What does it mean when a doctor hits your knee and move?

The knee-jerk reflex is a muscle stretch reflex involving the quadriceps muscles of the thigh. Its purpose is to preserve one's balance when the knee bends rapidly. With the patient's knee hanging freely (not touching anything), the doctor taps the patellar tendon with a reflex hammer. This stretches the quadriceps, activating a spinal reflex that causes them to contract. This is seen as a jerk of the lower leg. Damage to the sensory or motor neurons involved in this simple spinal reflex pathway can indicate various nervous system disorders.

By striking the patellar tendon, a neurologist can test for neural damage along this pathway. Absent, brisk, or asymmetric knee-jerk reflexes may be indicative of spinal cord lesions, peripheral neuropathy, or other problems in the motor or sensory pathways or a variety of other neurologic disorders. For instance, a brisker-than-normal reflex often indicates upper motor neuron damage that impairs the normal motor inhibitory function of the central nervous system.