The principle of cross training for golfers is to improve mobility, flexibility, stability, balance, strength, endurance, and power to help create a profienct swing. Cross training is something other than swinging the golf club.
The repetitive nature of the golf swing can lead to muscle imbalances, sore joints and muscles and injury. When you swing at a ball, the body is continually replicating the same movement with force that can lead to developing a series of opposing strengths and weaknesses.
For this reason it is important to cross-train and do exercises that help balance out your muscles. But not just any exercises: you need to do balancing exercises.
The NASM or National Academy of Sports Medicine recommends that people do a minimum of 2 days of resistance or strength training per week. Regular resistance training can help you improve your strength, flexibility, coordination and agility. All things needed for a efficient golf swing.
When you train to get stronger, first, your muscles are better able to tolerate the movements and stresses the golf swing puts on your joints. Second, strength training makes you stronger to swing, not only faster, but also smoother and easier. And third, strength training may help prevent injuries so that you can keep playing the game longer and better.