Dr Sasho MacKenzie is a professor at St Francis Xavier University in Canada. He has a Ph.D in kinesiology and publishes research papers on the golf swing and golf equipment.(1) Recently his work has gained mainstream attention and he’s appeared in the golfing media to discuss his findings.

His particular advice to swing under plane during the downswing, or “shallow the shaft”, can be seen in a video with golf instructor Chris Como.(2) The origin for this advice can be read in Dr MacKenzie’s 2011 research paper “Club position relative to the golfer’s swing plane meaningfully affects swing dynamics”.(3)

In this study, Dr MacKenzie had created a computer simulation of a golf swing. By changing the parameters of this model, he was able to test different downswing characteristics and see what affect they had.

Dr MacKenzie performed three experiments in this study, each comparing two different simulated swings.

During this experiment, the manual forces (i.e. simulated muscles) that rotate the wrist during the downswing were switched off. This meant the left wrist could only rotate anticlockwise through impact (the 3rd Power Accumulator) passively via momentum and not actively via muscular effort.

This experiment showed that the under plane swing was able to rotate the left wrist passively so at impact the clubface was square to the target line. For the over plane swing, the left wrist wasn’t able to rotate enough passively and at impact the clubface was open by over 50º. The under plane swing also recorded a higher clubhead speed through impact.

This experiment was to measure the difference between an on plane swing, and an “over the top” swing. It discovered both swings were able to square the clubface at impact, but clubhead speed was much higher at impact for the on plane swing. Simulation 3’s clubhead speed at impact was also much higher than Simulation 1’s. This demonstrates the contribution forearm muscles make to clubhead speed through impact.

From the results of this study, Dr MacKenzie has suggested golfers begin their downswing with the clubhead below the swing plane. This helps facilitate the left wrist rotating through impact with the help of angular momentum.
Results show a clubhead below the swing plane can mean less muscular effort is required and more clubhead speed can be produced through impact.

It would appear Dr MacKenzie’s findings oppose my advice to swing on plane. That’s not the case and I’ll explain why below, but first let’s go through the study.

When conducting experiments like above, one thing is paramount - precision. For your measurements to be worthwhile, the golf swings observed must be exact. In Dr MacKenzie’s experiments he’s tested: the clubshaft on plane, 10º above plane, 10º below plane and a swing plane steepened by 10º.

It would be impossible for a real golfer to be this precise. In fact, it’s near impossible for a golfer to repeat the exact swing twice in succession.

The solution is to use a model - a simplified replica of a golfer that can be precise and manipulated exactly. Dr Mackenzie’s model is a computer simulation. It’s comprised of a torso, a left arm, and a golf club.
The torso rotates about one axis which is fixed. The left arm can move across the chest and away from the chest (horizontal adduction / abduction). The left wrist can rotate clockwise and anticlockwise (pronation and supination). It can also cock and uncock (radial deviation and ulnar deviation).

So are there any issues with this study?

To start, the model used is an incredibly simplified version of a golfer. It only has three parts and four moving joints. To put this into perspective, a real golfer’s feet and ankle joints are more complicated than the whole of Dr MacKenzie’s golfer model.

Now compare the available movement in this model to a real golfer’s torso and left arm.

The table shows Dr MacKenzie’s model, although only comprised of a torso, left arm and club, is significantly simplified yet again.

Given this, we must be mindful when comparing the model golf swing to our own. They’re clearly not the same, and so the model’s behaviour and the measurements taken from it may not be applicable to our own swings.

The second issue with this study are the experiments conducted. Dr MacKenzie wanted to test, generally, what effect the club position had relative to the swing plane. Having discovered how beneficial having the clubhead below plane was, it would have made sense to test this further.

Dr MacKenzie tested the following swings:

• 10º below plane - left forearm rotation muscles disabled
• 10º above plane - left forearm rotation muscles disabled
• On plane - left forearm rotation muscles active

For a more thorough understanding he could have also tested:

• On plane - left forearm rotation muscles disabled
• 10º below plane - left forearm rotation muscles active
• 10º above plane - left forearm rotation muscles active

With these additional results, Dr MacKenzie’s study would have better shown the effects of swinging over, under and on plane. As it stands, his study shows it’s better to swing with the clubhead below plane compared to above it - if you have disabled forearm muscles… Swinging on plane with active forearm muscles is better yet.

The final issue with this study, and the crux of why its findings aren’t at odds with swinging on plane, is how Dr MacKenzie defines the swing plane.

In The Golfing Machine, the swing plane is described as the “…flat, inflexible, inclined plane which extends well beyond the circumference of the stroke - in every direction. The full length of the clubshaft remains unwaveringly on the face of this inclined plane - waggle to follow-through.”(4)

In other words, the swing plane is the desired two-dimensional path the golf club takes throughout the stroke. The swing plane may increase and decrease in steepness throughout the stroke, but the clubshaft must remain on plane. A visual clue as to whether a club is on plane or not is to see if it’s pointing at the ball from the “down the line” perspective.

This definition of the swing plane is universal in golf… except for Dr MacKenzie’s study. Here he states: “The golfer’s swing plane was defined by the path of the lead hand, which was represented by a single point at the center of the wrist joint.”

In his study, Dr MacKenzie traced the left wrist during the downswing. He plotted three locations of it during the downswing and from those created a two-dimensional plane.

Dr MacKenzie’s swing plane isn’t the famous “pane of glass” seen everywhere in golf instruction - it’s just the hand path. These are two different things.

Below are the downswings of Rory McIlroy, Charley Hull and Dustin Johnson. The red lines show their hand paths from the top of the swing down to their current location.

You’ll note a few things: the hand paths aren’t straight lines - they’re all curved, beginning steep and shallowing as the downswing progresses. This is typical of precision golf swings. I’ll explain why shortly.

Dr MacKenzie’s study ignores this curved hand path. Instead, his model’s hand travels in a straight line when viewed from down the line. He states: “To most clearly demonstrate the influence of club position relative to the path of the lead hand (the golfer’s swing plane), the path of the lead hand was constrained to move in a set plane.”
This is yet another simplification of the swing in Dr MacKenzie’s study.

The second thing of note is in each picture, the clubshaft is pointing at the ball. This shows us the golf club is on plane, albeit the commonly understood swing plane.

Finally, you’ll see how the direction of the hand path and the clubshaft’s orientation are different. The hand path is steeper than the clubshaft’s plane.

Dr MacKenzie has defined the hand path (red line) as the swing plane, and his study shows the clubshaft ought to be shallower than this. His study is correct. But again, what Dr MacKenzie calls the swing plane isn’t the common use of the term - he’s referring to the hand path.

Dr MacKenzie’s study has shown that the hand path ought to be steeper than the swing plane. For almost all swings, he’s right - and here’s why…

If you’ve read about the “normal to the clubface” in this website’s D Plane chapter, you’ll know that even when a clubface is square to the target, it may not be pointing at the target. To avoid this, the clubface not only needs to the square to the target line, but the leading edge must also be horizontal.

This horizontal leading edge places the club in its natural swing plane. When looking at a club’s statistics, you’ll find this measurement called the lie angle. For a 7 iron, its lie angle may be around 61 degrees. In other words, when the leading edge of the clubface is horizontal, the clubshaft is 61º from horizontal.

To swing this 7 iron so that the normal of the clubface and the leading edge are both pointing at the same direction, the swing plane must be 61 degrees.

In an ideal world, a golfer would set up at address for this 61 degree swing plane, and perform the whole swing on that plane angle. However, 61º, and indeed all golf club’s lie angles, are too shallow to be of much use for a full swing. The arms wouldn’t be able to rise very much during the backswing and it would limit the amount of power one could generate.

To get around this, most golfers steepen their swing plane during the backswing. When viewed from down the line, the clubshaft remains pointing at the ball, but the angle between horizontal and the clubshaft increases as the backswing reaches the top.

To compensate for this newly increased plane angle, during the downswing, the plane angle must return to its shallower plane until, in the case of our 7 iron, it’s back to 61 degrees.

It’s this shift in swing plane angle during the downswing, that requires the hand path to be steeper than the swing plane itself. The hand path and swing plane would only be the same if the golfer swung their club on one fixed plane angle - a true “one plane swing”.

Currently, the only golfer who does so with any success is Bryson DeChambeau. A look at his hand path on the downswing shows it line with the clubshaft for a substantial amount of distance.

So in summary, I agree with Dr MacKenzie’s findings. For any stroke where the swing plane angle increases during the backswing, and decreases during the downswing (i.e. most swings), the swing plane ought to be shallower than the hand path.
I disagree with calling the hand path the “swing plane” - it leads to confusion and implies Dr MacKenzie is suggesting golfers swing off-plane.