It's almost like a circus trick -- you hit that sweet spot on your dog's belly, and she can't help but kick her leg in a scratching motion. She's not telling you to stop, and it's likely she doesn't want you to. The leg movement is involuntary and normal.
When your dog moves her leg when you scratch her, it's called a scratch reflex. It's the same type of involuntary movement that occurs when the doctor knocks your knee to see if your leg jumps. It doesn't hurt your pup, and she's not trying to tell you to stop scratching. Quite the opposite -- most dogs love to be petted, and she'll just walk away if she's not interested. The origin of this reflex isn't completely clear, but it's likely the reflex developed as a way to stop annoying pests such as fleas from getting a foothold on your dog's skin.
Where It Happens
Scratching your dog's head is unlikely to result in some unexpected leg movement. Roll her over to her back and attack her belly with some loving scratches, and it's likely to be a different story. The most likely areas that cause the scratch reflex are her belly, her sides and low on her back. The exact sweet spot might be different on every dog, but most dogs have a spot that causes the reflex.
The scratching reflex itself is normal, but the lack of one might signify a problem. Vets often use the scratch reflex, or lack thereof, to determine if your dog has nervous system or spinal cord problems. If your pup suddenly stops exhibiting the scratch reflex or performs it at odd times, such as when she's standing still or walking, it could be the sign of a neurological or nervous system disorder. Talk with your vet if you're concerned about your dog's scratch reflex.
Your pooch isn't trying to hurt you when she's moving her leg, but watch your arm just in case. Some dogs kick exuberantly, and they aim toward the scratching sensation. Since this sensation is caused by your hand, your arm can get in the way of her nails and she might inadvertently scratch you in return. Keeping her nails well trimmed can help eliminate her scratching you while you're scratching her.