Research has shown that the key lower body muscle used in the golf swing is the gluteus maximus (butt). While the key upper body muscles are the pectoralis major (chest), latissimus dorsi (back), core and forearm muscles.
The gluteus maximus is shown to be responsible for hip external and internal rotation on the backswing and downswing as well as hip extension which is what gets you into a nice balanced posture during your finish. The pectoralis and latissimus are shown to create strong shoulder adductors which allows you to bring your arms across your body and raise them up in the air. The core which I consider the glue that holds the swing together transmits force from the lower body into the upper body and helps you rotate your torso, last but not least the forearms transmit all of the force created by the body into the club.
What this means to you is that if your thinking about getting your body in shape for the golf season then start with the King or your gluteus maximus followed by your chest, latissimus dorsi, your core and finally your forearms.