Playing tennis is the coordination of the whole body muscles, and it is unscientific to just say that you want to exercise those muscles. In fact, the landlord doesn't have to go to the gym to exercise which muscle. As long as he practices tennis against the wall at home, his main muscles will swell over time. If you want to hurry, you can find an iron bar (something you can catch) that is several times heavier than a tennis racket and hold it in your hand to imitate the jumping and waving of tennis. This effect is also very good.