Why do people have "knee jerk reaction"?

From the evolutionary point of view, knee jerk reflex is an anti-gravity reflex established after people walk upright. It centers on the spinal cord and aims to maintain the upright state of people. Because when people stand upright, the quadriceps femoris contracts, once stretched, it means that the knee joint will bend, so reflex contraction is necessary to maintain posture. Because this function is very important, it was gradually completed by the lower central spinal cord in the long evolution process.

In addition to the knee key, other muscles and muscle keys related to human anti-gravity posture also have similar reflexes, such as the heel key of human heel; Other animals, such as monkeys, have similar reflexes because their biceps are anti-gravity muscles because they hang on trees for a long time.