor neuron dysfunction 1

Technical Considerations

• Use consistent tapping force: 0-20 Newtons for suspected hyperreflexia, 21-50 Newtons for normal reflexes, and >50 Newtons for suspected hyporeflexia 3
• The Taylor hammer has a ceiling effect in the hyporeflexic range due to its small mass and short handle; use a Babinski or Queen Square hammer when hyporeflexia is suspected 3
• Mount a dome-shaped rubber pad on the patellar tendon at the most sensitive spot to reduce reflex variability significantly 6

Clinical Interpretation

To determine whether reflexes are pathologically increased or decreased, you must assess three critical comparisons: left-right symmetry, upper versus lower extremity balance, and overall limb reflex pattern. 2

Lower Motor Neuron Patterns

• Diminished or absent reflexes occur with peripheral nerve disorders, nerve root compression, or muscle diseases 1
• Common causes include lumbar radiculopathy (L4 for patellar, S1 for Achilles), peripheral neuropathy, or muscular dystrophy 1
• When hypotonia and weakness accompany diminished reflexes, measure serum creatine kinase—values >1000 U/L suggest Duchenne muscular dystrophy 1

Upper Motor Neuron Patterns

• Increased reflexes with abnormal plantar reflex (Babinski sign) indicate upper motor neuron dysfunction from spinal cord or brain lesions 1
• Hyperreflexia may be accompanied by spasticity, clonus, and loss of superficial abdominal reflexes 1

Asymmetry and Focal Findings

• Unilateral reflex changes suggest focal nerve root compression—absent patellar reflex indicates L4 radiculopathy, absent Achilles reflex indicates S1 radiculopathy 2
• Asymmetry or absence of protective reflexes suggests neuromotor dysfunction requiring further evaluation 1

Common Pitfalls and How to Avoid Them

Patient-Related Factors

• Paratonia/frontal rigidity in elderly patients makes standard techniques unreliable—if standard methods fail, immediately proceed to alternative techniques rather than documenting "absent reflexes" 4
• In elderly patients, using only standard methods incorrectly classified reflexes as absent in 37% of knee jerks and 84% of ankle jerks; supplementing with alternative methods reduced error rates to 19% and 21% respectively 4
• Inadequate muscle relaxation produces falsely diminished reflexes—use the Jendrassik maneuver (patient hooks fingers together and pulls) to enhance reflexes when initial attempts fail 2

Anatomical Variations

• When clinical findings don't match imaging, consider lumbosacral transitional vertebrae, nerve root malformations, or furcal nerve variants that alter the expected reflex pattern 2
• Perform reflex assessment after gait loading tests or standing extension loading tests to unmask latent lumbosacral neuropathy 2

Technical Errors

• Inconsistent tapping force produces unreliable results—maintain tapping velocities that are linearly proportionate to tapping angle for reproducible input 5
• Measure reflex loop delay from onset of tapping force to onset of reflex torque, not just reflex amplitude, for more precise quantification 6
• Never rely on a single reflex assessment method in elderly patients—if the standard technique doesn't produce a definite reflex, immediately use alternative positioning 4