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Post by dubiousgolfer on Apr 15, 2020 16:56:25 GMT -5
Hi UG
We've got to be careful with pressure plate information because remember 'Pressure = Force/Area'.
So for example: Force on right foot could be 350 Newtons Area of right foot in contact with pressure plate could be 0.02 sq metres
Pressure = 350/0.02 = 17500 pascals
Force on left foot could be 175 Newtons (ie. 50% less than right foot) Area of left foot in contact with pressure plate = 0.01 sq metres (ie. 50% less than right foot)
Pressure = 175/0.01 = 17500 pascals
So in the above example , the pressure plate will show 50:50 but in reality there is double the force being burdened via the right foot .
What we really need are force plate data on a big sample of PGA/LPGA tour players.
DG
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Post by utahgolfer on Apr 15, 2020 20:33:59 GMT -5
Hi DG,
So, is the AMG video expressing weight pressure correctly? How does their system calculate it? How is it meaningful to talk in terms of pressures when forces can vary so much based on surface area? How does 50/50 or 60/40 weight pressures provide any meaningful information?
UG
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Post by utahgolfer on Apr 15, 2020 21:13:59 GMT -5
Jeff, did you notice the visual evidence of the slow motion video of Jason Day that he re-centered his hips before his arms started moving downward? The weight pressure data is interesting, but it would have helped to see the changes in his weight pressure from p1 to p7.
UG
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Post by dubiousgolfer on Apr 15, 2020 21:36:07 GMT -5
Hi DG, So, is the AMG video expressing weight pressure correctly? How does their system calculate it? How is it meaningful to talk in terms of pressures when forces can vary so much based on surface area? How does 50/50 or 60/40 weight pressures provide any meaningful information? UG Hi UG - to be honest I don't know why they use pressure plates as one could easily misinterpret the dynamics going on. I'm sure they are correctly measuring ' percentage of total pressure' under each foot but its not very useful if you want to determine the actual dynamic stress forces being applied through each leg/foot. For example , if the golfer was standing on those pressure plates completely in balance , then lifted his right foot temporarily (without moving his COM) , then the left foot would show 100% pressure while the right foot 0%. If the instructor wasn't looking at the golfer or any 3D images/video , but just looking at the pressure readings, I wonder how he'd figure out what went on? Would he assume that the golfer moved 'his body mass' over the left foot or did the golfer just actively press very actively under his left foot using muscular contractions, or did he just lift his right foot up and nothing else? DG
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Post by imperfectgolfer on Apr 16, 2020 10:39:56 GMT -5
Jeff, Thanks for the input. I learn best by contrasting what I think I understand with new material. I can't remember every weight pressure illustration I've ever seen, which keeps me open minded, but inefficient. I appreciate being reminded, but I also need more evidence. Do we have evidence that a 50/50 weight pressure at p4 does not sufficiently activate the right hip external rotators? Do we have evidence that a 50/50 weight pressure at p4 is not an effective way to rotate the pelvis, since muscles on both legs can be activated to rotate the pelvis? Do we have evidence that this backswing re-centering concept proposed by AMG and SLAP is flawed, with no exceptions? Do we have evidence showing weight pressure changes, the kinematic sequence, the right arm adduction maneuver, flying wedge patterns, release patterns, etc. for the current top 100 players in the world? And, by evidence I mean many examples or data points. I like seeing all available examples, not just those that support our current belief. Because I believe an 80% loaded right foot at p4 helps to activate the right hip ERs, this shouldn't make me not want to know if a 50% load could work well, too, especially when I have trouble getting off of my right side. I would like to understand everything better, that is why I want more information and evidence. I don't care who is right and wrong, I want to dig up new information, not to only validate what I already believe, but to correct my mistaken concepts and to add to my knowledge base. imho, UG UG, Let's consider your questions. You asked-: " Do we have evidence that a 50/50 weight pressure at p4 does not sufficiently activate the right hip external rotators?"I presume that you are really asking whether a 50/50 COP distribution pattern at P4 will stabilise the right leg enough to allow the right-sided lateral pelvic rotator muscles to efficiently rotate the pelvis away from a pressure-stabilised right leg. The answer is unknowable, but I am not surprised that Boditrak's data base found that 80% of the pro golfers had >80% of their COP measurement under the right foot at P4. I cannot fathom why a pro golfer would prefer to have a 50/50 COP distribution pattern at P4.
You asked-: "Do we have evidence that a 50/50 weight pressure at p4 is not an effective way to rotate the pelvis, since muscles on both legs can be activated to rotate the pelvis?" I do not know of any muscles in the left leg that can be used to rotate the pelvis during the hip-squaring phase between P4 and P5. Please educate me on which muscles in the left leg can be used between P4 and P5 to efficiently rotate the pelvis counterclockwise.
You asked-: "Do we have evidence that this backswing re-centering concept proposed by AMG and SLAP is flawed, with no exceptions?" The re-centering phenomenon proposed by the AMG instructors - as demonstrated by Shaun Webb at the 14 minute time point of the video where he used an arch-extension maneuver to pull his upper swing center centrally - is the same biomechanical phenomenon that is used by pro golfers who perform a vertical-centralised (like Sean O'Hair), or leftwards-centralised (like Troy Matteson and Chalie Wi), upper torso loading pattern during their backswing action. It is obviously a workable solution, but is it the most optimum solution? I personally think that a rightwards-centralised upper torso loading pattern (as performed by Jack Nicklaus, Mickey Wright, Cameron Champ, Luke List, Henrik Stenson, Adam Scott, Jamie Sadlowski ---- and many, many pro golfers) makes much more sense. Here again are capture images of Jack Nicklaus. Note that his upper swing center (sternal notch area) is positioned well to the right of his lower swing center, and the center of his foot stance, at P4 and there is no evidence that he used an upper torso re-centering phenomenon during his backswing action. I cannot personally fathom why he would want to re-center his upper torso (as demonstrated by Shaun Webb in the AMG video) and SW did not provide a biomechanical explanation to justify the AMG's re-centering recommendation. Can you provide a rational justification for preferably performing that type of upper torso re-centering phenomenon during the late backswing? You state that you have always had a "hanging back" problem, but I personally don't think that it is due to you not re-centering your upper torso during the late backswing. I think that your "hanging back" problem happens because you efficiently shift-rotate your pelvis during the downswing, but you fail to rotate your upper torso counterclockwise by efficiently activating your abdominal oblique muscles and your upper torso simply "hangs back". You asked-: "Do we have evidence showing weight pressure changes, the kinematic sequence, the right arm adduction maneuver, flying wedge patterns, release patterns, etc. for the current top 100 players in the world?" No! And it would not change my thinking if we had all that data. So, for example, if the data showed that more pro golfers used a flipping subtype of non-DH hand release action (rather than a DH-hand release action), it would not change my thinking that a DH-hand release action is preferable. Another example - if the data showed that more pro golfers used a vertical-centralised/leftwards-centralised upper torso loading pattern (rather than a rightwards-centralised upper torso loading pattern) it would not change my thinking that a rightwards-centralised upper torso loading pattern makes more biomechanical sense. You also stated-: "Jeff, did you notice the visual evidence of the slow motion video of Jason Day that he re-centered his hips before his arms started moving downward?" No! I did not see him performing a separate (independent) re-centering phenomenon during his late backswing that was not an intrinsic part of his pelvic shift-rotation action that simply started before P4. I have previously stated that it is perfectly acceptable to start shift-rotating the pelvis during the late backswing between P3.75 => P4 if one has the requisite hula hula flexibility, but I believe that any left-lateral pelvic shift that happens during that time period must be an intrinsic part of the pelvic shift-rotation phenomenon and I do not believe that there must be a separate pelvic re-centering phenomenon that must precede the standard pelvic shift-rotation phenomenon (that is seen in Mickey Wright's, Jack Nicklaus', and Hideki Matsuyama's downswing starting at P4). The only difference is that Jason Day (like Bubba Watson) starts the pelvic shift-rotation phenomenon before P4, rather than at P4. Jeff.
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Post by dubiousgolfer on Apr 16, 2020 11:56:01 GMT -5
Hi UG Maybe the 'hanging back' problem is a symptom of trying to move a passive left arm with just your pivot from P4-P5.5 . The arms can be very heavy and add to your MOI so maybe your torso can't handle that type of action to keep in sync with your pelvic motion. Maybe its worth trying to use a bit more shoulder girdle muscular activity to 'lighten the load' :-) DG PS. There are ways to rotate the pelvis as per Dr Mann's article 'Educational Review: Michael Finney's Golf Swing Kinetics Seminar' but I wouldn't regard them as efficient . Sasho does suggest using GRF forces as per image below but absolutely zero detail on what muscles need to be activated (and when) to create those 'reaction' GRFs.
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Post by utahgolfer on Apr 16, 2020 15:38:40 GMT -5
You all are great. I really appreciate the discussion.
I think much of the confusion is how we define p3.75 to p4.5. At p3.75 to p4, for example, most of the weight pressure could be on the right foot, but AMG may define this as p3.8 to p3.9 and refer to this as the backswing, while everyone else might refer to this as p4. PGA pros have a transition leftward motion shift, like Jason Day, but we could call it either the backswing or the downswing, depending how we define p4.
Jeff,
You asked-: "Do we have evidence that a 50/50 weight pressure at p4 does not sufficiently activate the right hip external rotators?"
I presume that you are really asking whether a 50/50 COP distribution pattern at P4 will stabilise the right leg enough to allow the right-sided lateral pelvic rotator muscles to efficiently rotate the pelvis away from a pressure-stabilised right leg. The answer is unknowable, but I am not surprised that Boditrak's data base found that 80% of the pro golfers had >80% of their COP measurement under the right foot at P4. I cannot fathom why a pro golfer would prefer to have a 50/50 COP distribution pattern at P4.
At some point the COP is 50/50 from the top of the backswing to the downswing. I definitely agree with the boditrac's data, because somewhere between the takeaway and the top of the swing, over 80% of the COP is under the right foot. The key is that the COP doesn't remain on the right foot for too long during the downswing. I don't mean to be facetious, but I think it a definition of terms problem.
You asked-: "Do we have evidence that a 50/50 weight pressure at p4 is not an effective way to rotate the pelvis, since muscles on both legs can be activated to rotate the pelvis?"
I do not know of any muscles in the left leg that can be used to rotate the pelvis during the hip-squaring phase between P4 and P5. Please educate me on which muscles in the left leg can be used between P4 and P5 to efficiently rotate the pelvis counterclockwise.
You have mentioned the left adductor magnus before, but at some point in the downswing, when enough weight pressure is on the left foot, the left gluteus maximus and left rectus femoris can contract isotonically to move the left pelvis away from the target line and assist in CCW pelvic rotation.
You asked-: "Do we have evidence that this backswing re-centering concept proposed by AMG and SLAP is flawed, with no exceptions?"
The re-centering phenomenon proposed by the AMG instructors - as demonstrated by Shaun Webb at the 14 minute time point of the video where he used an arch-extension maneuver to pull his upper swing center centrally - is the same biomechanical phenomenon that is used by pro golfers who perform a vertical-centralised (like Sean O'Hair), or leftwards-centralised (like Troy Matteson and Chalie Wi), upper torso loading pattern during their backswing action. It is obviously a workable solution, but is it the most optimum solution? I personally think that a rightwards-centralised upper torso loading pattern (as performed by Jack Nicklaus, Mickey Wright, Cameron Champ, Luke List, Henrik Stenson, Adam Scott, Jamie Sadlowski ---- and many, many pro golfers) makes much more sense.
Shaun Webb may have shown an arch extension, but this is not what they are promoting. It is simple lateral motion of the upper and lower swing center. The right centralized upper torso loading pattern is not static. It is easy to see that pros don't hang back, and part of this leftward pressure shift is to move the upper and lower torso toward the target, some amount, during backswing or downswing, depending on how you define it.
Here again are capture images of Jack Nicklaus.
Note that his upper swing center (sternal notch area) is positioned well to the right of his lower swing center, and the center of his foot stance, at P4 and there is no evidence that he used an upper torso re-centering phenomenon during his backswing action. I cannot personally fathom why he would want to re-center his upper torso (as demonstrated by Shaun Webb in the AMG video) and SW did not provide a biomechanical explanation to justify the AMG's re-centering recommendation. Can you provide a rational justification for preferably performing that type of upper torso re-centering phenomenon during the late backswing?
In the second photo, it clear to see Jack's leftward motion, similar to Jason Day. This is what all the pros do at some point in the swing, depending on how you define your terms.
You state that you have always had a "hanging back" problem, but I personally don't think that it is due to you not re-centering your upper torso during the late backswing. I think that your "hanging back" problem happens because you efficiently shift-rotate your pelvis during the downswing, but you fail to rotate your upper torso counterclockwise by efficiently activating your abdominal oblique muscles and your upper torso simply "hangs back".
I agree, this is definitely one reason. I think I also don't get the COP to my left foot soon enough during my transition, which would help me rotate my upper torso more efficiently.
You asked-: "Do we have evidence showing weight pressure changes, the kinematic sequence, the right arm adduction maneuver, flying wedge patterns, release patterns, etc. for the current top 100 players in the world?"
No! And it would not change my thinking if we had all that data. So, for example, if the data showed that more pro golfers used a flipping subtype of non-DH hand release action (rather than a DH-hand release action), it would not change my thinking that a DH-hand release action is preferable. Another example - if the data showed that more pro golfers used a vertical-centralised/leftwards-centralised upper torso loading pattern (rather than a rightwards-centralised upper torso loading pattern) it would not change my thinking that a rightwards-centralised upper torso loading pattern makes more biomechanical sense.
Fair enough.
You also stated-: "Jeff, did you notice the visual evidence of the slow motion video of Jason Day that he re-centered his hips before his arms started moving downward?"
No! I did not see him performing a separate (independent) re-centering phenomenon during his late backswing that was not an intrinsic part of his pelvic shift-rotation action that simply started before P4. I have previously stated that it is perfectly acceptable to start shift-rotating the pelvis during the late backswing between P3.75 => P4 if one has the requisite hula hula flexibility, but I believe that any left-lateral pelvic shift that happens during that time period must be an intrinsic part of the pelvic shift-rotation phenomenon and I do not believe that there must be a separate pelvic re-centering phenomenon that must precede the standard pelvic shift-rotation phenomenon (that is seen in Mickey Wright's, Jack Nicklaus', and Hideki Matsuyama's downswing starting at P4). The only difference is that Jason Day (like Bubba Watson) starts the pelvic shift-rotation phenomenon before P4, rather than at P4.
I don't think anyone is saying it is a separate phenomenon. I definely think AMG could be more clear and state that some pros wait longer to move leftwards than other pros. There is no one size fits all. But, everyone wants to be the authority during their short video presentations.
DG, yes, it is difficult to keep the left arm relaxed with no independent pulling, while maintaining proper scapula activation.
Cheers!
UG
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Post by imperfectgolfer on Apr 17, 2020 9:49:48 GMT -5
UG, You wrote-: " Shaun Webb may have shown an arch extension, but this is not what they are promoting. It is simple lateral motion of the upper and lower swing center. The right centralized upper torso loading pattern is not static. It is easy to see that pros don't hang back, and part of this leftward pressure shift is to move the upper and lower torso toward the target, some amount, during backswing or downswing, depending on how you define it." If SW is simply promoting a leftwards shift of both the lower torso (pelvis) and upper torso, then I disagree with that golf instruction. I think that if one has used a rightwards-centralised loading pattern during the backswing, and then shifts the pelvis leftwards in either the late backswing (or early downswing) that it naturally/automatically creates secondary axis tilt, which is desirable. If the upper torso moved leftwards by the same amount then one could not create secondary axis tilt. Here again is Jack Nicklaus capture images. Image 3 shows his ~P5 position where he manifests obvious shift of his pelvis leftwards. However, his head is still positioned over his right foot and there is no evidence that he is also shifting his upper torso leftwards. If he did, then he would not be able to acquire the desirable amount of secondary axis tilt required for a driver swing action.
Regarding your "hanging back" problem you wrote-: "I agree, this is definitely one reason. I think I also don't get the COP to my left foot soon enough during my transition, which would help me rotate my upper torso more efficiently."
I don't believe that one needs to quickly get the COP to move over to the left foot in order to rotate the upper torso efficiently during the early downswing. For example, Luke List still has 56% of his COP measurement under his right foot, and only 44% under his left foot, at P5.5 and note how much he has rotated his upper torso between P4 and P5.5 without a "hanging back" problem.
Jeff.
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Post by utahgolfer on Apr 17, 2020 12:43:50 GMT -5
Jeff, draw a vertical line up JS's torso and watch his swing--a beautiful work of art. His upper body has significant forward motion during transition, yet he still has significant secondary axis tilt at impact.
Can this visual evidence be disputed?
UG
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Post by utahgolfer on Apr 17, 2020 22:56:14 GMT -5
Here is a video by Russell Heritage teaching the re-centering move during transition. Seems to say about what AMG stated, other than to keep most weight on the right foot during this process.
UG
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Post by dubiousgolfer on Apr 18, 2020 9:54:17 GMT -5
Hi UG - interesting micro move although it just looks like a full pelvic rotation to me and something I cannot physically do :-( RH still says that the pressure is on the right leg from 6:40- 7:27. DG PS . Here is a graph relating to JS's COM that might be useful PS. I now think that graph is very confusing and probably not useful at all . How can one have a percentage of the COM on the left foot? They must have meant % of the total COP on the left leg , not COM.
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Post by dubiousgolfer on Apr 18, 2020 20:00:36 GMT -5
Actually just found this on Dr Mann's website and there does seem to be a shift of COM towards the left foot but not by much. ----------------------------------- According to Doug Marsh, 51% of Jamie Sadlowski's body weight is situated over the left foot at impact and 49% over the right foot. Here is Doug Marsh's image showing Jamie Sadlowski's body mass distribution at the end-backswing position, and at impact. Note that Jamie Sadlowski has 38% of his body mass over his lead foot at his end-backswing position, and 51% of his body mass over his lead foot at impact. The large red arrow in the middle of the image shows the position of his COM. Note that there is very little shift of his COM during his downswing action. Minimising shift of the COM during the downswing action allows a golfer to more easily remain in balance between his feet. Using his motion capture analysis software program on many swing videos of professional golfers, Doug Marsh has been able to demonstrate how much the body mass shifts during the downswing action in professional golfers. Here is a smaller copy of a graph from his article. Note that the "average" professional golfer has approximately 41% (+/- 6%) of his body mass distributed over his lead foot at the end-backswing position, and 62% (+/- 8%) of his body mass distributed over his lead foot at impact. ---------------------------------- DG ps. Would be interesting to see JS's COP data and compare it with the graph for his COM .
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Post by utahgolfer on Apr 18, 2020 23:06:37 GMT -5
Thanks DG, that is nice to see those data.
I played golf with Jeff today and we talked over the topic. My main problem for hanging back was the spinning out my left foot too early in the downswing, with very little weight pressure shift on my left foot by impact. So, he got me doing a better squat move and a loaded left foot horizontal push back (toe to heel) sooner in the downswing. By the end of the round it was flowing nicely, and I had about 75% of my weight on my left foot at impact.
I think the re-center topic is okay for some players, but it is not a stock move that all players should emulate. It is appealing as a way to prevent hanging back, but Jeff was able to easily solve my hanging back problem once he noticed the premature spinning of my left foot. Jeff's instruction was definitely a much better way to fix my swing.
In contrast, the re-centering move beginning at p3 is too early for me and seems complicated to do. The simple squat move, the left foot push back, and a loaded left foot at impact definitely solves any hang back problem and was easy to integrate.
UG
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Post by dubiousgolfer on Apr 19, 2020 23:36:10 GMT -5
Here's a recent video with Sasho MacKenzie and Chris Como .
I found what he said at 3:08 quite surprising where he mentions at P6 a golfer is 'not strong enough' to perform 60 degrees of forearm supination in .04 secs (I am assuming he means with a neutral grip). So he is reinforcing his concept of using a passive torque (by having the clubshaft below his hand path by P5.5) to help square the clubface.
And he implies earlier in the video from 0:31 that even though Adam Scott swings 'On Plane' he is 'guessing' (because he swings hard left through impact) that Adam Scott is actually applying dynamic forces through the butt end to still create that same 'passive torque' effect. So I am assuming that if Adam Scotts hand path is not steeper than the clubshaft plane , he can still apply this passive torque force effect (even though it is invisible to the naked eye).
The 2nd half of the video implies that the downswing is limited unless there is an upward net ground force vector (ie. left foot force greater than right) ahead of the COM. I am assuming he is also implying that the COP must move more to the left before the downswing can commence . Then he reinforces his 2nd concept (regarding increasing clubhead speed) where the golfer must exert a high upward ground force through his left foot while shaft vertical in the downswing.
Chris Como did demonstrate that a targetwards slide of both upper and lower COM (such that net GRF force is directed through the COM) will limit the downswing (ie. not get angular momentum into the body) .
I still find this concept of moving the COP rapidly to the left foot confusing unless they are somehow working out what actual net forces are being applied through each foot (using the area of contact with the ground). But he does say that the COP is a "measure of the average spot that the vertical force is coming out of the ground" .
DG
PS. If I'm not mistaken Adam Scott shifts plane from TSP to elbow plane and shallowing the club with a right upper arm adduction and pitch elbow move.
So is SMK surmising that AS's left arm is applying a steeper downwards force to the grip of the club (than his clubshaft plane) while the right hand is preventing a 'toppling over' effect on the clubshaft (ie. an OTT)? That there are 2 torques cancelling each other out as the clubshaft plane shallows out (while still 'On Plane') with a build up in the magnitude of those equal and opposite torques up to P6? Then during PA#3 release, the right hand torque is relaxed allowing the 'ready-made built up' left hand supinatory torque to suddenly take over to apply the necessary 'squaring clubface' impetus by impact? I can only conclude that SMK is surmising that there is a build of potential torsional energy (like a torsional spring- see below explanation) in the left arm that can quickly be turned to KE later in the downswing.
---------------------- Energy Storage
Torsion springs store energy by twisting about their axes. The energy stored in the torsion spring, as with any spring, is potential energy. This potential energy differs from kinetic energy because is has the potential to do work, but is not doing work at the time. Kinetic energy is simply energy that is doing work. It is stored by increasing the tension of the spring, which just means making it tighter. The tightness of the spring is directly proportional to the amount of energy stored in the spring. To release the energy, tension must be let off the spring, at which time it quickly reverts to its original position. This means that the spring unwinds to the position in which is has the least potential energy and in which there is the least possible tension on the spring. The kinetic energy is released by the pressure the spring exerts on the object to which it is attached. ---------------------
I suspect one can see/feel this 'effect' in action if you generated a lot of twisting supinatory effort on your left forearm (with a clenched fist) but was preventing that supination by grabbing the left fist with your right hand . Now increase both the left forearm twisting force and the right hand 'restraining twist ' and you can feel the increased torsional strain in your left forearm . Then just relax your right hand grip and you will see a rapid supinatory rotation of the left forearm.
If you attempted to rotate your left forearm by the same amount (as quickly as possible) just using the muscles of your left forearm (ie. no right hand involved) , the rate of supination seems much smaller.
I'm finding the above explanation ,if that is what SMK is implying (and I'm probably wrong), very difficult to comprehend in a real golfers swing because it creates so much tension in the forearms and wrists that I cannot imagine it will allow an efficient release of PA#2.
With regards SMK's correlation findings between GRF under left foot while clubshaft vertical , I think that could be too simplistic without understanding other variables that might be involved. There could be another biomechanical factor like the creation of the straighter hand path from P4-P5.5 that might also necessitate a larger GRF force in the left foot to elevate the left shoulder socket and secondarily cause a more acute 'change' in the hand path direction to increase clubhead speed (so that both hands/clubhead are in the optimal position at impact for the golfers desired angle of attack).
Imho, there does seem to be too many variables in the golf swing to 100% confirm cause and effect on the 'kinetic' side of biomechanical research.
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Post by imperfectgolfer on Apr 20, 2020 9:42:11 GMT -5
UG, You asked-: " Jeff, draw a vertical line up JS's torso and watch his swing--a beautiful work of art. His upper body has significant forward motion during transition, yet he still has significant secondary axis tilt at impact.
Can this visual evidence be disputed?" I presume that you are asking this question because you believe that Jamie is moving his upper torso targetwards during the transition period - representing a re-centering move of the upper torso (as recommended by the AMG instructors and Russell Heritage). However, the "facts" show otherwise. Here are back view capture images of his P3.75 - P4.3 time period. Image 1 is at P3.75, image 2 is at P3.9, image 3 is at P4 and image 4 is at P4.2. I have drawn red lines on the outer border of his left hip and upper thigh at P3.75. I have drawn blue lines on the left side of his head at P3.75. I have drawn a yellow line where his upper swing center (top of his thoracic spine) is roughly located (which one cannot see because it is obscured by the right shoulder). Note that there is no targetwards swaying of the pelvis between P3.75 and P3.9 (image 2). Note that his head has not moved targetwards either. Note that his upper swing center has moved targetwards, but that is due to continued upper torso rotation of ~40 degrees and it is not due to upper torso re-centering (which would also involve the lower thoracic spine and lumbar spine). Note that JS starts squatting and rotating his pelvis counterclockwise between P3.9 and P4 - but without any targetwards sway of his pelvis or upper torso. His head is dropping due to increased hip joint flexion. At P4.2 (image 4) he has already rotated his pelvis counterclockwise by a large amount and there has been negligible sway (~1") of his pelvis/upper torso in a targetwards direction. JS has a nearly "pure" rotary pelvic motion (with no left-lateral pelvic shift) and he keeps his pelvis centered between his feet during his hip-squaring phase. Jeff.
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