Maintaining Trail Knee Flex in the Backswing

[iacas]

When you've been teaching golf as long as I have, you're going to find that you can teach some things better than you previously had, and you're probably going to find some things that you taught incorrectly. I don't see that as a bad thing — what would be worse is refusing to adapt and grow given new information. I've always said that my goal with my instruction isn't to be right, but it's to get things right. To that end, I'm about five years late in issuing a public proclamation on something…

When I first got my GEARS system, I immediately looked at the golf swings of the dozens and dozens of Tour players for which I suddenly had full 3D data. I created a huge spreadsheet showing how their bodies moved, how the club moved, at various points in the swing. I mapped knee and elbow angles, hand speeds, shoulder turns and pelvis turns… etc.

I re-considered what I thought I knew about the golf swing as performed by the best players.

One of those things dated back to the earliest days: that you extend (I never taught "straighten" and would avoid using that word unless in the context of saying "don't fully straighten") the trail knee/leg in the backswing. I was mislead by 2D photos from less-than-ideal camera angles — the trail leg rotates a bit during the backswing, and so when observing trail knee flex should also use a camera that moves to stay perpendicular to the plane of the ankle/knee/hip joint.

We have at least two topics here on this (here and here; both of which I'll be updating after publishing this) where @mvmac and I advise golfers to extend the trail knee.

Learning that this was not right is one of the reasons I'm glad to have a 3D system, as most golfers generally preserve the trail knee flex throughout the backswing.

Data

Here's a video showing an iron and a driver of someone who has won the career slam:

Here's what the graph of his right knee flex looks like. The solid lines I've positioned at the top of the backswing (GEARS aligns both swings at impact, the dashed line). Address is to the right, of course, and the graph shows knee flex from the two swings above. The data (17.56° and 23.20°) shows where this player is in both swings (orange being the yellow iron swing, pink the blue driver swing).

You can see that this golfer extends his trail knee 2-3°… before bending it even more than that through the late backswing and early downswing.

Months ago I created a quick Instagram video showing the trail knee flex in the backswing of several players (see the top for the larger number):

Erik J. Barzeski (@iacas) • Instagram reel

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GEARS shares expert advice on golf swing technique, focusing on the critical backswing phase. Tour winners and major champions reveal the key to a precise and powerful swing, highlighting the importance of...

Here are a few more graphs.

Two LIV players and major champions:

Two PGA Tour winners:

Two women's #1 ranked players:

Two more PGA Tour winners (one a major champ):

Two former #1s, the left one being a woman, the right a man, with a driver:

Two more PGA Tour players:

You'll notice a trend: they almost all maintain roughly the same flex throughout their backswing and downswing.

The Issues with Extending the Trail Knee

You can play good golf extending (again, not "straightening") the trail knee. Some Tour players do. But, as with many things, if 95 out of 100 Tour players do it, you're most likely better off doing similarly to what they do.

So, what are the issues with extending the trail knee in the backswing? To list a few:

Pelvic Depth and Rotation Quality Suffers

When the trail knee extends, the trail leg often acts like an axle on the backswing, with the pelvis rotating around the leg and the trail hip joint. This prevents the trail side from gaining depth, as is needed to keep the pelvis center from thrusting toward the ball. Most of the "early extension" (thrust) that I see occurs during the backswing.

Encourages Early Extension (Thrust) Patterns

When you've thrust and turned around the trail hip joint in the backswing, you often thrust a bit more in the downswing as the direction your pelvis is oriented is forward and "out" (to the right for a righty). Your trail leg can abduct to push you forward, but "forward" when your pelvis is turned like that is in the "thrust" direction.

Additionally, the trail knee "breaking" again at the start of the downswing often jumps the trail hip out toward the ball a bit too much or too quickly. While the trail hip does move in that direction, if it's too fast or too much, it can prevent the lead side hip from getting "back" at the right rate, or at a rate commensurate with the trail hip to keep the pelvis center from thrusting.

Disrupts the Pressure Shift/Transition

When the trail leg extends too much, it often can't "push" forward normally. The forward push begins much earlier than forward motion begins — pushing forward begins as early as about P1.5 to P2 in the swings of most good golfers. It can push forward by abducting, again, but that's a weaker movement that shoves the pelvis forward (toward the target) and turns it more than it generally should (see the next point).

Limits Internal Rotation of the Trail Hip

Internal rotation of the trail hip is a sort of "limiter" on the backswing. I have seen many golfers on GEARS whose trail knee extends, whose pelvis shifts forward (toward the target), and who turn over 50°, 60°, and rarely but not never, over 70° in the backswing. If you turn 60° in the backswing, it's going to be almost impossible to get "open enough" in the downswing to arrive at a good impact position.

Swaying/Lateral Motion

Occasionally a golfer who extends the trail knee too much will shift back too far, but more often the issue is that the golfer will shift forward too early in the backswing (sometimes even immediately to begin the backswing), leaving them "stuck forward" to begin the downswing. They'll push forward, stop, and have to restart around P4, disrupting the smooth sequence often seen in the game's best players.

Other Bits…

Reduces ground reaction force potential, compromises spine inclination and posture, makes transition sequencing harder, increases stress on the trail knee and lower back…

In short…

It's not athletic. We don't do many athletic things with "straight" or very extended legs (unless it's the end of the action, like a jump or a big push off like a step in a running motion).

[Phil McGleno]

Good for you for learning and admitting it-I think you and I both still let some higher handicappers do a little bit because they have other things to worry about first. But trail knee flex I would not have even imagined pros kept as consistent as your graphs show.-Surprising!

Good for you.

[iacas]

On 12/28/2025 at 9:33 PM, Phil McGleno said:

Good for you.

Thanks but honestly… I don't know any other way.

I don't mind being wrong so long as I know where to go from there. I don't like being wrong — I'd love to get things right (which is different than "being right").

I recommend grabbing a furniture slider or a paper plate or something, and doing something like this:

First, make a swing where you let your trail foot swing out as you turn, then twist that foot back in. From DL and FO, it'll look like this:

Then, during a regular backswing, try to twist your foot in slightly (demonstrated in the left image):

You'll notice a crease along the trail side of your hip, your pelvis will "fold" into that thigh (internal rotation of the hip joint), and your "bits" will be squished a little between your pelvis and your thigh.

Ben Hogan said once: "At the top of the back swing the groin muscle on the inside of your rt [sic] leg near your right nut will tighten," Hogan wrote. "This subtle feeling of tightness there tells you that you have made the correct move back from the ball."

I don't know about that, but you will probably feel something down around that area.

[Mystique]

It’s interesting how much a 3D system changes your understanding. Observing top players in full detail and realizing the trail knee shouldn’t fully extend is a great example of why being open to new info is so important. I like your approach: not trying to be “right,” but trying to teach correctly based on actual evidence. Makes sense why golfers would benefit from this perspective.