What does it mean when a kitten arches its back?
Stretching to make the body feel more comfortable; being startled and being in a state of high tension; hunting to "recharge"; being angry, at which point the fur on the back will stand up; scaring away enemies and wanting to appear stronger; playing and practicing, wanting their owner to play with them; seeking; being sick. 
Stretch
When cats sleep or lie in one position for a long time, their bodies can become numb. So, to make themselves more comfortable, cats will stretch, arch their backs, or press their backs down. This is just the cat stretching its muscles and is a normal phenomenon; owners don't need to worry too much.
startled
When a cat is startled, it becomes highly stressed. Arching its back allows it to react more quickly, preparing to fight or flee. This is often accompanied by puffed-up fur and hissing, all to scare away any perceived threat. For example, startling it with a toy, shouting at it, or seeing a dog can all cause a sudden fright, leading to an arched back. In this situation, the owner should not approach the cat, as it could easily attack. It's recommended to wait until the cat has calmed down before trying to soothe it.
hunting
Cats also arch their backs when hunting, mainly to conserve energy so that they have more explosive power when jumping or sprinting, thus increasing their success rate in hunting!
angry
When cats are angry, they will arch their backs and the fur on their backs will stand up to express their dissatisfaction. At this time, it is best for owners not to provoke them.
Scare away the enemy
When a cat encounters an animal stronger than itself, it will arch its back. This is because the cat wants to appear stronger and thus scare away the enemy.
Play and practice
If they are kittens, they might run around with their backs arched for no apparent reason, looking rather nervous. This is actually a signal from the kittens inviting their owners to play with them and experience the fun of hunting. If there are two or more kittens in the house, this is very common. The two kittens, with their fur puffed up and backs arched, will chase and play with each other, which is also a process of practicing hunting.
seek
When a cat wants its owner to pet it, it will usually arch its back and lean forward when you touch its head, meaning it wants to be petted on the back as well. Additionally, when a cat wants to mark its scent with its owner, it will arch its back and rub against them, trying to transfer its scent. Marking a scent also signifies possessiveness and indicates that the cat is very satisfied with its owner.
Sick
The arched back caused by the first few reasons is temporary. However, if a cat maintains an arched back while walking for an extended period, it may be sick or injured. If the cat also exhibits symptoms such as meowing, trembling, or loss of appetite, it indicates that the cat is in pain, and it is recommended that the owner take the cat to a veterinary clinic for examination as soon as possible.
