One way to improve the strength and speed of the muscles involved in the serve is to workout with a weighted fooball.
This is sometimes called a “medicine football” however you will find them faster on the Internet if you search for the term “weighted football”.
Most tennis videos will tell you that the serve motion in tennis is similar to the throwing motion of a baseball or football. By using a weighted football you can simulate this motion while working the muscles involved with the extra resistence of the weighted football.
Being a big guy I thought I should use the heaviest weighted football I could find which was in excess of 3 pounds. This was a mistake. Get a 2 pound or even use a regular football to start because the 3 pound football will tear your arm right out of the socket if you aren’t careful.