The Knee-Jerk Reflex: What the Doctor Is Testing

The knee-jerk reflex is a spinal stretch reflex involving the quadriceps. With the knee hanging freely, tapping the patellar tendon stretches the muscle, triggering a contraction that jerks the lower leg. This tests the integrity of the sensory and motor neurons in the reflex arc.

Abnormal responses (absent, brisk, asymmetric) can indicate spinal cord lesions, peripheral neuropathy, upper motor neuron damage, or other neurologic disorders.