Everyone who has ever owned a dog is familiar with it, and yet every time your dog does it you still laugh. When you scratch or rub your dog’s sides or belly, you will hit certain spots that make his or her back legs start kicking. It seems so comical to watch your dog kicking their hind legs like crazy while you give them a belly rub, but did you know that there is a scientific reason that it happens? It is an involuntary reflex that is built into their instincts, just like it is your involuntary reflex to pull you hand away from a flame as soon as pain starts. Your dog’s reflex is based upon self-preservation, just like yours is.
When your dog kicks their back legs during a belly rub, it is because you are triggering their brain to react to remove the source of the tickling sensation, because in nature this may be a tick, a flea or a dangerous parasite. Veterinarian opinions on the reflex have been published in magazines like Popular Science, where noted veterinarians attribute the reflex to the same kinds of triggers that cause humans to pull their hand away from pain, even before they realize it. If a person’s brain was to wait until the thought process realized that something is painful and potentially dangerous, then the damage has already been done. Brains are wired to react even before the conscious mind realizes what is going on in certain dangerous situations, just like your eyes automatically closing when you hear a loud noise. In dogs, the tickling sensation triggers their reaction to remove the source of the tickling, because it is usually something trying to harm the animal.
Many veterinarians use the scratch reflex to test for neurological issues in dogs, as when they stop reacting in this way it may illustrate neurological damage. Very much in the same way that your doctor tests your reflexes using a rubber hammer on your knee, your veterinarian may scratch your dog’s sides or belly during an exam, looking for the kicking reaction.
So now you know.