Why does your arm hurt when you play tennis?

Answer 1

It could be that you are using too heavy of a racquet, or even too light of a racquet.

If you are using too heavy of a racquet, then you are using your full force to just swing the racquet to hit the ball. If you are using too light of a racquet, then you are using all of your power just to return the ball because the racquet doesn't provide enough resistance to hit it back.

It could also be that you are using improper form.

Improper form leads to hurting arms, especially tennis elbow and hurt wrists. If you are only using your arms and not moving the rest of your body into position, that could be the reason your arm hurts - your form requires that your arm does all of the work.