Getting a full 90° shoulder turn on the backswing, as well as the forward swing, is very important in some sports in order to maximize hitting power. This requires a great deal of flexibility in the waist but also great strength of the abdominal oblique and lower back erector spinae muscles. The abdominal oblique muscles are responsible for rotating the shoulders forward while the lower back erector spinae muscles are responsible for rotating the shoulders to the rear.
This great flexibility and strength is especially important in sports such as golf when hitting drives, volleyball in spiking and baseball pitching. It is important in all other hitting and throwing sports skills but not to the same extent in regard to backward and forward rotation. In spite of the importance of midsection flexibility and strength, however, very few strength training programs address this issue.
This is especially true in golf. if you read the popular golf magazines or any books on fitness for golf, you will not find exercises that are specific to maximum midsection flexibility. Nor will you find exercises to strengthen the erector spinae muscles through a full range of motion for rotating the shoulders to the rear or exercises to strengthen the abdominal oblique muscles through a full 90° range of motion.
When these issues are addressed the recommendations are typically inadequate for full development of flexibility or strength . For example, it is often recommended that you keep the rear leg slightly bent and the kneecap directly over the foot when you rotate the shoulders to the rear. This is a very good suggestion for maintaining a stable leg position and for not allowing your body to shift laterally to the rear during the backswing.
However, when you rotate the shoulders to the rear there is almost always rotation of the hips. I have found very few golfers, even among professional golfers, capable of separating shoulder and hip rotation. In other words, when you do not have sufficient flexibility in the waist you must then rely on the hips to not only turn the shoulders to the rear, but to generate force to turn the shoulders in the downswing. As a result, you do not use the great power that can be generated from the abdominal oblique muscles.
It is important to understand is that it is not merely flexibility, but the strength of the lower back muscles that must turn the shoulders to the rear in the backswing. This is an often ignored fact and probably the main reason why most people do more flexibility and stretching exercises rather than strength and flexibility exercises. This is true of the abdominal oblique muscles and especially the erector spinae muscles of the lower back.
Thus to get a full 90° turn to the rear you should do exercises such as the reverse trunk twist which develops flexibility in the waist and is a great stretch for the erector spinae of the lower back. To strengthen the erector spinae of the lower back through the full range of motion you should do back raises with a twist.
For maximum power from the shoulder rotation in the downswing you should do exercises such as the Russian twist. This is perhaps one of the best strength exercises that develops both strength and flexibility as needed for a powerful swing. These exercises are demonstrated and explained in detail in the book Biomechanics and Kinesiology of Exercise.
See Explosive Golfhim him him him him for more information. For a video of how these exercises are executed see the Explosive Golf DVD.