[Yoda]

In another thread, Yoda stated that a golfer needs a i) pivot center and that the pivot center needs to be centralised between the feet.

Of all HKs' ideas I have never understood why there needs to be a pivot center for the swinging arms/clubshaft.

To maintain the Point of Compression (2-C-0) through Impact and for consistent Directional Control (2-D-0), there must be a Centered Arc. Or, a "compensating manipulation". In turn, a Centered Arc requires that both the Pivot and the Arm/Clubshaft Swing be Centered. In other words, the Pivot has its Center, and the Primary Lever Assembly (Left Arm and Club) has its Center. They are not the same.

I can understand a grandfather clock having a pivot center located at the fulcrum of the pendulum because i) the length/radius of the pendulum arm is constant and ii) the low point of the pendular arc is constant.

Yes, this is the mechanical ideal. A centered arc with a constant radius and a low point directly opposite the hinge pin.

However, golfers don't have a consistent ball position and/or a consistent low point - relative to the center of their stance.

A Centered Arc is indifferent to Ball Location and Stance. Hence, Low Point also is indifferent.

Also, the length (radius) of the left arm/clubshaft swinging structure varies continuously during the downswing.

Ideally, the Left Arm remains fully-extended from Start Up to the end of the Follow-Through (6-A-4). The Left Wristcock shortens the Radius (Left Arm and Club) to produce Power through mechanical advantage (2-P; The Glossary / Lever Assemblies and Wristcock).

Also, the fulcrum (left shoulder socket) of the left arm/clubshaft structure moves in space continuously and has a variable distance from the required low point (a set number of inches ahead of the ball).

Indeed, the Left Shoulder ideally is moving in a circle. This requires its own Center (either the Head or the 'Point between the Shoulders'). Otherwise, the Clubhead Arc becomes Uncentered (2-H).

Also, although the low point of the swing is roughly opposite the left shoulder socket area, the left arm/clubshaft length/radius is not necessarily at its greatest value at that time point. The left arm/clubshaft's length/radius is at its greatest value when the left arm is maximally straight and the clubshaft is maximally straight-in-line with the left arm, and that occurs at the end of the followthrough.

Actually, the Left Arm and Clubshaft Radius is at Full Extension within one inch past Impact (2-D-0 / 7th edition). It is this "Right" Timing (6-F-1) that produces Maximum Compression "near -- but prior to -- Full Extension". This Full Extension is independent of the Follow-Through, which by definition (8-11) requires that the Right Arm also be straight (6-A-4).

Maybe that is why HK didn't state that a stationary head is an imperative, but more imprecisely classified it as an essential requirement. Rhythm and balance are also imprecise concepts that are classified as essential requirements.

There is nothing imprecise in Mr. Kelley's terminology, classifications or concepts. The nearest dictionary will provide the necessary "standard of precision" (1-H) for the chosen terminology and classifications. Deliberately using that 'dictionary English' (Preface) -- as opposed to scientific or engineering jargon -- the book itself defines the concepts of Stationary Head, Balance and Rhythm. Its intentional brevity demands that the complete definition of any concept be the sum of all the available references (1-H). Nevertheless, an adequate definition for each is given in The Glossary.

To make any further discussion interesting/informative, I am including a series of photos showing the pivot center of different golfers. I drew red lines through the pivot center (defined as a point midway between the shoulders) at address. I then left the red line constant, and captured two additional images - at the end-backswing and at impact.

From this swing video -

Gary Player



Arnold Palmer



Jack Nicklaus



Mike Souchak



Here is another swing video that is pertinent to this discussion

From this Natalie Gulbis/Paula Creamer swing video -

Natalie Gulbis



Paula Creamer

Regarding the males, perhaps there is a mechanical reason why Mr. Souchak, as good as he was, never won a major championship. Regarding the females, well, let's just say that their actions are "less than ideal" and require a good deal of "compensating manipulation" (see 1-K and my first answer above).

If I were considering essential requirements (and not imperatives) for a golf swing, I would consider removing "stationary head" and inserting "set distance of the left shoulder socket from the ball at impact".

Mr. Kelley understood that the Left Shoulder is higher at Impact (7-8 ) than in the normal Adjusted Address position (8-3). Further, he gave us a comprehensive three-step routine for accomplishing that specific objective with unprecedented precision (2-J-1). Fortunately, he understood that these adjustments were a means to an end -- creating a uniform Arc -- and not the end in themselves.