The arm is a very mobile limb, and with that mobility comes the need for really good stability and control, which will allow you to use the arm in all sorts of manner, efficiently and successfully. Sadly, a lot of humans don’t have enough control over the shoulder for the things they want to do, which means they recruit more support from areas that don’t encourage efficiency. Like anything, if it’s not efficient, there’s a good chance it will break down, and Tennis Elbow is just one way the chain can break down.
The muscles that bend your wrist backwards run from your fingers and wrist, along the back of the forearm to the outside of your elbow. These are the ones that get upset when you have Tennis Elbow. They are often expected to do more than they can – go back to what was said above, something in the shoulder or the neck isn’t not functioning properly, so these muscles are compensating. When they are overused, they pull hard on their connection to the elbow and this creates irritation, leading to pain (in basic terms).