Imagine this: You’re at the doctor’s office, and instead of one quick tap with the reflex hammer—the kind that makes your leg kick out without you meaning to—they just…keep going. Tap, tap, tap, right on that same spot below your knee, for a solid two minutes straight. It wouldn’t hurt y ou. But after a while, you’d probably start feeling pretty strongly about wanting it to stop—and if they didn’t, you might start wanting to kick someone on purpose.  Now imagine you’re a dog, and someone finds your “sweet spot”—that one particular patch of skin on the side or back—and won’t quit scratching it. Suddenly, your back leg is kicking without your permission, over and over, like it’s got somewhere better to be. As adorable as it looks from the outside, it’s probably not exactly a party for you. “It’s an annoyance,” says Nancy Thomas, DVM, a veterinarian with Telavets, an online veterinary expert consultation service. “It doesn’t do any good to continue doing it. The spinal cord is trying to say ‘get off of me.’” So what is the mechanism behind your pup’s leg acting on autopilot? Your dog’s leg has a mind of its own  So it might look like your dog is practicing to bend it like Beckham, or maybe is angling for a spot with the Rockettes, but what is really happening is entirely out of your pet’s control. It’s something called the pruritic scratch reflex.  It’s a reflex arc, a shortcut in the nervous system that bypasses the brain altogether to allow for an instant response. When something (your hand, a flea, a burr caught in the fur) touches the skin in that spot, sensory nerves pick up on it and send a signal straight to the spinal cord. The spinal cord doesn’t pause to think it over, according to Thomas; it’s the same shortcut your body uses when you touch a hot stove, although your dog’s involuntary kicks do not involve pain. “You touch a stove, and the signal goes from the sensory nerves up to the spinal cord, and the spinal cord immediately sends a message back to your finger to get off the stove,” Thomas says. “It’s a millisecond kind of a deal.” When dogs twitch during pets, it’s not because they like it. Video: That’s the Spot, @TheDogRufferee The reason dogs have the pruritic scratch reflex basically comes down to pest control: that particular area along your dog’s flank is prime real estate for fleas, ticks, mites, and burrs—the kind of small, easy-to-miss irritants that can burrow in or bite before a dog even notices. Rather than relying on conscious awareness to catch the pest, dogs’ built in a shortcut: If something touches down in that zone, the leg kicks automatically on the off chance it’s actually a threat trying to dig in.  It’s the same basic reflex that makes a horse’s skin twitch to shake off a fly, says Thomas, who also created an emergency medical ID tag for horses and their riders. “It’s a mechanism designed to keep nefarious things off the body,” Thomas says. It’s not the same as tickling A very common misunderstanding around the pruritic scratch response is that it is a form of tickling, and it’s easy to see why: your dog is responding in a riotous, involuntary, and rather comical way to a physical trigger.  But as gleeful as it looks when your dog breaks into those rapid-fire leg kicks, the experience is probably neutral at best for your dog. By continuing to do it, you could be “perpetuating the annoyance,” according to Thomas. Even tickling isn’t universally and unequivocally loved. “A lot of people hate being tickled,” she says.  Related 'Ask Us Anything' Stories Do dogs smile? Not like us. Why your dog eats grass Test your dog or cat’s IQ using these simple tricks Why do dogs tilt their heads? It isn’t just cute. Could raccoons become the new dogs? Are some dog breeds really easier to train? Not really. Why your dog gets so excited to see you Do dogs dream? The answer might make you appreciate your pup even more. A scratch your dog will actually enjoy To be clear, this doesn’t mean your dog is necessarily having a miserable time every time you find that spot. In general, the reflexive leg kick is just proof that your dog’s built-in protective wiring is working exactly the way it’s supposed to. But there’s one pattern worth keeping an eye on. If your dog reacts almost anywhere you touch them, or seems unusually sensitive even on their belly (which is normally a calmer, less reactive area due to its lack of contact with the ground), that’s worth mentioning to your vet.  “That’s a possible red flag for atopy,” Thomas says, referring to allergies or skin inflammation that leaves the whole body more sensitive. It’s similar to how a patch of poison ivy makes even light touch uncomfortable. “That might be a bit of a sign,” Thomas says. “These dogs might be a little extra sensitive, due to a skin condition.” To make bonding with your dog more enjoyable for both of you, Thomas advises sticking to areas not commonly associated with the pruritic scratch reflex, such as the belly. She also recommends watching your dog’s body language to ensure they’re having as much fun as you. It’s a much better way to ‘get your kicks’ without turning your dog into a full-on kickboxer.  “If you’re rubbing their belly, and they’re all spread out and relaxed-looking, that’s a sign you’re on the right track,” she says.  In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us. The post Why do dogs’ legs shake during pets? It’s not because they like it. appeared first on Popular Science. ...read more read less