What are the reflexes in the lower extremities?

Lower Extremity Reflexes

The key lower extremity reflexes include the patellar (knee jerk), Achilles (ankle jerk), medial hamstring, lateral hamstring, and plantar reflexes, which are essential for neurological assessment and diagnosing various conditions affecting the lumbosacral spine and peripheral nervous system.

Primary Lower Extremity Reflexes

Patellar Reflex (L2-L4)

• Also known as the knee jerk reflex
• Tests the L2-L4 nerve roots, primarily L4
• Technique: Patient seated with legs dangling freely; tap the patellar tendon just below the patella
• Normal response: Extension of the knee (contraction of quadriceps)
• Assessment methods:
  • Standard method: Patient seated with legs dangling
  • Superior patellar supine method: Particularly useful in elderly patients with paratonia 1

Achilles Reflex (S1-S2)

• Also known as the ankle jerk reflex
• Tests the S1-S2 nerve roots, primarily S1
• Technique: Foot held in slight dorsiflexion; tap the Achilles tendon
• Normal response: Plantar flexion of the foot
• Assessment methods:
  • Standard method: Patient kneeling or with foot dorsiflexed
  • Achilles strike elevated method: Most effective in elderly patients 1
  • Plantar strike method: Also highly effective in difficult-to-assess patients 1

Medial Hamstring Reflex (L5-S1)

• Tests primarily the L5 nerve root
• Technique: Tap the tendon of the semimembranosus and semitendinosus muscles at the medial side of the popliteal fossa
• Normal response: Flexion of the knee

Lateral Hamstring Reflex (L5-S1)

• Tests primarily the S1 nerve root
• Technique: Tap the tendon of the biceps femoris at the lateral side of the popliteal fossa
• Normal response: Flexion of the knee

Plantar Reflex (L4-S2)

• Tests the integrity of the corticospinal tract
• Technique: Stroke the lateral aspect of the sole from heel to ball of foot, curving medially across the ball
• Normal response: Plantar flexion of the toes
• Abnormal response: Babinski sign (dorsiflexion of the great toe with fanning of other toes) indicates upper motor neuron lesion

Clinical Significance

Diagnostic Value

• Reflexes provide objective neurological assessment that is less dependent on patient cooperation than sensory or strength testing 2
• Asymmetry between sides or upper/lower extremities can indicate pathology
• Hyperreflexia suggests upper motor neuron lesions
• Hyporeflexia or areflexia suggests lower motor neuron lesions, peripheral neuropathy, or radiculopathy

Specific Patterns in Lumbar Spine Disorders

• L4 radiculopathy: Diminished or absent patellar reflex
• L5 radiculopathy: May have normal reflexes (as L5 has no specific reflex)
• S1 radiculopathy: Diminished or absent Achilles reflex 2

Special Considerations

Age-Related Changes

• Reflex development follows a non-linear pattern in early development
• Reflex irradiation (responses in muscles other than the one being directly stimulated) is maximal during the first year of life and progressively declines 3
• Elderly patients may require alternative techniques to elicit reflexes due to paratonia/frontal rigidity 1

Technical Challenges

• If standard methods fail to elicit reflexes in elderly patients, alternative methods should be used:
  • For knee reflexes: Superior patellar supine method
  • For ankle reflexes: Achilles strike elevated or plantar strike methods 1
• Jendrassik maneuver (patient interlocking fingers and pulling) can be used to reinforce and enhance reflexes

Common Pitfalls

• Incorrectly labeling reflexes as absent when they are actually present but difficult to elicit
• Failure to recognize that absent reflexes in elderly patients may be due to technical difficulties rather than pathology
• Not considering the overall pattern of reflexes in context with other neurological findings

Specialized Reflexes and Testing

Crossed Adductor Reflex

• Tapping one patellar tendon causes adduction of the contralateral thigh
• Indicates upper motor neuron lesion when present

Clonus

• Rhythmic, involuntary contractions when a muscle is suddenly stretched and maintained in that position
• Most commonly tested at the ankle by brisk dorsiflexion
• Sustained clonus (more than 3-4 beats) indicates upper motor neuron pathology

By properly assessing these reflexes, clinicians can gain valuable information about the integrity of the nervous system and localize lesions affecting the lumbosacral spine and peripheral nerves.