The knee jerk reflex is a common physical examination in neurology and orthopedics. It is a tendon reflex, a response in which a rapid tap on the knee tendon while the knee is half bent and the lower leg is free to drop can cause the lower leg to kick forward sharply. If the knee reflex is weak or absent, it is usually seen in patients with spinal cord or peripheral neuropathy, which is a sign of lower motor neuron palsy; if the knee reflex is hyperactive, it indicates upper motor neuron palsy, and some patients with mental stress or blood calcium abnormalities, as well as those with tetanus and hyperthyroidism may also have hyperactive knee reflexes. The knee-jerk reflex may also be hyperactive.