- Email: [email protected]
Golf players exhibit changes to grip speed parameters during club release in response to changes in club stiffness
Human Movement Science 31 (2012) 91–100
Contents lists available at ScienceDirect
Human Movement Science journal homepage: www.elsevier.com/locate/humov
Golf players exhibit changes to grip speed parameters during club release in response to changes in club stiffness Sean T. Osis, Darren J. Stefanyshyn ⇑ Human Performance Laboratory, University of Calgary, Calgary, AB, Canada, T2N 1N4
a r t i c l e
i n f o
Article history: Available online 5 August 2011 PsycINFO Calssification: 2320 Keywords: Golf Shaft Flex Proprioception Vibration
a b s t r a c t The influence of golf club stiffness on driving performance is currently unclear, and it is possible that this ambiguity is due in part to golfer adaptation to equipment. The purpose of the current study was to elucidate mechanisms of adaptation to club stiffness, during the golf swing, by employing tendon vibration to distort proprioceptive feedback. Vibration (50 Hz, 1 mm amplitude) was applied to the upper extremities of 24 golfers using DC motors with eccentric weights. Golfers hit golf balls in a laboratory setting using three clubs of varying shaft stiffness, and club kinematics were recorded using high speed (180 Hz) digital cameras. The results demonstrated significant slowing of the club grip during club release for a high-stiffness shaft with vibration. This suggests that, when proprioceptive feedback is available, players adapt to changes in club stiffness by modifying the release dynamics of the club late in the downswing. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In order to hit a golf ball as far as possible, it is often suggested that golf club construction must be optimized for a golfer’s swing. This is evident in the plethora of available products and services, designed to provide innumerable equipment choices to the player, with the implication that some subset of these choices will provide the player with maximum distance. Given the wide breadth of available equipment, being able to accurately judge, a priori, which subset is optimal for a particular player is critically important. However, there is evidence that current procedures of club-player matching, with ⇑ Corresponding author. Address: Human Performance Laboratory, University of Calgary, 2500 University Drive NW, Calgary AB, Canada T2N 1N4. Tel.: +1 403 220 8637; fax: +1 403 284 3553. E-mail address: [email protected] (D.J. Stefanyshyn). 0167-9457/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.humov.2011.02.006
92
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
respect to club shaft stiffness, result in sub-maximal performance (Worobets & Stefanshyn, 2008). Additionally, studies of the effect of shaft stiffness on ball distance have demonstrated mixed results, and only some golfers seem to benefit from changes to shaft stiffness (Pelz, 1990; Stanbridge, Jones, & Mitchell, 2004; Worobets & Stefanyshyn, 2008). This raises the question as to why only some players benefit from changes in club shaft stiffness. One answer could be that players adapt their technique to their equipment, effectively negating the effects of mechanical changes in the equipment. This idea has been previously suggested in a study of the effects of string tension on ball placement in tennis (Bower & Cross, 2008), and a study of putting accuracy when using putters of varying mass (Karlsen & Nilsson, 2007). In both instances, large changes in the mechanical properties of the equipment resulted in little to no change in accuracy for the given task. This suggests that the athlete is adapting to the equipment, and may be doing so despite limited awareness of changes to their equipment (Bower & Cross, 2008). Recently, evidence has been presented that golf players adapt their swing kinematics in response to changes in shaft stiffness (Osis & Stefanyshyn, 2010; Fig. 1). In this study, tendon vibration was applied to the upper extremities of golfers during the swing. Due to the sensory-impairing nature of tendon vibration, it was concluded that this intervention impaired proprioceptive feedback, which in turn prevented any adaptation response by the players to their equipment. However, the exact nature of player response to the shaft is currently unclear, which raises the question of how players modify their swing to achieve consistency with different clubs. Osis and Stefanyshyn (2010) demonstrated that when proprioceptive feedback was impaired in golfers, club head speed was decreased in a stiff-shaft condition, but not for a compliant shaft-condition (Fig. 1). This implies that, when sensory feedback is intact, players change their swing kinematics to increase club head speed in a stiff-shaft condition. These changes in swing kinematics must therefore be: (1) shaft-specific and (2) related to the generation of club head speed. Club release, defined as the un-cocking of the wrists during the downswing, has been linked to the generation of club head speed at impact. In particular, it has been previously shown that delaying the timing of release can result in an increase in club head speed of 1.6–2.9% (Jorgensen, 1994; Pickering & Vickers, 1999; Sprigings & Mackenzie, 2002). These differences in club head speed, while small, could easily account for the changes reported by Osis and Stefanyshyn (2010), which were of the order of 1.3%. Therefore, modification of club release represents a potential player response to shaft stiffness; however, club release has not yet been examined. In addition to affecting club head speed, club release also has observable effects on the speed of the club grip (or grip speed) during the downswing. Several studies have shown that club release causes grip speed to decrease prior to impact (Miao, Makato, Kawaguchi, & Ikeda, 1998; Milburn, 1982; Miura & Naruo, 1998; Vaughan, 1979). This decrease occurs due to the forces acting upon the club as it rotates about the wrist (Jorgensen, 1994; Miura & Naruo, 1998). In terms of generating club head speed,
Fig. 1. Club head speed at impact as a function of club shaft stiffness (X-flex being the most stiff, and L-flex being the least). These data show that, with intact proprioception (white bars), players maintained similar kinematics at impact despite being given shafts of different stiffness. When proprioception was impaired (grey bars), players experienced a decrease in club head speed for a stiff shaft, but not for a compliant shaft. With permission from Osis and Stefanyshyn (2010).
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
93
both the timing, and the magnitude of peak grip speed have been linked with club head speed at impact. It has been observed that peak velocities of the upper extremities occur significantly closer to ball contact in professional players who generate higher club head speeds (Zheng, Barrentine, Fleisig, & Andrews, 2008). It has also been shown that players who generate higher club head speeds demonstrate higher peak grip speeds (Miura & Naruo, 1998). Golfers could potentially respond to changes in shaft stiffness by modifying club release in a way so as to increase their club head speed in a shaft-specific manner. If this is the case then, based on the current understanding of club release dynamics, concomitant changes in the timing and/or magnitude of peak grip speed should be observed. Grip speed, as a descriptor of club release, has not yet been examined. Therefore, the purpose of this study was to determine whether the timing and/or magnitude of peak grip speed change with increased shaft stiffness, in response to impairment of upper extremity proprioception during the golf swing. The following hypotheses were systematically tested: H1: For a high-stiffness shaft, peak grip speed will be reduced with tendon vibration. H2: For a high-stiffness shaft, peak grip speed will occur earlier with respect to ball contact with tendon vibration. H3: For a low stiffness-shaft, no differences will be observed in either the magnitude or timing of peak grip speed with tendon vibration. 2. Methods Twenty-four golf players, ranging in play level from recreational to professional/instructional, were recruited for this study. All players were free from musculo-skeletal injury and all gave informed consent prior to participating. This study received ethical approval from the Conjoint Health Research Ethics Board at the University of Calgary. Vibration was applied using a method adapted from Bock et al. (2007). Eight small vibrating motors were fixed to the skin using medical tape, 2–5 cm proximal to the wrist and elbow joints (Fig. 2). The motors were situated in order to stimulate the primary effectors of the wrist and elbow, while preventing any interference with the normal range of motion during the swing. Each vibrating motor was pre-fabricated and consisted of a 12 V DC motor (19.8 mm 15.0 mm 25.4 mm) and an eccentric weight (half-moon, 18.0 mm diameter 5.1 mm thick) attached to the motor shaft. The motors were each fit into a housing made from 1 inch PVC pipe, in order to protect the subject from the moving parts. The total mass of each housing/motor unit was 45.0 g. A pilot test, using a force-replication task, was conducted to ensure that the vibrating motors had their intended effect of impairing proprioceptive feedback. The force replication task required participants to learn to apply 1000 g of force-equivalent to a digital scale, using their wrist flexors. The subjects were then asked to repeat the force application without feedback as to the actual force they applied. This was performed with the vibrating motors taped over their wrist flexor and extensor
Fig. 2. Photographs showing the placement of vibrating motors on the upper extremity.
94
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
tendons, under two conditions; with the motors off (no vibration) or the motors on (vibration). The results of the pilot demonstrated that replication of applied forces was not impaired under the no vibration condition but was significantly compromised with vibration. During pilot testing, local vibration in the tissue adjacent to one motor was characterized using an accelerometer; the frequency of vibration was 50 Hz, while the amplitude was approximately 1 mm. This frequency and amplitude are within the range of values from previous studies using vibration to impair proprioceptive feedback (Bock, 2007; Cordo, Gurfinkel, Bevan, & Kerr, 1995; Roll & Vedel, 1982). The clubs consisted of three interchangeable driver shafts (Aldila NV 55), and one driver head (TaylorMade CGB Max Ltd.). These shafts were chosen as the manufacturer was able to maintain consistent mass and inertial properties while varying only the shaft stiffness (Table 1). The shafts were representative of the range of stiffness commercially available: L-flex (least stiff), R-flex, and X-flex (most stiff). With the vibrating motors in place, but not activated, each subject was given a five minute period for a self-directed warm-up using clubs not included in the testing protocol. Following warm-up, the protocol consisted of 10 swings, in each of three shaft conditions (X-flex, R-flex, and L-flex), and two vibration conditions (no vibration, vibration) for a total of 60 swings. The experiment was a full-block design, with randomization for the order of shaft stiffness, but not for the order of vibration conditions. Each subject performed 10 swings in each stiffness with no vibration, whereupon the subject performed 10 more in each stiffness with vibration turned on. After every 10 swings, the club was taken out of sight of the player and the shaft was changed, blinding the subjects to the club conditions. For the vibration trials, the motors were activated once the subject had addressed the ball and was prepared to swing, and they were then deactivated after the player reached the end of follow-through. During each swing, the player hit a standard golf ball from a rubber tee approximately 2 m into a net. The only criterion for rejection of a trial was radically errant ball flight after impact. An artificial turf surface was used to simulate practice conditions, and players were encouraged to wear footwear they normally would for play or practice. The driver head had three spherical retro-reflective markers (11 mm diameter) affixed to pins that were welded to the club head. The grips of the shafts each had a further three spherical retro-reflective markers that were affixed to pegs, which were glued to the grips using epoxy. Club kinematics was measured using nine high-speed digital video cameras which collected 2D images of the club markers at 180 Hz. The 2D images from all cameras were then transformed into 3D marker positions for all six club markers using DLT algorithms. Using the known geometry of the configuration of markers for each club, the orientations, positions, and velocities of the club head center-face and a point 14 cm distal to the club butt (center-grip) were calculated from the marker data. The kinematic variables addressed in this investigation are shown in Table 2. Players were categorized based on their response to the tendon vibration, using the same method applied by Osis and Stefanyshyn (2010). Study of the individual responses to vibration revealed a pattern whereby certain individuals exhibited greater grip speed in the L-flex compared with the X-flex. Based on how each subject’s shaft-specific trend in grip speed changed with vibration, they were divided into two groups. This was done by ranking subjects according to change in grip speed, calculated using Equation (1):
DGS ¼ ðGSX
nov ibe
GSL
nov ibe Þ
ðGSX v ibe GSL v ibe Þ
ð1Þ
Table 1 Selected driver shaft characteristics reported by the distributor.
Butt Flex (mm) Tip Flex (mm) Frequency (Hz) Swing Weight (g) Length (cm) Mass (g) Moment of Inertia (kg m2)
X-flex
R-flex
L-flex
70.3 97.7 255 34.7 111.8 57.7 0.0061
91.1 125.7 228 33.6 111.8 56.7 0.0060
114.5 151.0 203 34.0 111.8 54.1 0.0057
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
95
Table 2 List of variables addressed in this investigation. Symbol
Variable name
Description
HSimp GSimp GSpeak GSpeak-imp TGSpeak
Club head speed at impact Grip speed at impact Peak grip speed Grip speed decrease Time of peak grip speed
Magnitude of the center-face velocity vector at impact Magnitude of the center-grip velocity vector at impact Maximum magnitude of center-grip velocity Difference between peak grip speed and grip speed at impact Time from peak grip speed to impact (impact = 0)
Where GSX_novibe, GSL_novibe, GSX_vibe, and GSL_vibe indicate grip speed at impact for the following respective conditions: X-flex with no vibration, L-flex with no vibration, X-flex with vibration and L-flex with vibration. Using the average standard error of measurement for grip speed from all data (0.03 ms1), the standard error in DGS was calculated as 0.13 ms1. Subjects were then divided into two groups: a ‘change’ group for whom DGS > 0.13 ms1 (CHG group) and a ‘no change’ group for whom DGS < 0.13 ms1 (NOCHG). In order to verify the repeatability of this classification, the experiment was repeated with 3 subjects. DGS was again calculated and all 3 subjects were classified into the same groups as they had been previously. For the purposes of this study, only CHG players were analyzed as they demonstrated shaft-specific changes in club kinematics. Selected subject characteristics for this group are shown in Table 3. For each variable two-way analysis of variance (ANOVA) with repeated measures were run for each level of shaft and vibration condition. For those variables where a significant interaction effect was seen between shaft and vibration, simple effects testing using multivariate analysis of variance (MANOVA) was implemented to determine the main effects. Mauchly’s Test of Sphericity was used to ensure assumptions were met for the use of parametric statistics. Bonferroni-adjusted post hoc tests were conducted where appropriate. Significance was set at alpha < .05. 3. Results There was a significant decrease in HSimp for the X-flex with vibration (F = 10.29, df = 1, p = .004; Fig. 3). With vibration, there was a significant effect of stiffness on GSimp (F = 12.96, df = 2, p < .001), and players demonstrated decreased GSimp with increased shaft stiffness in the vibration condition. With vibration, there was a significant increase in GSpeak-imp with increased stiffness (F = 8.902, df = 2, p < .001). 4. Discussion The results for HSimp are in good agreement with published values. Club head speeds in the literature have ranged from 32–58 ms1 (Fradkin, Sherman, & Finch, 2004), with average values for low handicap players in the range of 40–46 ms1 (Lephart, Smoliga, Myers, Sell, & Tsai, 2007; Miura & Naruo, 1998; Nesbit, 2005; Watanabe, Kuroki, Hokari, & Nishizawa, 1998). The current values for GSpeak and GSimp were also within the range of values previously reported: 7–12 ms1 for GSpeak, and 4–8 ms1 for GSimp (Miura & Naruo, 1998; Nesbit, 2005). Data reporting the timing of peak grip speed is sparse, however Miura and Naruo (1998) presented values ranging from approximately 50–100 ms
Table 3 Selected subject characteristics for the CHG group. N
11
Height (cm) Mass (kg) Age (yrs) Handicap index
182 (6.3) 82.4 (11.5) 35.8 (13.0) 7.0 (2.6)
96
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
Fig. 3. Club head and grip kinematics by shaft flex and vibration. White bars are swings without vibration while grey bars are swings with vibration. Symbols indicate significance for Bonferroni-adjusted post hoc tests.
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
97
prior to impact. These are earlier in the swing than the data presented here, however, Miura and Naruo (1998) used a limited number of data points to fit a quadratic function, a procedure that could have produced errors in the timing of the peak. The first hypothesis tested was that for the X-flex shaft, GSpeak would be reduced in response to tendon vibration. No support was found for this hypothesis, as there were no significant changes in GSpeak with shaft stiffness. While it has been shown that higher GSpeak may coincide with higher HSimp between players (Miura & Naruo, 1998), it is possible that this is not the case for within-player comparisons. Therefore, the current study suggests that GSpeak may be an inappropriate variable for characterizing within-subject changes in HSimp. Without vibration, the ‘‘deceleration’’ of the grip during club release is the same across shaft conditions (Fig. 4). This ‘‘deceleration’’ of the grip is due to the reaction forces and torques exerted by the club upon the arms during club release (Jorgensen, 1994; Miura & Naruo, 1998; Neal & Wilson, 1985), and it is presumed that forces and torques needed to affect rotation of a high-stiffness shaft are higher than for a low-stiffness shaft, as a stiffer spring effectively constitutes a higher elastic load. Player behaviour with no vibration therefore suggests that the player response to changes in the shaft serves to maintain the same ‘‘deceleration’’ despite changes in reaction torques that arise due to shaft stiffness. When vibration was applied, the results in GSpeak-imp indicate that the grip ‘‘decelerated’’ more in the stiffer shaft and, ‘‘decelerated’’ less with the low-stiffness shaft (Fig. 4). Since the relative stiffness of the shafts represents the relative magnitude of the elastic load on the player, the trend in GSpeak-imp with vibration seems consistent with the mechanical properties of the clubs (high ‘‘deceleration’’ with X-flex, low ‘‘deceleration’’ with L-flex). The apparent dominance of club mechanics with impaired sensory feedback (grey bars), as compared to the consistency in kinematics when feedback is intact (dotted line bars), strongly suggests that golfers are capable of using sensory feedback to detect and respond to changes in shaft stiffness. It is speculated that, when shaft stiffness increases, players may respond by increasing shoulder and/or torso muscle activity, which compensates for the increase in reaction torques due to the shaft. An increase in shoulder torque, for example, would provide increased power flow to the arm segment to compensate for additional power flow away from the arm segment as work is done on the club. A mathematical model by Kaneko and Sato (2000) estimated how joint torques might change in the context of an increased load on the player due to an increase in club inertia. Interestingly, their results suggested that shoulder and torso torques would increase in the latter half of the downswing, during
Fig. 4. Hypothesized link between reaction torques due to the shaft and grip kinematics. Left panel: large reaction torque due to a stiff shaft (dark grey) and a smaller reaction due to a compliant shaft (light grey). Middle panel: decrease in grip speed (or ‘‘deceleration’’) from peak to impact, for stiff and compliant shafts, without vibration (dotted line bars) and with vibration (dark and light grey bars). Right panel: sample time-histories of grip speed for one subject demonstrating the decrease in grip speed prior to impact at 0 ms; traces are each one trial: one with vibration and a stiff shaft (dark grey), with vibration and a compliant shaft (light grey); and a single trial without vibration for a stiff shaft, provided for comparison (dotted).
98
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
club release. Although they did not address shaft stiffness directly, their result supports the speculation that players compensate for increases in reaction torques during release by modifying shoulder and torso muscular activity. Motor control research has provided similar interpretations regarding external loads during movement tasks. It is known that reaction forces due to external loads are important variables in the control of the upper extremity (Debicki & Gribble, 2004; Gribble & Ostry, 1999; Yamasaki, Tagami, Fujisawa, Hoshi, & Nagasaki, 2008, 2009). In order to achieve consistent movement, it has been suggested that the central controller is capable of learning and predicting reaction loads that will arise during the course of a movement task, and will adjust muscle activity proactively to compensate (Debicki & Gribble, 2004; Gribble & Ostry, 1999; Hirashima, Kudo, & Ohtsuki, 2003). In addition, when proprioception of the limb is compromised, as in the current study, compensation for changes in reaction loads is poor, and kinematic changes result (Ghez & Sainburg, 1995; Messier, Adamovich, Berkinblit, Tunik, & Poizner, 2003; Sainburg, Ghilardi, Poizner, & Ghez, 1995). In this context, it may be speculated that proprioceptive feedback allowed the players in this study to modify shoulder and/or torso muscle activity in response to the increased load imposed by a stiffer shafted club. This would explain why subjects in the current study were unable to maintain consistent ‘‘deceleration’’ across shafts in the vibration conditions, as the vibration would have distorted the sensory feedback required to program muscular responses during club release. While this is currently a speculation, motor control research supports the idea, and further study should be done to clarify how muscular output changes in response to shaft stiffness. The second hypothesis stated that for the X-flex shaft, GSpeak would occur earlier with respect to impact in response to tendon vibration. This hypothesis was not supported by the data, as no significant differences were found in TGSpeak. However, TGSpeak averaged 7 ms earlier in the X-flex shaft with impaired proprioception, and although not significant (p = .054), this result supports the concept that delayed release of the club is associated with higher club head speeds, a finding that has been previously suggested by mathematical models (Jorgensen, 1994; Pickering & Vickers, 1999; Sprigings & Mackenzie, 2002). In addition, this trend supports the hypothesis that the players increased their shoulder and/or torso muscular output as shaft stiffness increased. Increased power flow to the arms could cause their velocity to peak later in the downswing. Since this additional muscle activity must be programmed using proprioceptive feedback, timing changes could occur with impaired proprioception, as suggested by the 7 ms discrepancy in TGSpeak for the X-flex shaft. The third hypothesis was supported by the results, as no significant differences were found in either GSpeak or TGSpeak with vibration in the low-stiffness shaft. Interestingly, although HSimp was not significantly different with vibration, GSimp and GSpeak-imp both showed significant differences for the L-flex club. This suggests that even if shoulder or torso activity were increased with vibration, it may not produce additional club head speed. This result is difficult to interpret as the relationship between GSpeak-imp and HSimp is complex and could be affected not only by changes in shaft stiffness and player torques, but also ball position. It has been shown, for example, that changes in applied torques can increase club head speed, however, ball position must also be adjusted in order to maximize the effect (Chen, Inoue, & Shibara, 2007). Clearly, the relationship between grip ‘‘deceleration’’ and club head speed is intricate and requires further study. A limitation of the current study is the lack of measurement of player inputs to the swing. An explanation has been offered for the differences in GSimp and GSpeak-imp, based on how the player might vary muscular output, however, the way in which these changes relate to club head speed is complex, and requires an understanding of how work is done on the club. For example, although GSimp was increased in the more compliant shaft with vibration, there was no corresponding change in HSimp. This suggests that even though there was an increase in the kinetic energy of the arm segment, this energy did not transfer to the club by the time ball contact occurred. For the stiffer shaft, GSimp was decreased with vibration, and GSpeak-imp was increased. This may imply an increase in the transfer of kinetic energy to the club as shown by the findings of Chen et al. (2007), however Chen and others have chosen fixed values for shoulder torques (Chen et al., 2007; Jorgensen, 1994; Pickering & Vickers, 1999), an assumption which may not be valid. In particular, the current findings, which show systematic changes in GSpeak-imp, imply that shoulder and/or torso torque profiles could change between shaft conditions. Clearly further study is needed to elucidate the connection between player muscular activity, grip
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
99
speeds and club head speeds. Additionally, given that this study was conducted on a subset of players based on their response to vibration, the validity of current findings extends only to those players that can be similarly categorized. Further study is needed to clearly describe this particular group of players. In summary, the current findings indicate that the shaft-specific differences observed at impact with impaired proprioception arise from changes in the ‘‘deceleration’’ of the arm segment during club release. In addition, there is compelling evidence that, when given clubs of different stiffness, players respond by maintaining a consistent ‘‘deceleration’’ of the grip during release, and this response is programmed using proprioceptive feedback from muscular sources. The benefit to the player in employing this response seems to be the maintenance of high club head speed in the face of a changing external load due to club mechanical properties. Further study is needed to understand how player inputs affect the ‘‘deceleration’’ of the grip during club release, and to understand how changes in grip kinematics ultimately lead to differences in club head speed. Finally, these results may have implications for any study of sport performance involving equipment, and the detection and study of adaptation mechanisms in other activities may be of significant value. Acknowledgments The authors would like to acknowledge the financial support provided by the Natural Sciences and Engineering Research Council of Canada, Alberta Scholarship Programs and the Alberta Ingenuity Fund. The authors also thank Dr. Ian Wright for his technical assistance and TaylorMade for providing equipment for the study. References Bock, O., Pipereit, K., & Mierau, A. (2007). A method to reversibly degrade proprioceptive feedback in research on human motor control. Journal of Neuroscience Methods, 160, 246–250. Bower, R., & Cross, R. (2008). Elite tennis player sensitivity to changes in string tension and the effect on resulting ball dynamics. Sports Engineering, 11, 31–36. Chen, C., Inoue, Y., & Shibara, K. (2007). Numerical study on the wrist action during the golf downswing. Sports Engineering, 10, 23–31. Cordo, P., Gurfinkel, V. S., Bevan, L., & Kerr, G. K. (1995). Proprioceptive consequences of tendon vibration during movement. Journal of Neurophysiology, 74, 1675–1688. Debicki, D. B., & Gribble, P. L. (2004). Inter-joint coupling strategy during adaptation to novel viscous loads in human arm movement. Journal of Neurophysiology, 92, 754–765. Fradkin, A. J., Sherman, C. A., & Finch, C. F. (2004). How well does club head speed correlate with golf handicaps? Journal of Science and Medicine in Sport / Sports Medicine Australia, 7, 465–472. Ghez, C., & Sainburg, R. (1995). Proprioceptive control of interjoint coordination. Canadian Journal of Physiology and Pharmacology, 73, 273–284. Gribble, P. L., & Ostry, D. J. (1999). Compensation for interaction torques during single- and multijoint limb movement. Journal of Neurophysiology, 82, 2310–2326. Hirashima, M., Kudo, K., & Ohtsuki, T. (2003). Utilization and compensation of interaction torques during ball-throwing movements. Journal of Neurophysiology, 89, 1784–1796. Jorgensen, T. P. (1994). The physics of golf. New York: Springer. Kaneko, Y., & Sato, F. (2000). The adaptation of golf swing to inertia property of golf club. In S. Haake (Ed.), The engineering of sport–Research, development and innovation (pp. 469–476). London: Blackwell. Karlsen, J., & Nilsson, J. (2007). Club shaft weight in putting accuracy and perception of swing parameters in golf putting. Perceptual and Motor Skills, 105, 29–38. Lephart, S. M., Smoliga, J. M., Myers, J. B., Sell, T. C., & Tsai, Y. (2007). An eight-week golf-specific exercise program improves physical characteristics, swing mechanics, and golf performance in recreational golfers. Journal of Strength and Conditioning Research / National Strength & Conditioning Association, 21, 860–869. Messier, J., Adamovich, S., Berkinblit, M., Tunik, E., & Poizner, H. (2003). Influence of movement speed on accuracy and coordination of reaching movements to memorized targets in three-dimensional space in a deafferented subject. Experimental Brain Research. Experimentelle Hirnforschung. Expérimentation Cérébrale, 150, 399–416. Miao, T., Makato, W., Kawaguchi, M., & Ikeda, M. (1998). A study of clubhead speed as a function of grip speed for a variety of shaft flexibility. In M. R. Farrally & A. J. Cochran (Eds.), Science and golf III: Proceedings of the 1998 World Scientific Congress of Golf (pp. 554–561). London: Human Kinetics. Milburn, P. D. (1982). Summation of segmental velocities in the golf swing. Medicine and Science in Sports and Exercise, 14, 60–64. Miura, K., & Naruo, T. (1998). Acceleration and deceleration phases of the wrist motion of the golf swing. In S. Haake (Ed.), The engineering of sport: Design and development (pp. 455–463). Cambridge: Blackwell Science. Neal, J., & Wilson, B. (1985). 3D kinematics and kinetics of the golf swing. International Journal of Sport Biomechanics, 1, 221–232. Nesbit, S. M. (2005). A three dimensional kinematic and kinetic study of the golf swing. Journal of Sports Science and Medicine, 4, 499–519.
100
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
Osis, S. T., & Stefanyshyn, D. J. (2010). Vibration at the wrist and elbow joints during the golf swing reveals shaft-specific swing kinematics. Procedia Engineering, 2, 2637–2642. Pelz, D. (1990). A simple scientific shaft test: Steel versus graphite. In A. J. Cochran (Ed.), Science and folf: Proceedings of the World Scientific Congress of Golf (pp. 264–269). London: Chapman & Hall. Pickering, W. M., & Vickers, G. T. (1999). On the double pendulum model of the golf swing. Sports Engineering, 2, 161–172. Roll, J. P., & Vedel, J. P. (1982). Kinaesthetic role of muscle afferents in man, studied by tendon vibration and microneurography. Experimental Brain Research. Experimentelle Hirnforschung. Expérimentation Cérébrale, 47, 177–190. Sainburg, R. L., Ghilardi, M. F., Poizner, H., & Ghez, C. (1995). Control of limb dynamics in normal subjects and patients without proprioception. Journal of Neurophysiology, 73, 820–835. Sprigings, E., & Mackenzie, S. (2002). Examining the delayed release in the golf swing using computer simulation. Sports Engineering, 5, 23–32. Stanbridge, K., Jones, R., & Mitchell, S. (2004). The effect of shaft flexibility on junior golfers’ performance. Journal of Sports Sciences, 22, 457–464. Vaughan, C. L. (1979). A three-dimensional analysis of the forces and torques applied by a golfer during the downswing. In A. Morecki, K. Fedelius, K. Kedzior, & A. Wit (Eds.), Biomechanics VII-B (pp. 325–331). Warsaw: University Park Press. Watanabe, K., Kuroki, S., Hokari, M., & Nishizawa, S. (1998). Golf swing and skill. In M. R. Farrally & A. J. Cochran (Eds.), Science and golf III: Proceedings of the 1998 World Scientific Congress of Golf (pp. 29–39). London: Human Kinetics. Worobets, J. T., & Stefanyshyn, D. J. (2008). Shaft stiffness: Implications for club fitting. In E. Thain (Ed.), Science and golf IV: Proceedings of the World Scientific Congress of Golf (pp. 431–437). London: Routledge. Yamasaki, H., Fujisawa, H., Hoshi, F., & Nagasaki, H. (2009). Incomplete posture adjustment during rapid arm movement. Perceptual and Motor Skills, 108, 915–932. Yamasaki, H., Tagami, Y., Fujisawa, H., Hoshi, F., & Nagasaki, H. (2008). Interaction torque contributes to planar reaching at slow speed. Biomedical Engineering Online, 7, 27. Zheng, N., Barrentine, S. W., Fleisig, G. S., & Andrews, J. R. (2008). Kinematic analysis of swing in pro and amateur golfers. International Journal of Sports Medicine, 29, 487–493.
Contents lists available at ScienceDirect
Human Movement Science journal homepage: www.elsevier.com/locate/humov
Golf players exhibit changes to grip speed parameters during club release in response to changes in club stiffness Sean T. Osis, Darren J. Stefanyshyn ⇑ Human Performance Laboratory, University of Calgary, Calgary, AB, Canada, T2N 1N4
a r t i c l e
i n f o
Article history: Available online 5 August 2011 PsycINFO Calssification: 2320 Keywords: Golf Shaft Flex Proprioception Vibration
a b s t r a c t The influence of golf club stiffness on driving performance is currently unclear, and it is possible that this ambiguity is due in part to golfer adaptation to equipment. The purpose of the current study was to elucidate mechanisms of adaptation to club stiffness, during the golf swing, by employing tendon vibration to distort proprioceptive feedback. Vibration (50 Hz, 1 mm amplitude) was applied to the upper extremities of 24 golfers using DC motors with eccentric weights. Golfers hit golf balls in a laboratory setting using three clubs of varying shaft stiffness, and club kinematics were recorded using high speed (180 Hz) digital cameras. The results demonstrated significant slowing of the club grip during club release for a high-stiffness shaft with vibration. This suggests that, when proprioceptive feedback is available, players adapt to changes in club stiffness by modifying the release dynamics of the club late in the downswing. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In order to hit a golf ball as far as possible, it is often suggested that golf club construction must be optimized for a golfer’s swing. This is evident in the plethora of available products and services, designed to provide innumerable equipment choices to the player, with the implication that some subset of these choices will provide the player with maximum distance. Given the wide breadth of available equipment, being able to accurately judge, a priori, which subset is optimal for a particular player is critically important. However, there is evidence that current procedures of club-player matching, with ⇑ Corresponding author. Address: Human Performance Laboratory, University of Calgary, 2500 University Drive NW, Calgary AB, Canada T2N 1N4. Tel.: +1 403 220 8637; fax: +1 403 284 3553. E-mail address: [email protected] (D.J. Stefanyshyn). 0167-9457/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.humov.2011.02.006
92
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
respect to club shaft stiffness, result in sub-maximal performance (Worobets & Stefanshyn, 2008). Additionally, studies of the effect of shaft stiffness on ball distance have demonstrated mixed results, and only some golfers seem to benefit from changes to shaft stiffness (Pelz, 1990; Stanbridge, Jones, & Mitchell, 2004; Worobets & Stefanyshyn, 2008). This raises the question as to why only some players benefit from changes in club shaft stiffness. One answer could be that players adapt their technique to their equipment, effectively negating the effects of mechanical changes in the equipment. This idea has been previously suggested in a study of the effects of string tension on ball placement in tennis (Bower & Cross, 2008), and a study of putting accuracy when using putters of varying mass (Karlsen & Nilsson, 2007). In both instances, large changes in the mechanical properties of the equipment resulted in little to no change in accuracy for the given task. This suggests that the athlete is adapting to the equipment, and may be doing so despite limited awareness of changes to their equipment (Bower & Cross, 2008). Recently, evidence has been presented that golf players adapt their swing kinematics in response to changes in shaft stiffness (Osis & Stefanyshyn, 2010; Fig. 1). In this study, tendon vibration was applied to the upper extremities of golfers during the swing. Due to the sensory-impairing nature of tendon vibration, it was concluded that this intervention impaired proprioceptive feedback, which in turn prevented any adaptation response by the players to their equipment. However, the exact nature of player response to the shaft is currently unclear, which raises the question of how players modify their swing to achieve consistency with different clubs. Osis and Stefanyshyn (2010) demonstrated that when proprioceptive feedback was impaired in golfers, club head speed was decreased in a stiff-shaft condition, but not for a compliant shaft-condition (Fig. 1). This implies that, when sensory feedback is intact, players change their swing kinematics to increase club head speed in a stiff-shaft condition. These changes in swing kinematics must therefore be: (1) shaft-specific and (2) related to the generation of club head speed. Club release, defined as the un-cocking of the wrists during the downswing, has been linked to the generation of club head speed at impact. In particular, it has been previously shown that delaying the timing of release can result in an increase in club head speed of 1.6–2.9% (Jorgensen, 1994; Pickering & Vickers, 1999; Sprigings & Mackenzie, 2002). These differences in club head speed, while small, could easily account for the changes reported by Osis and Stefanyshyn (2010), which were of the order of 1.3%. Therefore, modification of club release represents a potential player response to shaft stiffness; however, club release has not yet been examined. In addition to affecting club head speed, club release also has observable effects on the speed of the club grip (or grip speed) during the downswing. Several studies have shown that club release causes grip speed to decrease prior to impact (Miao, Makato, Kawaguchi, & Ikeda, 1998; Milburn, 1982; Miura & Naruo, 1998; Vaughan, 1979). This decrease occurs due to the forces acting upon the club as it rotates about the wrist (Jorgensen, 1994; Miura & Naruo, 1998). In terms of generating club head speed,
Fig. 1. Club head speed at impact as a function of club shaft stiffness (X-flex being the most stiff, and L-flex being the least). These data show that, with intact proprioception (white bars), players maintained similar kinematics at impact despite being given shafts of different stiffness. When proprioception was impaired (grey bars), players experienced a decrease in club head speed for a stiff shaft, but not for a compliant shaft. With permission from Osis and Stefanyshyn (2010).
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
93
both the timing, and the magnitude of peak grip speed have been linked with club head speed at impact. It has been observed that peak velocities of the upper extremities occur significantly closer to ball contact in professional players who generate higher club head speeds (Zheng, Barrentine, Fleisig, & Andrews, 2008). It has also been shown that players who generate higher club head speeds demonstrate higher peak grip speeds (Miura & Naruo, 1998). Golfers could potentially respond to changes in shaft stiffness by modifying club release in a way so as to increase their club head speed in a shaft-specific manner. If this is the case then, based on the current understanding of club release dynamics, concomitant changes in the timing and/or magnitude of peak grip speed should be observed. Grip speed, as a descriptor of club release, has not yet been examined. Therefore, the purpose of this study was to determine whether the timing and/or magnitude of peak grip speed change with increased shaft stiffness, in response to impairment of upper extremity proprioception during the golf swing. The following hypotheses were systematically tested: H1: For a high-stiffness shaft, peak grip speed will be reduced with tendon vibration. H2: For a high-stiffness shaft, peak grip speed will occur earlier with respect to ball contact with tendon vibration. H3: For a low stiffness-shaft, no differences will be observed in either the magnitude or timing of peak grip speed with tendon vibration. 2. Methods Twenty-four golf players, ranging in play level from recreational to professional/instructional, were recruited for this study. All players were free from musculo-skeletal injury and all gave informed consent prior to participating. This study received ethical approval from the Conjoint Health Research Ethics Board at the University of Calgary. Vibration was applied using a method adapted from Bock et al. (2007). Eight small vibrating motors were fixed to the skin using medical tape, 2–5 cm proximal to the wrist and elbow joints (Fig. 2). The motors were situated in order to stimulate the primary effectors of the wrist and elbow, while preventing any interference with the normal range of motion during the swing. Each vibrating motor was pre-fabricated and consisted of a 12 V DC motor (19.8 mm 15.0 mm 25.4 mm) and an eccentric weight (half-moon, 18.0 mm diameter 5.1 mm thick) attached to the motor shaft. The motors were each fit into a housing made from 1 inch PVC pipe, in order to protect the subject from the moving parts. The total mass of each housing/motor unit was 45.0 g. A pilot test, using a force-replication task, was conducted to ensure that the vibrating motors had their intended effect of impairing proprioceptive feedback. The force replication task required participants to learn to apply 1000 g of force-equivalent to a digital scale, using their wrist flexors. The subjects were then asked to repeat the force application without feedback as to the actual force they applied. This was performed with the vibrating motors taped over their wrist flexor and extensor
Fig. 2. Photographs showing the placement of vibrating motors on the upper extremity.
94
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
tendons, under two conditions; with the motors off (no vibration) or the motors on (vibration). The results of the pilot demonstrated that replication of applied forces was not impaired under the no vibration condition but was significantly compromised with vibration. During pilot testing, local vibration in the tissue adjacent to one motor was characterized using an accelerometer; the frequency of vibration was 50 Hz, while the amplitude was approximately 1 mm. This frequency and amplitude are within the range of values from previous studies using vibration to impair proprioceptive feedback (Bock, 2007; Cordo, Gurfinkel, Bevan, & Kerr, 1995; Roll & Vedel, 1982). The clubs consisted of three interchangeable driver shafts (Aldila NV 55), and one driver head (TaylorMade CGB Max Ltd.). These shafts were chosen as the manufacturer was able to maintain consistent mass and inertial properties while varying only the shaft stiffness (Table 1). The shafts were representative of the range of stiffness commercially available: L-flex (least stiff), R-flex, and X-flex (most stiff). With the vibrating motors in place, but not activated, each subject was given a five minute period for a self-directed warm-up using clubs not included in the testing protocol. Following warm-up, the protocol consisted of 10 swings, in each of three shaft conditions (X-flex, R-flex, and L-flex), and two vibration conditions (no vibration, vibration) for a total of 60 swings. The experiment was a full-block design, with randomization for the order of shaft stiffness, but not for the order of vibration conditions. Each subject performed 10 swings in each stiffness with no vibration, whereupon the subject performed 10 more in each stiffness with vibration turned on. After every 10 swings, the club was taken out of sight of the player and the shaft was changed, blinding the subjects to the club conditions. For the vibration trials, the motors were activated once the subject had addressed the ball and was prepared to swing, and they were then deactivated after the player reached the end of follow-through. During each swing, the player hit a standard golf ball from a rubber tee approximately 2 m into a net. The only criterion for rejection of a trial was radically errant ball flight after impact. An artificial turf surface was used to simulate practice conditions, and players were encouraged to wear footwear they normally would for play or practice. The driver head had three spherical retro-reflective markers (11 mm diameter) affixed to pins that were welded to the club head. The grips of the shafts each had a further three spherical retro-reflective markers that were affixed to pegs, which were glued to the grips using epoxy. Club kinematics was measured using nine high-speed digital video cameras which collected 2D images of the club markers at 180 Hz. The 2D images from all cameras were then transformed into 3D marker positions for all six club markers using DLT algorithms. Using the known geometry of the configuration of markers for each club, the orientations, positions, and velocities of the club head center-face and a point 14 cm distal to the club butt (center-grip) were calculated from the marker data. The kinematic variables addressed in this investigation are shown in Table 2. Players were categorized based on their response to the tendon vibration, using the same method applied by Osis and Stefanyshyn (2010). Study of the individual responses to vibration revealed a pattern whereby certain individuals exhibited greater grip speed in the L-flex compared with the X-flex. Based on how each subject’s shaft-specific trend in grip speed changed with vibration, they were divided into two groups. This was done by ranking subjects according to change in grip speed, calculated using Equation (1):
DGS ¼ ðGSX
nov ibe
GSL
nov ibe Þ
ðGSX v ibe GSL v ibe Þ
ð1Þ
Table 1 Selected driver shaft characteristics reported by the distributor.
Butt Flex (mm) Tip Flex (mm) Frequency (Hz) Swing Weight (g) Length (cm) Mass (g) Moment of Inertia (kg m2)
X-flex
R-flex
L-flex
70.3 97.7 255 34.7 111.8 57.7 0.0061
91.1 125.7 228 33.6 111.8 56.7 0.0060
114.5 151.0 203 34.0 111.8 54.1 0.0057
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
95
Table 2 List of variables addressed in this investigation. Symbol
Variable name
Description
HSimp GSimp GSpeak GSpeak-imp TGSpeak
Club head speed at impact Grip speed at impact Peak grip speed Grip speed decrease Time of peak grip speed
Magnitude of the center-face velocity vector at impact Magnitude of the center-grip velocity vector at impact Maximum magnitude of center-grip velocity Difference between peak grip speed and grip speed at impact Time from peak grip speed to impact (impact = 0)
Where GSX_novibe, GSL_novibe, GSX_vibe, and GSL_vibe indicate grip speed at impact for the following respective conditions: X-flex with no vibration, L-flex with no vibration, X-flex with vibration and L-flex with vibration. Using the average standard error of measurement for grip speed from all data (0.03 ms1), the standard error in DGS was calculated as 0.13 ms1. Subjects were then divided into two groups: a ‘change’ group for whom DGS > 0.13 ms1 (CHG group) and a ‘no change’ group for whom DGS < 0.13 ms1 (NOCHG). In order to verify the repeatability of this classification, the experiment was repeated with 3 subjects. DGS was again calculated and all 3 subjects were classified into the same groups as they had been previously. For the purposes of this study, only CHG players were analyzed as they demonstrated shaft-specific changes in club kinematics. Selected subject characteristics for this group are shown in Table 3. For each variable two-way analysis of variance (ANOVA) with repeated measures were run for each level of shaft and vibration condition. For those variables where a significant interaction effect was seen between shaft and vibration, simple effects testing using multivariate analysis of variance (MANOVA) was implemented to determine the main effects. Mauchly’s Test of Sphericity was used to ensure assumptions were met for the use of parametric statistics. Bonferroni-adjusted post hoc tests were conducted where appropriate. Significance was set at alpha < .05. 3. Results There was a significant decrease in HSimp for the X-flex with vibration (F = 10.29, df = 1, p = .004; Fig. 3). With vibration, there was a significant effect of stiffness on GSimp (F = 12.96, df = 2, p < .001), and players demonstrated decreased GSimp with increased shaft stiffness in the vibration condition. With vibration, there was a significant increase in GSpeak-imp with increased stiffness (F = 8.902, df = 2, p < .001). 4. Discussion The results for HSimp are in good agreement with published values. Club head speeds in the literature have ranged from 32–58 ms1 (Fradkin, Sherman, & Finch, 2004), with average values for low handicap players in the range of 40–46 ms1 (Lephart, Smoliga, Myers, Sell, & Tsai, 2007; Miura & Naruo, 1998; Nesbit, 2005; Watanabe, Kuroki, Hokari, & Nishizawa, 1998). The current values for GSpeak and GSimp were also within the range of values previously reported: 7–12 ms1 for GSpeak, and 4–8 ms1 for GSimp (Miura & Naruo, 1998; Nesbit, 2005). Data reporting the timing of peak grip speed is sparse, however Miura and Naruo (1998) presented values ranging from approximately 50–100 ms
Table 3 Selected subject characteristics for the CHG group. N
11
Height (cm) Mass (kg) Age (yrs) Handicap index
182 (6.3) 82.4 (11.5) 35.8 (13.0) 7.0 (2.6)
96
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
Fig. 3. Club head and grip kinematics by shaft flex and vibration. White bars are swings without vibration while grey bars are swings with vibration. Symbols indicate significance for Bonferroni-adjusted post hoc tests.
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
97
prior to impact. These are earlier in the swing than the data presented here, however, Miura and Naruo (1998) used a limited number of data points to fit a quadratic function, a procedure that could have produced errors in the timing of the peak. The first hypothesis tested was that for the X-flex shaft, GSpeak would be reduced in response to tendon vibration. No support was found for this hypothesis, as there were no significant changes in GSpeak with shaft stiffness. While it has been shown that higher GSpeak may coincide with higher HSimp between players (Miura & Naruo, 1998), it is possible that this is not the case for within-player comparisons. Therefore, the current study suggests that GSpeak may be an inappropriate variable for characterizing within-subject changes in HSimp. Without vibration, the ‘‘deceleration’’ of the grip during club release is the same across shaft conditions (Fig. 4). This ‘‘deceleration’’ of the grip is due to the reaction forces and torques exerted by the club upon the arms during club release (Jorgensen, 1994; Miura & Naruo, 1998; Neal & Wilson, 1985), and it is presumed that forces and torques needed to affect rotation of a high-stiffness shaft are higher than for a low-stiffness shaft, as a stiffer spring effectively constitutes a higher elastic load. Player behaviour with no vibration therefore suggests that the player response to changes in the shaft serves to maintain the same ‘‘deceleration’’ despite changes in reaction torques that arise due to shaft stiffness. When vibration was applied, the results in GSpeak-imp indicate that the grip ‘‘decelerated’’ more in the stiffer shaft and, ‘‘decelerated’’ less with the low-stiffness shaft (Fig. 4). Since the relative stiffness of the shafts represents the relative magnitude of the elastic load on the player, the trend in GSpeak-imp with vibration seems consistent with the mechanical properties of the clubs (high ‘‘deceleration’’ with X-flex, low ‘‘deceleration’’ with L-flex). The apparent dominance of club mechanics with impaired sensory feedback (grey bars), as compared to the consistency in kinematics when feedback is intact (dotted line bars), strongly suggests that golfers are capable of using sensory feedback to detect and respond to changes in shaft stiffness. It is speculated that, when shaft stiffness increases, players may respond by increasing shoulder and/or torso muscle activity, which compensates for the increase in reaction torques due to the shaft. An increase in shoulder torque, for example, would provide increased power flow to the arm segment to compensate for additional power flow away from the arm segment as work is done on the club. A mathematical model by Kaneko and Sato (2000) estimated how joint torques might change in the context of an increased load on the player due to an increase in club inertia. Interestingly, their results suggested that shoulder and torso torques would increase in the latter half of the downswing, during
Fig. 4. Hypothesized link between reaction torques due to the shaft and grip kinematics. Left panel: large reaction torque due to a stiff shaft (dark grey) and a smaller reaction due to a compliant shaft (light grey). Middle panel: decrease in grip speed (or ‘‘deceleration’’) from peak to impact, for stiff and compliant shafts, without vibration (dotted line bars) and with vibration (dark and light grey bars). Right panel: sample time-histories of grip speed for one subject demonstrating the decrease in grip speed prior to impact at 0 ms; traces are each one trial: one with vibration and a stiff shaft (dark grey), with vibration and a compliant shaft (light grey); and a single trial without vibration for a stiff shaft, provided for comparison (dotted).
98
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
club release. Although they did not address shaft stiffness directly, their result supports the speculation that players compensate for increases in reaction torques during release by modifying shoulder and torso muscular activity. Motor control research has provided similar interpretations regarding external loads during movement tasks. It is known that reaction forces due to external loads are important variables in the control of the upper extremity (Debicki & Gribble, 2004; Gribble & Ostry, 1999; Yamasaki, Tagami, Fujisawa, Hoshi, & Nagasaki, 2008, 2009). In order to achieve consistent movement, it has been suggested that the central controller is capable of learning and predicting reaction loads that will arise during the course of a movement task, and will adjust muscle activity proactively to compensate (Debicki & Gribble, 2004; Gribble & Ostry, 1999; Hirashima, Kudo, & Ohtsuki, 2003). In addition, when proprioception of the limb is compromised, as in the current study, compensation for changes in reaction loads is poor, and kinematic changes result (Ghez & Sainburg, 1995; Messier, Adamovich, Berkinblit, Tunik, & Poizner, 2003; Sainburg, Ghilardi, Poizner, & Ghez, 1995). In this context, it may be speculated that proprioceptive feedback allowed the players in this study to modify shoulder and/or torso muscle activity in response to the increased load imposed by a stiffer shafted club. This would explain why subjects in the current study were unable to maintain consistent ‘‘deceleration’’ across shafts in the vibration conditions, as the vibration would have distorted the sensory feedback required to program muscular responses during club release. While this is currently a speculation, motor control research supports the idea, and further study should be done to clarify how muscular output changes in response to shaft stiffness. The second hypothesis stated that for the X-flex shaft, GSpeak would occur earlier with respect to impact in response to tendon vibration. This hypothesis was not supported by the data, as no significant differences were found in TGSpeak. However, TGSpeak averaged 7 ms earlier in the X-flex shaft with impaired proprioception, and although not significant (p = .054), this result supports the concept that delayed release of the club is associated with higher club head speeds, a finding that has been previously suggested by mathematical models (Jorgensen, 1994; Pickering & Vickers, 1999; Sprigings & Mackenzie, 2002). In addition, this trend supports the hypothesis that the players increased their shoulder and/or torso muscular output as shaft stiffness increased. Increased power flow to the arms could cause their velocity to peak later in the downswing. Since this additional muscle activity must be programmed using proprioceptive feedback, timing changes could occur with impaired proprioception, as suggested by the 7 ms discrepancy in TGSpeak for the X-flex shaft. The third hypothesis was supported by the results, as no significant differences were found in either GSpeak or TGSpeak with vibration in the low-stiffness shaft. Interestingly, although HSimp was not significantly different with vibration, GSimp and GSpeak-imp both showed significant differences for the L-flex club. This suggests that even if shoulder or torso activity were increased with vibration, it may not produce additional club head speed. This result is difficult to interpret as the relationship between GSpeak-imp and HSimp is complex and could be affected not only by changes in shaft stiffness and player torques, but also ball position. It has been shown, for example, that changes in applied torques can increase club head speed, however, ball position must also be adjusted in order to maximize the effect (Chen, Inoue, & Shibara, 2007). Clearly, the relationship between grip ‘‘deceleration’’ and club head speed is intricate and requires further study. A limitation of the current study is the lack of measurement of player inputs to the swing. An explanation has been offered for the differences in GSimp and GSpeak-imp, based on how the player might vary muscular output, however, the way in which these changes relate to club head speed is complex, and requires an understanding of how work is done on the club. For example, although GSimp was increased in the more compliant shaft with vibration, there was no corresponding change in HSimp. This suggests that even though there was an increase in the kinetic energy of the arm segment, this energy did not transfer to the club by the time ball contact occurred. For the stiffer shaft, GSimp was decreased with vibration, and GSpeak-imp was increased. This may imply an increase in the transfer of kinetic energy to the club as shown by the findings of Chen et al. (2007), however Chen and others have chosen fixed values for shoulder torques (Chen et al., 2007; Jorgensen, 1994; Pickering & Vickers, 1999), an assumption which may not be valid. In particular, the current findings, which show systematic changes in GSpeak-imp, imply that shoulder and/or torso torque profiles could change between shaft conditions. Clearly further study is needed to elucidate the connection between player muscular activity, grip
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
99
speeds and club head speeds. Additionally, given that this study was conducted on a subset of players based on their response to vibration, the validity of current findings extends only to those players that can be similarly categorized. Further study is needed to clearly describe this particular group of players. In summary, the current findings indicate that the shaft-specific differences observed at impact with impaired proprioception arise from changes in the ‘‘deceleration’’ of the arm segment during club release. In addition, there is compelling evidence that, when given clubs of different stiffness, players respond by maintaining a consistent ‘‘deceleration’’ of the grip during release, and this response is programmed using proprioceptive feedback from muscular sources. The benefit to the player in employing this response seems to be the maintenance of high club head speed in the face of a changing external load due to club mechanical properties. Further study is needed to understand how player inputs affect the ‘‘deceleration’’ of the grip during club release, and to understand how changes in grip kinematics ultimately lead to differences in club head speed. Finally, these results may have implications for any study of sport performance involving equipment, and the detection and study of adaptation mechanisms in other activities may be of significant value. Acknowledgments The authors would like to acknowledge the financial support provided by the Natural Sciences and Engineering Research Council of Canada, Alberta Scholarship Programs and the Alberta Ingenuity Fund. The authors also thank Dr. Ian Wright for his technical assistance and TaylorMade for providing equipment for the study. References Bock, O., Pipereit, K., & Mierau, A. (2007). A method to reversibly degrade proprioceptive feedback in research on human motor control. Journal of Neuroscience Methods, 160, 246–250. Bower, R., & Cross, R. (2008). Elite tennis player sensitivity to changes in string tension and the effect on resulting ball dynamics. Sports Engineering, 11, 31–36. Chen, C., Inoue, Y., & Shibara, K. (2007). Numerical study on the wrist action during the golf downswing. Sports Engineering, 10, 23–31. Cordo, P., Gurfinkel, V. S., Bevan, L., & Kerr, G. K. (1995). Proprioceptive consequences of tendon vibration during movement. Journal of Neurophysiology, 74, 1675–1688. Debicki, D. B., & Gribble, P. L. (2004). Inter-joint coupling strategy during adaptation to novel viscous loads in human arm movement. Journal of Neurophysiology, 92, 754–765. Fradkin, A. J., Sherman, C. A., & Finch, C. F. (2004). How well does club head speed correlate with golf handicaps? Journal of Science and Medicine in Sport / Sports Medicine Australia, 7, 465–472. Ghez, C., & Sainburg, R. (1995). Proprioceptive control of interjoint coordination. Canadian Journal of Physiology and Pharmacology, 73, 273–284. Gribble, P. L., & Ostry, D. J. (1999). Compensation for interaction torques during single- and multijoint limb movement. Journal of Neurophysiology, 82, 2310–2326. Hirashima, M., Kudo, K., & Ohtsuki, T. (2003). Utilization and compensation of interaction torques during ball-throwing movements. Journal of Neurophysiology, 89, 1784–1796. Jorgensen, T. P. (1994). The physics of golf. New York: Springer. Kaneko, Y., & Sato, F. (2000). The adaptation of golf swing to inertia property of golf club. In S. Haake (Ed.), The engineering of sport–Research, development and innovation (pp. 469–476). London: Blackwell. Karlsen, J., & Nilsson, J. (2007). Club shaft weight in putting accuracy and perception of swing parameters in golf putting. Perceptual and Motor Skills, 105, 29–38. Lephart, S. M., Smoliga, J. M., Myers, J. B., Sell, T. C., & Tsai, Y. (2007). An eight-week golf-specific exercise program improves physical characteristics, swing mechanics, and golf performance in recreational golfers. Journal of Strength and Conditioning Research / National Strength & Conditioning Association, 21, 860–869. Messier, J., Adamovich, S., Berkinblit, M., Tunik, E., & Poizner, H. (2003). Influence of movement speed on accuracy and coordination of reaching movements to memorized targets in three-dimensional space in a deafferented subject. Experimental Brain Research. Experimentelle Hirnforschung. Expérimentation Cérébrale, 150, 399–416. Miao, T., Makato, W., Kawaguchi, M., & Ikeda, M. (1998). A study of clubhead speed as a function of grip speed for a variety of shaft flexibility. In M. R. Farrally & A. J. Cochran (Eds.), Science and golf III: Proceedings of the 1998 World Scientific Congress of Golf (pp. 554–561). London: Human Kinetics. Milburn, P. D. (1982). Summation of segmental velocities in the golf swing. Medicine and Science in Sports and Exercise, 14, 60–64. Miura, K., & Naruo, T. (1998). Acceleration and deceleration phases of the wrist motion of the golf swing. In S. Haake (Ed.), The engineering of sport: Design and development (pp. 455–463). Cambridge: Blackwell Science. Neal, J., & Wilson, B. (1985). 3D kinematics and kinetics of the golf swing. International Journal of Sport Biomechanics, 1, 221–232. Nesbit, S. M. (2005). A three dimensional kinematic and kinetic study of the golf swing. Journal of Sports Science and Medicine, 4, 499–519.
100
S.T. Osis, D.J. Stefanyshyn / Human Movement Science 31 (2012) 91–100
Osis, S. T., & Stefanyshyn, D. J. (2010). Vibration at the wrist and elbow joints during the golf swing reveals shaft-specific swing kinematics. Procedia Engineering, 2, 2637–2642. Pelz, D. (1990). A simple scientific shaft test: Steel versus graphite. In A. J. Cochran (Ed.), Science and folf: Proceedings of the World Scientific Congress of Golf (pp. 264–269). London: Chapman & Hall. Pickering, W. M., & Vickers, G. T. (1999). On the double pendulum model of the golf swing. Sports Engineering, 2, 161–172. Roll, J. P., & Vedel, J. P. (1982). Kinaesthetic role of muscle afferents in man, studied by tendon vibration and microneurography. Experimental Brain Research. Experimentelle Hirnforschung. Expérimentation Cérébrale, 47, 177–190. Sainburg, R. L., Ghilardi, M. F., Poizner, H., & Ghez, C. (1995). Control of limb dynamics in normal subjects and patients without proprioception. Journal of Neurophysiology, 73, 820–835. Sprigings, E., & Mackenzie, S. (2002). Examining the delayed release in the golf swing using computer simulation. Sports Engineering, 5, 23–32. Stanbridge, K., Jones, R., & Mitchell, S. (2004). The effect of shaft flexibility on junior golfers’ performance. Journal of Sports Sciences, 22, 457–464. Vaughan, C. L. (1979). A three-dimensional analysis of the forces and torques applied by a golfer during the downswing. In A. Morecki, K. Fedelius, K. Kedzior, & A. Wit (Eds.), Biomechanics VII-B (pp. 325–331). Warsaw: University Park Press. Watanabe, K., Kuroki, S., Hokari, M., & Nishizawa, S. (1998). Golf swing and skill. In M. R. Farrally & A. J. Cochran (Eds.), Science and golf III: Proceedings of the 1998 World Scientific Congress of Golf (pp. 29–39). London: Human Kinetics. Worobets, J. T., & Stefanyshyn, D. J. (2008). Shaft stiffness: Implications for club fitting. In E. Thain (Ed.), Science and golf IV: Proceedings of the World Scientific Congress of Golf (pp. 431–437). London: Routledge. Yamasaki, H., Fujisawa, H., Hoshi, F., & Nagasaki, H. (2009). Incomplete posture adjustment during rapid arm movement. Perceptual and Motor Skills, 108, 915–932. Yamasaki, H., Tagami, Y., Fujisawa, H., Hoshi, F., & Nagasaki, H. (2008). Interaction torque contributes to planar reaching at slow speed. Biomedical Engineering Online, 7, 27. Zheng, N., Barrentine, S. W., Fleisig, G. S., & Andrews, J. R. (2008). Kinematic analysis of swing in pro and amateur golfers. International Journal of Sports Medicine, 29, 487–493.