To get a full 90º turn of the shoulders on the backswing it is recommended that you keep the rear leg slightly bent and the kneecap directly over the foot. The goal is to turn as far as possible without letting the knee drift out of position. 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.
The key to getting a full shoulder turn with a limited hip turn, however, is to have ample flexibility in the waist. There is general agreement on this point and many believe that conditioning is the key to doing this. Most often you will hear that you must increase flexibility in the waist, lats and shoulders in order to make the full 90º turn while maintaining stability in the hips and rear knee.
However, having more flexibility in the lats and shoulders has little to do with turning the shoulders. These joints are most involved in how far the arms can go upward and to the rear after the shoulders have fully turned or simultaneously with the shoulder turn.
Regardless of the belief that physical conditioning is important, many pros do not bring out which exercises are needed. Merely becoming more fit does not mean that you will have the flexibility to execute a better turn. In fact, even if you did develop the flexibility needed, it does not mean that you will then have a better swing.
It is important to understand is that it is not merely flexibility, but the strength of the muscles that must turn the shoulders to the rear in the backswing that determines the range of motion in the shoulder turn. This is an often ignored fact and probably the main reason why many golfers do more flexibility and stretching exercises rather than strength exercises that can also develop flexibility at the same time.
Thus to get a full 90° turn to the rear, in addition to having ample flexibility you must also have the needed strength of the muscles involved to turn the shoulders not only on the backswing but also on the downswing. It does little good to be able to execute a full 90° turn to the rear if you don’t have the strength to rotate the shoulders forward in the downswing to create additional force.
The best exercise to gain both flexibility in the waist as develop well as strength of the abdominal rotational muscles as used in the downswing, is the reverse trunk twist. A more advanced exercise is the Russian twist done on the Yessis Glute Ham Back machine. To strengthen the muscles that are used in rotating the shoulders to the rear to get a full shoulder turn you should do back raises with a twist. This exercise involves the lower back erector spinae muscles located in the lumbar region.
See Explosive Golf for more information on the shoulder turn and the exercises that you can do to ensure a full turn. To see a video of technique execution of these exercises, see the Explosive Golf DVD in our store.