Why Dogs Move Their Leg When Scratched

Most veteran dog owners are aware of a sweet spot on their dog’s stomach or lower body that, when scratched or rubbed, causes their dog’s leg to go crazy. Most people are able to produce this kicking or cycling motion in their dog at will. It’s a special kind of belly rub that brings about this distinctive reaction, and its origin is not as foreign to humans as we might think. 

When you go to the doctor for a physical, one of the tests involves your physician striking your knee with a blunt, hammer-like object. If all is good and well, your leg probably jerks out involuntarily, just like your dog’s leg seems to do when you scratch its belly. If you’ve ever wondered what’s really going on when you scratch that sweet spot, here are some reasons and explanations for it.

Natural reflexes are responsible for both your dog’s leg kicking when scratched, and your knee-jerk response when struck. The reflex related to your dog’s kicking is called a “scratch reflex,” and a similar reflex can be found in most other mammals and land-based species. It is completely normal, nearly universal in dogs, and is in fact more concerning if not present. By definition, reflexes are involuntary, so you may notice that your dog seems confused or unaware of that fact that its leg is kicking out wildly. Even when your dog is sick or exhausted, you can still activate the scratch reflex and cause the reaction. 

Scientists believe that the leg-jerk response is a natural defense mechanism against insects and small parasites. Dogs have nerves under their skin that cause their legs to shake when tickled, irritated, or stimulated. Other animals, including horses and large land animals, have similar mechanisms to combat flies and small irritants that land on their bodies. If you have ever involuntarily swatted at a fly that landed on a sensitive part of your body, then you have also demonstrated similar built-in defense mechanisms. Dogs that suffer from allergies are more likely to have a strong scratch reflex response, since they tend to suffer from more surface irritants than dogs without allergies.

Many people automatically assume that their dogs love being scratched in that special location—typically somewhere on the belly, chest or sides of the body. While it is true that some dogs seem to like having their scratch reflex stimulated, this is not the case across the board. Dogs may or may not enjoy it when you stimulate their scratch reflex, and different dogs will react differently. Some people don’t mind having their knee-jerk response tested, but this doesn’t mean that it would feel good if the doctor continued to repeat the test over and over again. The only way to check whether or not your dog genuinely enjoys being rubbed or scratched in that special spot is to watch its body language closely.

Your dog’s body language will tell you whether or not being scratched in the sweet spot is enjoyable or not. Generally, a dog that enjoys being scratched where its scratch reflex resides will stay relaxed, open its mouth, wag its tail, and have its tongue hanging out while its leg is shaking all over the place. If your dog doesn’t enjoy you striking the scratch reflex, it may tense up, become stiff, close its mouth, flatten its ears, and attempt to roll over or move away from you. Despite the natural assumption that dogs enjoy having their scratch reflex activated, it is more accurate to assume that dogs tolerate or dislike it.

Sometimes, you will notice that your dog exhibits the scratch reflex reaction without you being there to stimulate it. At times, seemingly randomly, your dog might shake its leg out while lying down or lean over and kick repeatedly. Sometimes, you may even incidentally strike a new scratch reflex zone that wasn’t there before. In all of these cases, there may be something else going on with your dog causing it to move its legs. As nature intended, the presence of fleas or irritating parasites on your dog’s body may activate the scratch reflex and alleviate your dog’s pain. If you are causing the reaction in an area that typically does not bring on the scratch reflex, your dog may be experiencing soreness or sensitivity in the area that you should be aware of.

Veterinarians use the scratch reflex as a test to check for neurological health. Since the nerves responsible for the scratch reflex are closely connected to the spine and brain, an absence of reaction could be cause for concern. Nerve damage is the most likely cause of a lack of a scratch reflex in your dog. If this is the case, it should be addressed sooner rather than later. Imagine if you could not immediately feel an imminent threat to your body—something burning your skin—and failed to reflexively react to the threat. Reflexes and instant reactions are a part of physical health and wellbeing, and you can become aware of your dog’s nerve condition before your veterinarian if you are observant.

Next time you rub your dog’s belly and cause its leg to spasm like a jackhammer, watch closely to make sure that your dog is having as good a time as you are. A dog wouldn’t approach you and flip over if it didn’t want a good belly rub, but you should still know whether or not you are scratching an itch, or simply causing a reflexive reaction to occur.