Just like humans get tense when anxious or scared, a dog will stiffen up for the same reason. In frustrating, stressful, or even exciting situations, a dog might scratch (or bite, especially their paws) even if they're not itchy.

What does it mean when your dog tenses up?

When your dog participates in strenuous physical activities or doesn't get enough fluids, muscle contractions may be interrupted, resulting in localized spasms. Spasms are often a sign of muscle strain or physical damage.

Why does my dog tense up when I touch his back?

Ask any expert and the majority will agree that there is no deep hidden meaning behind your dog acting weird when you scratch his or her back. In fact, the answer most often given is that it simply feels good. ... This area at the base of your dog's tail is full of sensitive nerve endings.