135 Do Expert Golfers Keep Their Heads Still While Putting? Timothy D. Lee 1, Tadao Ishikura 2, Stefan Kegel 1, Dave Gonzalez 1 and Steven Passmore 1 1 Department of Kinesiology, McMaster University Hamilton, Ontario, Canada, L8S 4K1 scapps@mcmaster.ca 2 Doshisha University, Kyoto, Japan ABSTRACT The putting patterns of five expert and 11 less-skilled golfers were examined to understand the nature of head movement that occurred during the putting stroke. As expected, the less-skilled golfers moved their heads in the direction of the backswing, then reversed and moved it in the direction of the downstroke during the putt. In contrast, and contrary to expectations, the experts did the reverse they moved their head in a direction that was opposite to the direction of the putter. The strokes of both the experts and lessskilled golfers revealed closely matched timing constraints, indicating a strong preference to organize these actions as a single coordination pattern. Key words: Allometric, Degrees of Freedom, Egocentric, Expertise, Golf Putting INTRODUCTION The golf swing illustrates a concept known in motor control research as the degrees of freedom problem [1]. In golf, the term, degrees of freedom, generally means that independently moving body parts are in motion during the swing. Ankles, knees, hips, shoulders, elbows, wrists, and head are just the primary observable degrees of freedom. When we consider all of the muscles that span these joints and the fact that each may be active to varying extents during the swing, then the total number of individual elements, or degrees of freedom, that the golfer must control becomes very large. Quite simply, the problem is that the degrees of freedom involved in the swing are far too numerous for the golfer to voluntarily control at a conscious level. One solution to the degrees of freedom problem is to reduce the number of parts that must be consciously controlled during the swing. An analogy might be useful Reviewer: Paul Glazier (University of Wales Institute Cardiff, UK)
136 Annual Review of Golf Coaching 2008 here (see [2] for a more detailed explanation). The front wheels of a car represent two moving parts. Imagine how difficult it would be to drive your car if the wheels were independent you would need two steering devices, one for each wheel, and both devices would need to be simultaneously controlled by the driver. The task of coordinating the two wheels would reside with the driver. But, of course, the front wheels are not independent. Movements of the steering device enable us to control both front wheels because a fixed linkage in the car s steering mechanism has reduced the number of degrees of freedom from two to one. All we really need to think about is the direction in which we want to go, and both wheels conform to our steering actions. Our central nervous system works in an analogous way to help perform coordinated actions without overburdening our brains with the details regarding how each degree of freedom must be controlled. But, instead of fixed linkages like the one used in our steering mechanism example, the central nervous system uses neural mechanisms, which we will simply call coordination constraints. These constraints serve essentially the same purpose as the fixed linkage in the car s steering mechanism the individual moving parts are commanded by specific parts of the central nervous system that respond to our intentions. The following demonstration (after [3]) nicely illustrates how the central nervous system coordinates two simultaneously moving degrees of freedom. Use your hands to form two pistols, with the index fingers pointing straight ahead, and hold them as if you were shooting at a target directly in front of you. Now, rhythmically start to waggle your right finger back and forth. After five seconds or so start the left finger waggling too, so that both fingers are now in motion. What happened? Almost certainly, the rhythmic waggling of the left finger quickly fell in synchrony with the rhythm of the right finger the combined rhythm of the two fingers formed a coordination pattern. The reason for doing so is complex and researchers are still discovering answers regarding how and why we spontaneously form simple coordination patterns such as these. Note that you were not obligated to coordinate your two fingers in the pattern that emerged. Indeed, a large number of patterns could have been formed. But the pattern that you spontaneously adopted was likely done so in order that the central nervous system could establish a coordination pattern that produced a reliable continuation of each finger s rhythm and, at the same time, minimized the cost of conscious monitoring (attention) of the pattern. Like the steering mechanism example, the constraint reduced the degrees of freedom to be voluntarily controlled from two to one, and did so in a reliable and efficient manner. A review of the coordination research by Swinnen [4] revealed that the central nervous system typically coordinates degrees of freedom according to one of two constraints in environmental space: an allocentric constraint or an egocentric constraint. An allocentric constraint refers to a preference for the central nervous system to synchronize the relative timing of limbs that move in the same direction. Simultaneously tapping your index fingers on the table is easy to do if they move together in the same direction and at the same time. Similarly, tapping a finger of one hand while tapping your foot is also easy to perform together in time, because the allocentric constraint allows the pattern to be maintained with minimal attention. An egocentric constraint refers to the tendency to coordinate the relative timing of
Head Movement During Putting 137 movements on either side of a body midline in opposing, mirror-image directions. The pistol-shooting example provided earlier is a good example of an egocentric constraint for degrees of freedom on mirror-image sides of the body s saggital plane. In this paper we report the findings of a study that explored the role of constraints in the coordination of the golf stroke 1. Specifically, we examined how golfers of limited playing ability compared to golfers of exceptional ability while coordinating the movements of the golf club and their head during the putting stroke. From a golf instruction point of view, a common fault, which is particularly apparent amongst poor golfers, is movement of the head during the putt. High-handicap golfers tend to sway their upper body during the putt and to look up before the ball is struck to see where the putt will go. Both the sway and the quick look are likely to cause head movement. However, perhaps there is another reason for head movements. If we consider the upper-body/putter pendulum action as one degree of freedom to be controlled during the putt, and the movement of the head as another degree of freedom, then from a coordination perspective it would make good sense for the central nervous system to constrain these two degrees of freedom so that the golfer is functionally performing a single, coordinated action. An allocentric constraint, in which the putter and head move in the same direction, provides a reasonable solution to the degrees of freedom problem faced by high-handicap golfers during the putt. Therefore, we hypothesized that high-handicap golfers would couple their putter and head movements in a tightly coordinated allocentric pattern. We mentioned earlier that allocentric and egocentric constraints were two of the many possible ways to coordinate two or more degrees of freedom. For example, a polyrhythm is an example of tapping in which the temporal phases of the two limbs is neither allocentric nor egocentric (e.g., timings of 2:3, 3:4, 3:5). And yet skilled drummers can maintain a coordination constraint that allows them to perform a polyrhythm. To do so however, requires learning, and evidence exists that learning to perform a new coordination constraint requires considerable practice along with augmented feedback, such as provided by a teacher, instructor or coach [5]. Many golf instructors advise their students to keep their head still while putting. The ability to keep the head completely motionless during the putting stroke requires a unique coordination constraint that must be learned. In essence, the golfer must decouple the motion of the head from the motion of the upper-body/putter. Therefore, a second purpose of the study that we report here was to examine how experts achieve such a coordination constraint. The findings revealed some results that did not conform to expectations. EXPERIMENT Golfers of varying ability performed 60 putting strokes on an indoor putting surface. They putted balls toward regulation-sized holes drawn in chalk on the carpet, located 1, 3 and 5 metres from the start location. We asked the golfers to stop their putts as close to the hole as possible, and the final resting position of the ball was marked for later analysis. Each trial involved a practice stroke and a putt. The golfers were asked to make the practice stroke as similar as possible to the putt. Measurements of the 1 A detailed description of the methodology and results can be found in the July 2008 issue of the Journal of Motor Behavior.
138 Annual Review of Golf Coaching 2008 motions of the putting stroke and head were performed by an Optotrak motion analysis system. This system records the position of infrared diodes (ireds) once every 8.3 msec, in three dimensions (in the direction of the putt, at right angles to the direction of the putt, and in the vertical dimension). For the analyses in this experiment, we placed ireds on the bill of a hat, on the bottom of the shaft of the putter, and on the tip of the blade of the putter. Another ired was placed on the putting surface, directly opposite the point corresponding to the back of the ball. In this setup, data collection began when the putter initiated the backstroke and was completed at the point opposite to the back of the ball (regardless of whether the stroke contacted a ball or not). The golfers ranged from 21 to 56 years of age and had varying years of experience playing the game. We categorized 11 of the golfers as less skilled, as they possessed handicap indexes that ranged between 12 and 40. The five remaining golfers were active members of a regional development golf tour, three as professionals and two as amateurs. The amateurs had handicap indexes of 1 and 5. Each of the professionals also had experience on other pro tours. We categorized these golfers as experts, but not at the elite level amongst the very best in the world. 2 As anticipated, the experts putts were closer to the hole on average, and more consistent from trial to trial, than the less-skilled golfers. This finding held true for distances of 3 and 5 meters. At the 1-metre distance however, the experts tended to putt the ball slightly beyond the hole on average, compared to the less-skilled golfers. This finding might reflect a strong tendency not to leave the ball short of the hole on putts of minimal distances, or perhaps, other strategic differences. The analyses that were the focus of our study, however, concerned the head and putter movements during the execution of the putting stroke. Throughout all of our analyses, there were relatively few statistical interactions involving either the length of the putt or whether the golfer was taking a practice putt or actually stroking the ball. In other words, the putter-head relationships held true regardless of the length of the putt or whether or not a ball was contacted. Putts of varying distances resulted in total average stroke durations of 845 msec to 1038 msec. Based on the Optotrak sampling rate of 120 Hz, this resulted in between 101 and 125 positional data points recorded per trial. Therefore, the motions of the putter and the head provided positional data over the entire duration of the stroke, from which we then derived the velocity data that are plotted in the graphs in Figures 1 and 2. In looking at these graphs, keep the following points in mind. The putter velocity curves are indexed by the y-axis on the left; the head velocity curves by the y-axis on the right. Any point on the curve that corresponds to a negative number reflects a movement away from the hole; points indexed by positive values are movements toward the hole. Portions of curves that have non-zero slopes generally reflect accelerations; flat portions of the curves (slopes near zero) reflect constant velocities; and the point where a curve crosses the 0 mark on its corresponding axis reflects a reversal in direction. All 60 trials performed by two of the less-skilled golfers are plotted in the graphs in Figure 1. The results from these two golfers were generally representative of the other nine less-skilled golfers. As can be seen in Figure 1, the less-skilled golfers 2 As well, expertise, as defined by handicap index or professional status, does not guarantee that the golfer will necessarily be an expert putter.
Head Movement During Putting 139 showed a consistent relationship between the motion of the putter and the motion of the head in the direction of the putt (the x dimension): the putter and the golfer s head generally moved in the same direction during the execution of the swing. In Figure 1 (less-skilled #5), the golfer showed only a little backward motion of the head during the putter backswing, but began a rapid forward motion of the head velocity as the velocity of the putter accelerated during the downswing. Less-skilled golfer #8 showed a more strongly coupled relationship between the head and putter the initial increase in negative velocity (i.e., velocity away from the hole, in the backswing) was matched by a negative velocity in the movement of the head. Both the velocity of the head and the putter produce a pause at the end of the backswing, and both produce rapid rises in positive velocity in the direction of the putt. Essentially, the less-skilled golfers solved the problem of controlling the two degrees of freedom by recruiting an allocentric coordination pattern moving both in the same direction with similar relative timings. Figure 1. Overlay of Velocity Profiles on Head and Putter Movement During 60 Real (With Ball) and Simulated (No Ball) Putts to Holes at 1-m, 3-m and 5- m Distances for Two Less-Skilled Golfers The upper horizontal dotted lines refer to zero head velocity; the lower dotted lines to zero putter velocity. We statistically analyzed the coupling of head and putter velocity patterns using Pearson correlations. Each pair of velocity data points within a trial (recall that the Optotrak sampled the ired positions every 8.3 msec) was used to compute correlations for each stroke, which were then averaged over trials and across golfers. The analysis resulted in an average correlation value of +0.78, meaning that there was a highly positive relationship between the velocity profiles of the putter and head during the putting stroke. The head and putter velocity profiles of all 60 strokes for each of the experts are presented in Figure 2. Recall that we expected the highly skilled golfers to keep their head relatively motionless during the putt. If our experts were performing according to these expectations, then the velocity profiles for the motion of the head in the figures would be expected to be relatively straight lines with slopes = 0. The graphs in Figure 2 reveal this not to be so, but the experts were not coordinating their actions
140 Annual Review of Golf Coaching 2008 Figure 2. Overlay of Velocity Profiles on Head and Putter Movement During 60 Real (With Ball) and Simulated (No Ball) Putts to Holes at 1-m, 3-m and 5-m Distances for the Five Expert Golfers The upper horizontal dotted lines refer to zero head velocity; the lower dotted lines to zero putter velocity. similar to the less-skilled golfers either. Instead, the experts showed a very consistent and tightly coupled coordination pattern between the putter and head velocity that conformed to an egocentric constraint (i.e., moving each in opposite directions but with similar relative timing). Experts # 1, 3 and 5 show the most typical egocentric pattern. For these golfers, the head: a) moved with a high positive velocity (i.e., toward the hole) while the putter moved in a negative velocity during the backswing, b) attained maximum velocity in the direction opposite the putter at approximately the same point in time, then c) revealed a negative velocity (i.e., away from the hole) as the putter increased its positive velocity toward the hole in the downstroke. Expert #4 showed a pattern that was mostly similar to these experts, except that the head did a slight reversal to a positive velocity near the end of the stroke. The early portion of
Head Movement During Putting 141 the putts performed by Expert #2 revealed a pattern that we had expected for the entire putt by all of the skilled golfers. The backswing was accompanied by a relatively motionless head Expert #2 maintained a zero velocity head movement during about the first two-thirds of the putt. However, as the putter began its downstroke (indicated by crossing the zero-putter-velocity dotted line in Figure 2), the head showed a rapid rise in negative velocity. This golfer was the only expert to reveal any sign of an out-of-phase coupling of the head and putter, and this was the case only for the backswing the completion of the putt corresponded to the same egocentric constraint shown by the other experts. We performed correlations on these data in a manner similar to that described for the less-skilled golfers. The average correlation was 0.70. This strong, negative correlation value means that the changes in velocity values for the putter were accompanied by changes in the opposite direction for head velocities. The figures also revealed an interesting phenomenon characteristic of all the golfers who took part in our study. Note that virtually every velocity profile plotted in these figures showed a slight reversal at the end of the putt. The change in the rate of velocity simply suggests that the putter was slowing down (decelerating) just prior to ball contact. However, this was the case regardless of whether or not a ball was present. The accompanying head movement also revealed a similar change in velocity regardless of whether the head was moving with a positive or negative velocity, it decelerated just prior to ball contact. This finding is interesting because it suggests a very tight coupling of the velocities of the head and putter at the point of ball contact. However, these couplings do not suggest an underlying cause does the head decelerate because it is coupled to the deceleration of the putter, or does the putter decelerate because it is coupled to the motions of the head, or are both simply conforming to a constraint that reduces the degrees of freedom at the critical moment of ball contact? DISCUSSION The putt requires that a golfer control at least two important degrees of freedom the motions of the putter and the head. Research tells us that the central nervous system tries to find the most economical and effective solution to the simultaneous movement of multiple degrees of freedom by forming action patterns. The present results suggest that golfers organize a solution to the degrees of freedom problem in a manner commensurate with their level of skill. For less-skilled golfers, the motions of the head and putter conformed to an allocentric constraint moving both in the same direction provided a workable solution for controlling head movement while the putter was in motion. For expert golfers, the degrees of freedom were constrained by an egocentric pattern moving the head and putter in opposite directions provided the most appropriate constraint. On the one hand, these results simply confirm what many have suspected that poor putters move their head during the putt. A finding that was somewhat more surprising, but perhaps not counterintuitive, was the tightly coupled nature of the coordination pattern that existed between the head and putter. They were virtually locked together in relative time, both moving with similar velocity profiles. The data also indicated that the concern is not a matter of simply taking a quick look at the putt.
142 Annual Review of Golf Coaching 2008 Recall that data collection for a trial was completed at the point of ball contact, so all of the coordination activity occurred prior to any movement of the ball. Moreover, the same coordination pattern existed regardless of whether a ball was present or not. We argue that these findings reflect a central nervous system constraint that solves the problem of how to control the head during the putt, and is consistent with a considerable body of research that have found allocentric constraints to provide workable solutions to the degrees of freedom problem. The experts results were unexpected. With the exception of the backswing portion of the putt by Expert #2, all revealed a rather tight coupling of the head and putter that conformed to an egocentric constraint. Two questions that are raised immediately by these findings are: a) why does the egocentric constraint provide a better solution than an allocentric constraint, and b) why did the experts not just keep their head motionless during the putt? The first question requires some speculation and additional research. Assuming that allocentric and egocentric patterns are generally reflective of weak and better putters respectively, our speculation is that an egocentric constraint provides better stability than an allocentric constraint. The backswing of the putter may be considered as a perturbation (or disruption) to the equilibrium established in the posture of the setup. Moving the head in the same direction as the putter by the less-skilled golfers likely increased the magnitude of that perturbation. We suggest that a movement of the head in the opposite direction might serve to counteract the destabilization of posture resulting from the perturbation caused by the putter. Have another look at Figure 2. With the exception of Expert #2, all of the golfers had a positive head velocity at the time when the putter initiated the backswing. Therefore, these experts might have already been anticipating the perturbation caused by the putter by initiating a head action in the opposite direction. These findings are reminiscent of other research that shows compensatory postural anticipations prior to self-initiated perturbations [6]. The finding that weak golfers used allocentric constraints and that experts used egocentric constraints begs the question, how did this rather startling change occur? 3 Of course, the question is circular: did the improvement in skill level bring about the change in coordination constraints or did the change in constraints give rise to the improvement in skill level? The likely answer is that both share some of the truth. Research suggests that changes to a coordination pattern do not occur in a simple, linear fashion. Rather, breaking away from one coordination pattern usually results in moving into the other dominant pattern [7]. You may recall that an egocentric pattern dominated the coordination of your hands in the finger waggling example provided in the introduction of this paper. Try the example again. Now, try to do something other than defaulting to an egocentric pattern. What will probably happen is that the solution you choose will involve moving the fingers in alternation they are now moving according to an allocentric constraint. The solution to breaking away from one pattern that is preferred by the central nervous system usually involves moving into another pattern that is also preferred by the central nervous system; not to an out-of-phase pattern that falls somewhere between these two preferred patterns. The fact that our experts revealed an egocentric pattern might not be so surprising 3 Presumably, the expert golfers had been weak golfers at one time.
Head Movement During Putting 143 after all they know that the allocentric pattern is not optimal and the egocentric constraint provides the most natural and best alternative solution to the degrees of freedom problem. CONCLUSION So, is an out-of-phase solution to the problem, where the head is kept completely motionless during the putt, a necessary coordination pattern to learn? We don t know. Is it an impossible coordination pattern to learn? We don t think so. Research on learning new coordination patterns suggests that the process is neither simple nor easy, but it is possible. Our central nervous system wants to make things simple for us, and we are trying to make things difficult instead. To achieve that requires motor learning, and to achieve motor learning requires practice that is augmented by very specific feedback about the putting patterns [8]. It is likely that the experts in our study have never been made aware of the egocentric nature of their coordination patterns. We suspect that any significant change away from egocentricity toward a pattern characterized by a motionless head during the putt will require specific practice involving feedback about these motions. Our current research is focused on the conditions necessary for such learning to occur. REFERENCES 1. Bernstein, N.A., The Coordination and Regulation of Movements, Pergamon Press, Oxford, 1967. 2. Turvey, M.T., Coordination, American Psychologist, 1990, 45, 938-953. 3. Kelso, J.A.S., Phase Transitions and Critical Behavior in Human Bimanual Coordination, American Journal of Physiology: Regulatory, Integrative and Comparative Physiology, 1984, 15, R1000- R1004. 4. Swinnen, S.P., Intermanual Coordination: From Behavioural Principles to Neural-Network Interactions, Nature Reviews Neuroscience, 2002, 3, 350-361. 5. Summers, J.J., Rosenbaum, D.A., Burns, B.D. and Ford, S.K., Production of Polyrhythms, Journal of Experimental Psychology: Human Perception and Performance, 1993, 19, 416-428. 6. Lee, W.A., Anticipatory Control of Postural and Task Muscles During Rapid Arm Flexion, Journal of Motor Behavior, 1980, 12, 185-196. 7. Lee, T.D., Swinnen, S.P. and Verschueren, S., Relative Phase Alterations During Bimanual Skill Acquisition, Journal of Motor Behavior, 1995, 27, 263-274. 8. Swinnen, S.P., Lee, T.D., Verschueren, S., Serrien, D.J. and Bogaerds, H., Interlimb Coordination: Learning and Transfer Under Different Feedback Conditions, Human Movement Science, 1997, 16, 749-785.