Proper knee-flex reduces the "moving parts" of your golf swing, increases repeatability
When working with golfers of all abilities, it's important to develop a swing that is dependable and repeatable. There is a lot of talk among golf instructors about achieving full shoulder turns and more hip turn in the backswing to generate more power. Although a full shoulder turn is vital to a powerful golf swing, be sure not to compromise one key fundamental: right knee flex (for right-handed golfers).
The role of the right knee in the backswing is very simple, but incredibly important. The importance of the flex in the right knee is undeniable. The flex in the right knee helps keep the hips more level in the backswing. If the right knee straightens, it pushes the right hip up and consequently the left hip down resulting in too much left-hip tilt in the backswing.
To visualize this, stand in front of a mirror and make a backswing with your right knee staying flexed, and then make a backswing with your right leg locking. You can clearly see the change in how much your hips tilt in the backswing.
Where there is too much hip tilt to the left in the backswing, it's difficult to shift your weight properly to the right in the backswing. Therefore, the player inadvertently creates a reverse pivot, or reverse weight shift with too much weight on the left foot at the top of the swing.
Another key reason to keep the right knee flexed is how it is used in your transition into your downswing. Your right knee is a key source of driving power of your lower body in the downswing. It's very difficult to drive off a straightened right leg in the transition. Therefore, the upper body tends to dominate the downswing resulting in poor swing plane and loss of power.
In order to keep the right knee flexed in your backswing, try this drill. Put a basketball-sized ball between your knees and hold it very stable as you rotate into your backswing. Keep the ball squeezed between your knees as you complete your backswing. This will help you feel how the knee must stay flexed throughout the backswing.
June 2, 2008