Human Movement Science 32 (2013) 1270–1287 Contents lists available at ScienceDirect Human Movement Science journal homepage: www.elsevier.com/locate/humov How to detect the yips in golf Martin K. Klämpﬂ a,⇑, Babett H. Lobinger a, Markus Raab a,b a Department of Performance Psychology, Institute of Psychology, German Sport University Cologne, Am Sportpark Müngersdorf 6, Cologne NRW 50933, Germany b Department of Applied Sciences, London South Bank University, 103 Borough Road, London SE1 0AA, United Kingdom a r t i c l e i n f o Article history: Available online 7 September 2013 PsycINFO classiﬁcation: 2223 2225 2320 2330 3100 3200 3700 Keywords: Focal dystonia Choking under pressure Kinematics Electromyography Putting a b s t r a c t The yips is a multi-aetiological phenomenon that is characterized by an involuntary movement that can affect a golfer’s putting performance. Diagnostics are crucial for a better understanding of what causes the yips but are still lacking. The purpose of the present study was therefore to identify sensitive methods for detecting the yips and evaluating its aetiology. Forty participants, 20 yipsaffected golfers and 20 nonaffected golfers, completed a psychometric testing battery and performed a putting session in the laboratory. They answered questions about their golﬁng and yips experience and ﬁlled in standardized questionnaires measuring trait anxiety, perfectionism, stress-coping strategies, somatic complaints, and movement and decision reinvestment. In the laboratory, they had to putt in ﬁve different conditions that might elicit the yips: as usual with both arms, under pressure, with one (the dominant) arm, with a unihockey racket, and with latex gloves. Measures included putting performance, situational anxiety, kinematic parameters of the putter, electromyography of the arm muscles, and electrocardiography. The groups were separated only by putting performance and kinematic parameters when putting with the dominant arm. Future research should use kinematics to investigate the aetiology of the yips and possible interventions. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Tony Jackling, Sam Snead, Arnold Palmer, Tom Watson, and Bernhard Langer are very successful golfers, and they all suffered from the infamous yips. The yips occurs mostly in putting and consists ⇑ Corresponding author. Tel.: +49 (0)221 4982 5721; fax: +49 (0)221 4982 8320. E-mail address: m.klaempﬂ@dshs-koeln.de (M.K. Klämpﬂ). 0167-9457/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.humov.2013.04.004 M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 1271 of involuntary movements appearing shortly before hitting the ball that result in loss of control and usually missing the putt (McDaniel, Cummings, & Shain, 1989; Smith et al., 2000; Stinear et al., 2006). Such loss of control has a large impact on golf performance and has consequences for the athlete’s career, as the putt represents the most important stroke in high-level golf. The yips is a common phenomenon in golf, affecting 28 to 48% of golfers (McDaniel et al., 1989; Smith et al., 2000). However, the use of samples covering different performance levels as well as reliance on subjective reports may explain the high discrepancy in reported prevalence rates. A standardized method for diagnosing the yips is needed to form the basis of a scientiﬁc approach dealing with this controversial phenomenon. We therefore propose psychometric measurements and a putting experiment as a sensitive method for detecting yips-affected golfers. 1.1. Aetiology of the yips—Neurological origin The literature examining the aetiology of yips is equivocal. Contemporary research places the yips on a continuum between a neurological origin connected to focal dystonia and a psychological origin linked to choking under pressure (Smith et al., 2003; Stinear et al., 2006). The yips was ﬁrst categorised as a task-speciﬁc focal dystonia (Adler, Crews, Hentz, Smith, & Caviness, 2005; McDaniel et al., 1989; Sachdev, 1992; Smith et al., 2000). Focal dystonia describes a neuromuscular movement disorder whose symptoms include involuntary muscular contractions resulting in twisting and repetitive movements or abnormal postures occurring exclusively in one body part and during the performance of a task (Pont-Sunyer, Martí, & Tolosa, 2010). Commonly affected tasks are writing, playing a musical instrument, and others requiring highly repetitive ﬁne motor skills (Torres-Russotto & Perlmutter, 2008). Reported dystonia-affected sports other than golf include table tennis (Le Floch et al., 2010), pistol shooting (Sitburana & Ondo, 2008), petanque (Lagueny et al., 2002), and tennis (Mayer, Topka, Boose, Horstmann, & Dickhuth, 1999). The mechanisms of dystonia are still unclear but are assumed to involve abnormalities within the basal ganglia, inhibitory and processing dysfunction of the sensorimotor system, and abnormal plasticity (Rosenkranz et al., 2008). Clinical signs of task-speciﬁc focal dystonia include the presence of a phasic dystonic movement and the following accompanying signs (Albanese & Lalli, 2009): First, an overﬂow that describes a coactivation of neighbouring muscles not normally involved. The occurrence of such cocontractions has been shown in some yips-affected golfers (Adler et al., 2005, 2011). Second, mirror dystonia deﬁned as the appearance of the dystonic muscle activation in the affected limb even if the movement is performed with the opposite side. Third, effective sensory tricks that cause a temporal absence of symptoms due to a change in the sensory pathways through additional tactile input (Abbruzzese & Berardelli, 2003; Tinazzi, Rosso, & Fiaschi, 2003). For instance, symptoms are reduced when affected pianists play with gloves (Altenmüller & Jabusch, 2009). Besides clinical signs, speciﬁc rating scales are used to diagnose dystonia, such as Fahn’s Arm Dystonia Disability Scale (ADDS; Burke et al., 1985). The genetic disposition of an affected patient can be assessed by the occurrence of dystonia-like symptoms in the family (Schmidt et al., 2009). Reported yips prevalence rates are about 30 times higher than the 1% prevalence of musician’s dystonia (Altenmüller, 2003) and up to 5,000 times higher than those of other forms of focal dystonias such as writer’s cramp and facial dystonia (Fukuda, Kusumi, & Nakashima, 2006; Nutt, Muenter, Aronson, Kurland, & Melton, 1988), indicating that dystonia might only partially explain the yips. 1.2. Aetiology of the yips—Psychological origin Yips-affected golfers reported that the symptoms often occur in pressure situations (McDaniel et al., 1989; Philippen & Lobinger, 2012; Smith et al., 2003). At the same time, performance anxiety is thought to play a major role in both triggering the yips and exacerbating its symptoms (McDaniel et al., 1989; Smith et al., 2000). The yips is therefore also associated with choking under pressure, which is deﬁned as the ‘‘process, whereby the individual perceives their resources are insufﬁcient to meet the demands of the situation, and concludes with a signiﬁcant drop in performance—a choke’’ (Hill, Hanton, Fleming, & Matthews, 2009, p. 209). The yips can be seen as a severe form of choking (Masters, 1992) or at least exhibits ‘‘many characteristics similar to a severe form of choking’’ (Bawden 1272 M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 & Maynard, 2001, p. 937). However, rather than describing an acute or sudden drop in performance, the yips may represent a chronic form of severe choking because its symptoms occur more steadily over time. The existence of a chronic form of choking in golf was ﬁrst mentioned without the connection to the yips by Gucciardi, Longbottom, Jackson, and Dimmock (2010). Two types of attentional theories have been proposed to explain the mechanisms of choking under pressure (see Hill, Hanton, Matthews, & Fleming, 2010, for a review): distraction theories and self-focus theories. Distraction theories, such as Eysenck and Calvo’s (1992) processing efﬁciency theory, claim that a choke results from pressure-induced anxiety that shifts the focus of attention of the individual away from task-relevant information. This mechanism is thought to apply primarily to cognitive tasks. Self-focus theories perhaps come closer to explaining the occurrence of the yips because they assume that performance anxiety causes the athlete to shift the focus of attention inward or to consciously monitor the skill, which detrimentally affects the well-learned, automated action (Baumeister, 1984). This is also the basis of reinvestment theory, which tries to unite all self-focus theories (see Masters & Maxwell, 2008, for a review). Reinvestment is deﬁned as the ‘‘manipulation of conscious, explicit, rule based knowledge, by working memory, to control the mechanics of one’s movements during motor output’’ (Masters, 1992, p. 208). The more explicit knowledge the athlete has, the more likely it is that the movement will be disrupted when attempting to consciously control it. This usually happens when the athlete perceives pressure and tries to ensure high performance by consciously intervening in a movement that normally runs automatically. Rotheram, Maynard, Thomas, and Bawden (2007) found through a questionnaire-based study that yips-affected athletes had an increased tendency both to consciously control (reinvest in) their movements and to be perfectionist. The occurrence of choking under pressure also depends on two appraisals, according to the classic stress model (Lazarus, 1974). The primary appraisal clariﬁes if the situation represents an individual threat. The secondary appraisal focuses on the resources the individual needs to be able to cope with the situation. Therefore, positively perceived stress-coping strategies such as positive self-instruction, situational control, and reaction control seem to be beneﬁcial in the avoidance of choking (Janke & Erdmann, 2008). Yips-affected golfers might therefore use more negative or maladaptive stress-coping strategies, such as cognitive rumination, resignation, and self-blaming, than their nonaffected colleagues. 1.3. Gaps in yips research and aims of the present study From the descriptions of dystonia and choking under pressure it appears that the yips can be explained by two independent theories. We assume that both dystonia and a more severe form of choking under pressure might result in involuntary movements during the execution of the skill—the yips. To the best of our knowledge, no difference in the appearance of the yips depending on its aetiology has been postulated. Separating the yips according to the two presented aetiologies is difﬁcult to do, and the distinction between them has been evaluated rather than studied so far (Smith et al., 2000, 2003; Stinear et al., 2006). These studies grouped the yips as either neurologically based (Type I) or psychologically based (Type II), relying only on the subjective descriptions of the participants. In the following we will not use such a classiﬁcation because its validity has not yet been conﬁrmed. Alternative explanations of the yips might be possible. For instance, Marquardt (2009) described the yips as a contextual movement disorder, which can be learned by fatal movement strategies such as practicing the wrong movement technique. Furthermore, earlier studies selected their yips-affected participants by relying on self-reports of the participants about their yips symptoms. No standardized selection criterion, for example, on the basis of the phenomenological appearance of the yips, has yet been used. Many different measurement methods have been used to investigate the yips, including recording muscle activity, grip force, heart rate, putting performance, and kinematic parameters. Yips-affected golfers exhibited an increased muscle activity (Smith et al., 2000; Stinear et al., 2006) and cocontractions of the lower forearm muscles (Adler et al., 2005, 2011). While Smith et al. (2000) found that yipsaffected golfers had a decreased performance accuracy compared to nonaffected golfers, Stinear et al. (2006) could not ﬁnd a difference in performance. Moreover, yips-affected golfers showed an increased grip force and increased mean heart rate during the execution of putts (Smith et al., 2000). 1273 M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 Although Marquardt and Fischer (2008) found a more objective way (compared to self-reports) to identify the yips using a vague kinematic template of yips-affected golfers, they examined only kinematic differences between affected and nonaffected golfers. They discovered that especially the rotation rate and the face angle of the putter around ball impact are inconsistent in yips-affected golfers. Other studies on the yips focused on psychometric measures. For instance, McDaniel et al. (1989) found a tendency toward obsessional thinking in yips-affected golfers. Sachdev (1992) used a battery of different psychometric measures to look at trait anxiety, somatic complaints, depression, and obsession. No differences between the affected and nonaffected golfers were found, but once again, the study did not use clear criteria to distinguish between the golfers. For instance, it may not be necessary for golfers to have played at least 5 years to be categorized as yips affected. Philippen, Klämpﬂ, and Lobinger (2012) argued that beginners and golfers with less experience and a higher handicap can also suffer from the yips. Rotheram et al. (2007) reported differences between yips-affected and nonaffected athletes in perfectionism and reinvestment, but they did not include a description of their selection criterion. To sum up, ﬁndings have been reported so far with no standardized method to identify yips-affected golfers in the ﬁrst place. A standardized method is, however, required for further investigations of the yips. We sought to overcome the limitations of previous studies by comparing yips-affected golfers with nonaffected golfers who were grouped according to a more reliable selection criterion based on the phenomenological appearance of the yips. This is the ﬁrst study in this ﬁeld that combines a wide range of diagnostic tools, including psychometric as well as psychophysiological, behavioral, and kinematic measures. The main purpose of the present study was to ﬁnd sensitive methods to diagnose the yips independent of its aetiology. The application of these diagnostic tools in ﬁve different putting conditions, which are explained in the Methods section, might further reveal the aetiology of the yips (Table 1). We hypothesized that yips-affected golfers would have higher values than nonaffected golfers in psychometric measures, including trait performance anxiety, perfectionism, reinvestment, and somatic complaints. We also expected that they would use more negative and fewer positive stress-coping strategies than nonaffected golfers. In regard to the putting experiment, we hypothesized that yips-affected golfers would exhibit higher muscle activity, higher inconsistency in the kinematic parameters, and lower putting performance than nonaffected golfers. Yips-affected golfers were expected to show higher values in the kinematic and psychophysiological parameters and a decrease Table 1 Overview of measures and methods. Pre-experiment (online survey) Experiment (putting session in laboratory) General measures Standardised questionnaires Measures and scales Pre-putting Putting Post-putting Informed consent Trait-anxiety (WAI-T) Heart activity (LF/HF ratio, heart rate) EMG (MVC 2) Demographics Decision reinvestment (DSRS) Movement reinvestment (MSRS) Perfectionism MPS-F Somatic complaints (SCL-90) Putting test with one arm Heart activity at rest EMG (MVC 1) EMG (RMS, cocontraction) FeedbackInterview Golf experience Yips experience Neurological questions and ADDS Stress-coping strategies (SVF-78) Kinematics (SDs of rotation, face angle velocity, and acceleration at impact) Anxiety Thermometer State-anxiety (WAI-S) Performance accuracy (holed putts, distance) Note. ADDS: Fahn’s Arm Dystonia Disability Scale; DSRS: Decision-Speciﬁc Reinvestment Scale; EMG: electromyography. LF/HF: low frequency/high frequency; MPS-F: German version of Frost’s Multidimensional Perfectionism Scale; MSRS: MovementSpeciﬁc Reinvestment Scale; MVC: maximum voluntary contraction; RMS: root mean square; SCL-90: subscale of the Symptom Checklist; SVF-78: German Stress-coping Questionnaire; WAI-S: German State Competition Anxiety Inventory; WAI-T: German Trait Competition Anxiety Inventory. 1274 M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 Table 2 Participant characteristics. Characteristic Yips M ± SD No yips M ± SD Age (in years) Handicap Golf experience In years Hours per week Sex ratio (female/male) 53.9 ± 13.9 27.4 ± 17.5 51.3 ± 14.1 33.5 ± 18.7 7.6 ± 5.2 6.0 ± 3.5 2/18 12.0 ± 13.1 5.7 ± 5.4 2/18 Note. MANOVA showed no signiﬁcant differences between groups. in putting performance in the pressure and one-arm putting condition compared to the control putting condition. The opposite was expected for the sensory trick and context change conditions, as explained in the following section. 2. Methods 2.1. Participants The sample size was limited to 40 participants for practical reasons. The 40 golfers (36 males, 4 females) were assigned to either a yips-affected group (n = 20) or a nonaffected group (n = 20). Participant characteristics are presented in Table 2. It was explicitly searched for yips-affected and nonaffected golfers. They were recruited by ﬂyers in local golf clubs and from previous studies. The yips criterion was established according to the advice of a professional golf coach who had approximately 15 years’ experience dealing with the yips in golf. Following his advice, we identiﬁed golfers as yips-affected when they exhibited obvious twists of the wrist while putting up to ﬁve times with only the dominant arm in a pre-test. A video clip of a one-handed putt by a yips-affected golfer can be seen in the supplementary content. The assignment of participants to either the yips-affected or nonaffected group was completed by the ﬁrst author, whereas the golfers were not told about their grouping. In contrast to previous investigations, not only expert golfers, but also golfers with higher handicaps, that is, at a lower performance level, were included in the study, because the yips is not restricted to expert golfers (Philippen et al., 2012). All participants were identiﬁed as right-handed with the Edinburgh Handedness Inventory (Oldﬁeld, 1971). They all putted with their right arm in the laboratory experiment. Each participant was informed of the requirements of the investigation and all provided informed consent before testing commenced. Ethical clearance to conduct the study was provided by the national psychology association and the authors’ university ethics board. 2.2. Online survey The online survey consisted of items to obtain the informed consent, demographic information, and golﬁng and yips experience. In the yips experience section, the participants were provided with a yips deﬁnition by Smith et al. (2000) and subsequently had to indicate whether they had ever experienced the yips. The survey also contained the ADDS (Burke et al., 1985) and questions about known neurological disorders in the family and current medication to determine if there was a link to focal dystonia. The neurological scale contained questions about impairment in daily activities, such as grasping an object or writing. Participants had to rate their impairment on a Likert scale from 0 (no impairment) to 3 (strong impairment). The participants ﬁlled in randomized, standardized questionnaires about the following psychometric measures. Trait anxiety was measured with the German Trait Competition Anxiety Inventory (WAIT; Brand, Ehrlenspiel, & Graf, 2009); participants had to answer such items as ‘‘Before competitions, I worry about failing under pressure’’. The reported Cronbach’s alpha values for the three subscales were M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 1275 all higher than .72. Perfectionism was estimated with the German version of Frost’s Multidimensional Perfectionism Scale (MPS-F; Altstötter-Gleich & Bergemann, 2006). Cronbach’s alpha values for the six subscales ranged from .70 to .90. A sample item for the subscale performance-related doubts would be ‘‘Even if I do something very thoroughly, I often have the feeling that it is not fully correct’’. Validated German versions of the movement-speciﬁc (a = .77) and decision-speciﬁc reinvestment (a = .84) scales (MSRS and DSRS; Laborde, Musculus, Kalicinski, Klämpﬂ, & Lobinger, 2013) were used to determine the reinvestment level of the golfers. Sample items for the reinvestment scales include ‘‘I am always trying to think about my movements when I carry them out’’ and ‘‘I rarely forget the times when I have made a bad decision, even about minor things’’, respectively. The short form of the German stress-coping questionnaire (SVF-78; Janke & Erdmann, 2008) covered positive (a = .89) and negative (a = .94) coping strategies. Positive coping strategies such as distraction are characterized by, for instance, the item ‘‘I try to shift my thoughts to something different’’. Negative strategies are characterized by, for example, ‘‘I wonder what I did wrong again’’ for the subscale self-blaming. Finally, somatic complaints were obtained with the subscale (a = .83) of the Symptom Checklist (SCL-90; Derogatis, Lipman, & Covi, 1973). There, the participants were asked to rate how much they had suffered from, for example, headaches or muscle pain within the last 7 days. For data analysis, only the main scales of the questionnaires were considered. 2.3. Putting experiment The experimental part took place in a laboratory. The participants putted on an artiﬁcial putting green from a distance of 1.5 m. This distance is within the range most likely to elicit symptoms (Smith et al., 2000). The participants had to putt in ﬁve different conditions, which have the potential to reveal or alleviate the yips. 2.3.1. Putting conditions In the control condition, participants were instructed to putt as they would on the putting green. Thepressure condition consisted of putting while being exposed to three deﬁned stressors: monetary incentive (Mullen & Hardy, 2000; Wang, Marchant, Morris, & Gibbs, 2004) video taping of putts (Mesagno, Marchant, & Morris, 2009), and the monotonous repeating of a soccer audience booing (Laborde, Brüll, Weber, & Anders, 2011). The rationale behind using all these stressors was to ensure that at least one would increase perceived pressure in the participants. The detailed instructions for the pressure condition were as follows: ‘‘Try to hole the next 15 putts. You have 15 euros in your account. For every missed putt, you lose one euro. Any money you still have at the end we will donate to a charity for kids in Africa. One euro is enough to ensure that a child survives one week. Every putt counts. The putts will be conducted under hindered conditions. Noise from an audience will be played throughout the whole condition. A video camera placed behind the hole will record your movements, which will be assessed later by a golf professional with respect to the putting technique.’’ Losing one euro was acoustically accompanied by the sound of the researcher dropping a coin into a metal box. The raised money was donated to a charity. In the sensory trick condition, participants were required to putt with latex gloves on both hands. In the context change condition, the golfers putted with a unihockey racket using their normal putting grip. With this condition, we tested how stable the yips is when small changes in the context are made but the movement remains the same. A unihockey racket was chosen because it was assumed that all participants had little or no experience in using it, guaranteeing no bias from this aspect. In the one-arm condition, the golfers putted with only their dominant (right) arm, while the left arm was held motionless at the side of the body. Yips-affected golfers would consequently have no chance to compensate for the jerk by applying the second supporting arm. 2.3.2. Putting performance Putting performance was measured by both the number of holed putts and the distance from the ball to the hole in centimeters after each putt. A webcam ﬁxed on the ceiling above the hole captured 1276 M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 the putting performance. Footage was analyzed via the video-analysis software utiliusÒ easyinspect (ccc Gmbh, Markkleeberg, Germany). Participants were instructed to use their own putter and preferred putting grip. Forcing the participants to use a standardized putter and the conventional grip instead could have inﬂuenced the yips behavior. 2.3.3. Kinematic analysis Kinematic parameters of the putters were derived by means of the ultrasound-based SAM PuttLab Pro Wireless 2010 System (Science&Motion GmbH, Munich, Germany), which was used in earlier putting studies (Karlsen, Smith, & Nilsson, 2008; Marquardt, 2007; Marquardt & Fischer, 2008). Data was processed and analyzed with the accompanying software. The standard deviations of directionally relevant parameters, including rotation rate and face angle, and speed-related parameters, speciﬁcally velocity and acceleration, of the putter at impact were calculated and indicate the movement consistency. 2.3.4. Electromyography Electromyograms (EMGs) of the ﬂexor carpi radialis (FCR), extensor carpi radialis (ECR), and biceps brachii (BB; Smith et al., 2000; Stinear et al., 2006) were bilaterally recorded at 1024 Hz with ASA Lab (ANT B.V., Enschede, The Netherlands), a neurophysiological multirecording system. Bipolar surface electrodes (Ag/AgCl) were placed according to the European Recommendations for Surface Electromyography (Hermens et al., 1999). Recordings were stored for each putting condition. These were processed ofﬂine (bandpass ﬁlter: 20–250 Hz; bandstop ﬁlter: 49–51 Hz) with ASA software (ANT B.V., Enschede, The Netherlands) and subsequently in Matlab (The MathWorks, Inc., Natick, Mass.). The yips is usually observed in the front swing shortly before ball contact during the putt. All participants, across all putting trials, had a forward-swing-to-impact phase lasting at least 150 ms (M = 316 ms, SD = 68 ms). Therefore, the root mean square (RMS) of the muscle activity was estimated for the time frame of 150 ms before ball contact to ball contact for each trial and muscle. Ball contact was triggered by a synchronized recording microphone. To enable a group comparison, the RMS was put in relation to the maximum RMS derived in individual maximum voluntary contraction (MVC) tests. This test was conducted twice for each muscle, once before and once after the putting session, to control for muscle fatigue. For data analysis, the mean relative RMS for each muscle and the cocontraction ratio (Lohse, Sherwood, & Healy, 2011) of the ECR and FCR for each putting condition were used. 2.3.5. Manipulation check, stress level, and state anxiety State anxiety was measured with the validated Anxiety Thermometer (Bakker, Vanden Auweele, & van Mele, 2003; Houtman & Bakker, 1989; Oudejans & Pijpers, 2010), in which the participants had to subjectively rate their anxiety on a scale from 0 (not at all) to 10 (panic). In each condition, participants were asked twice, once before the ﬁrst and once after the 10th putt, how nervous they felt at the moment. The mean of these two values counted as the subjective nervousness indicator for each condition. Electrocardiograms (ECGs) were derived as an objective index of stress with a recording rate of 256 Hz with the portable multi-recording system NeXus-4 (Mind Media BVÒ, Roermond-Herten, The Netherlands). Raw ECG signals were stored in Biotrace software (Mind Media BVÒ, Roermond-Herten, The Netherlands) and processed with Matlab-based Kubios HRV software (Biosignal Analysis and Medical Imaging Group, University of Kuopio, Finland). The last 3 min of each recording, each representing a putting condition, were used to calculate mean heart rate and the low- and high-frequency band ratio of the heart rate variability (HRV). Temperature and breathing frequency as possible confounding factors for ECG were recorded (Neumann & Thomas, 2011). The standardised German State Competition Anxiety Inventory (WAI-S; Ehrlenspiel, Brand, & Graf, 2009) served as an additional manipulation check for the pressure putting condition. 2.4. Procedure A link to a personalized online survey was sent to recruited participants. After completing the questionnaire, they were invited to the laboratory for the putting experiment. Filling in the online survey and participating in the putting experiment took place on different days. Golfers gave their informed M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 1277 consent and assurance that they had followed the preparation instructions for avoiding possible confounding factors for the physiological measurements, such as following their usual sleeping routine, not training the day before, and not consuming caffeine or tobacco (Laborde et al., 2011; Nakahara, Furuya, Francis, & Kinoshita, 2010). After the golfers performed a one-handed putting pre-test, they were immediately grouped without telling them as either yips-affected or nonaffected without consideration of their indication on the online survey. As previously mentioned, they were grouped as yips-affected, when they exhibited obvious twists of the wrist while performing the putting pre-test. They were then prepared for the EMG and ECG measurements. Afterwards, the participants performed the ﬁrst MVC test, in which they had to isometrically contract each muscle (consecutively) twice for 3 s. The actual putting experiment started with 10 warm-up putts followed by 15 putts in each of the ﬁve putting conditions presented in an incomplete counterbalanced order by means of two mirrored Latin squares. In each condition, the golfers had to twice rate their level of anxiety on the Anxiety Thermometer. They ﬁlled in the WAI-S questionnaire about state anxiety after receiving the instructions for the pressure and the control condition. The video camera was set up in the pressure putting condition only and then removed again. Putting performance was captured after each putt. EMG and ECG were recorded for each condition. After putting, the second MVC test containing the same script as the ﬁrst one followed. Finally, a short, recorded feedback interview was completed to get information about the effectiveness of each single stressor used in the pressure condition. The participants were asked to shortly describe, how they perceived the applied stressors. After the completion of the study, the participants received via email individual feedback on their performance and a putting technique report created by the professional golf coach using the kinematic data derived in the control putting condition. The individual putting technique report included no indication of yips, but general advices on how to improve the putting technique. 2.5. Statistical analysis Multivariate analyses of variance (MANOVAs) were used to ﬁnd differences between the yips-affected and nonaffected groups regarding demographics and psychometric measures. In all calculated MANOVAs, Pillai’s trace was chosen to indicate the critical value because of its conservativeness and robustness to assumption violations, such as no homogeneity of covariances, if equal sample sizes are compared. Main effects and interaction effects were tested for group (yips, no yips) as the betweengroups factor and condition (control, pressure, one arm, sensory trick, context change) as a withingroup factor via repeated-measures MANOVAs for the various parameters derived from the putting experiment. A main effect of group would indicate that the measure distinguishes the groups independent of the putting condition and could be powerful in detecting the yips. A main effect of condition would signal that conditions differed from each other according to the measure used. An interaction effect between group and condition would reveal that speciﬁc putting conditions cause different putting behaviors in the groups and might be useful to diagnose the yips. In the case of sphericity violation, the Greenhouse–Geisser correction was applied. Follow-up analyses of variance (ANOVAs) were conducted after signiﬁcant ﬁndings in the main analysis to gain further insight into the parameters causing the signiﬁcant effect. Signiﬁcant ANOVAs followed planned simple contrasts with the control condition as the reference putting condition. The signiﬁcance level was set to .05. Outliers (4.1%) were corrected by Winsorisation, the replacement of the discarded values by the most extreme retained values (Wilcox, 2005), here represented by the values two standard deviations away from the group mean. Missing values (3.2%) including all derived data were then replaced by the group mean. Deleting these values would not have changed the results. 3. Results 3.1. Online survey The 40 participants needed on average 47 min to complete the survey. Eighteen (yips group 17; nonaffected group: 1) indicated in the online survey having already experienced the yips, meaning 1278 M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 Table 3 Scores on the standardized questionnaires. Psychometric measure Trait anxiety Somatic complaints Movement-speciﬁc reinvestment Decision-speciﬁc reinvestment Perfectionism Positive stress coping Negative stress coping Questionnaire Scale WAI-T SCL-90 MSRS DSRS MPS-F 1 0 1 1 1 to to to to to 4 4 6 5 6 SVF-78 0 to 4 Scale scores Yips No yips 2.09 ± 0.42 0.34 ± 0.32 3.16 ± 0.94 2.61 ± 0.72 2.27 ± 0.47 2.02 ± 0.44 1.43 ± 0.72 2.05 ± 0.57 0.27 ± 0.24 3.36 ± 0.78 2.50 ± 0.77 2.30 ± 0.57 2.16 ± 0.49 1.35 ± 0.53 Note. DSRS: Decision-Speciﬁc Reinvestment Scale; MPS-F: German version of Frost’s Multidimensional Perfectionism Scale; MSRS: Movement-speciﬁc Reinvestment Scale; SCL-90: subscale of the Symptom Checklist; SVF-78: German Stress-Coping Questionnaire; WAI-T: German Trait Competition Anxiety Inventory. that 15% of the yips-affected participants as detected by the pre-test were not aware of being affected and 5% of the nonaffected participants rated themselves wrongly as yips affected. At the same time, 35% of the nonaffected golfers reported having putting problems unrelated to the yips. One participant who was formerly affected by the yips did not meet the yips grouping criterion and was consequently put into the nonaffected group. 3.1.1. Neurological questions No impairment in the execution of daily activities could be detected within the self-reported yipsaffected golfers. The mean score on the ADDS was 1.03 ± 0.21. None of the participants indicated that they had to cope with a neurological disorder. One participant in each group reported a family member having hand tremors. 3.1.2. Standardised questionnaires A one-factorial MANOVA with trait anxiety, perfectionism, movement reinvestment, decision reinvestment, somatic complaints, negative coping strategies, and positive coping strategies as dependent variables showed no main effect of group on these psychometric measures (Table 3), F(7, 32) = 0.42, p > .05, gp2 = .083. 3.2. Putting experiment 3.2.1. Manipulation check of pressure condition A 2 (Group: yips, no yips) 2 (Condition: control, pressure) repeated-measures MANOVA with the subscales of the WAI-S as dependent variables showed only a signiﬁcant main effect of group. However, there was no signiﬁcance on the univariate level because of apparent correlations between the WAI-S subscales. Only a tendency of an effective pressure manipulation was found, indicated by scores on the subscales somatic anxiety and concern, but not optimism. An overview of the descriptive statistics of all derived parameters can be seen in Table 4. F values and effect sizes are given in Table 5 and statistical parameters of the planned simple contrasts in Table 6. 3.2.2. Stress level and state anxiety in all conditions A 2 (Group: yips, no yips) 5 (Condition: control, pressure, one arm, sensory trick, context change) repeated-measures MANOVA with LF/HF ratio, heart rate, and Anxiety Thermometer as dependent variables revealed main effects of group and condition, but no interaction effect. Follow-up ANOVAs of between-subjects effects indicate that the yips-affected group had across all conditions a lower heart rate (M = 78.4, SE = 2.2) than the nonaffected group (M = 88.3, SE = 2.2). The main effect of condition occurred due to heart rate and the score on the Anxiety Thermometer. Simple contrasts uncovered that heart rate was substantially higher in the pressure condition (M = 85.9, SE = 1.7) and lower in the one arm putting condition (M = 81.7, SE = 1.6) than in the control condition (M = 83.6, SE = 1.5). Table 4 Descriptive statistics of parameters derived in the putting experiment. Measure Putting condition Yips WAI-S Somatic anxiety Concern Optimism Anxiety Thermometer ECG LF/HF ratio [%] HR [bpm] EMG Cocontraction right Cocontraction left FCR right [%MVC] ECR right [%MVC] BB right [%MVC] FCR left [%MVC] ECR left [%MVC] BB left [%MVC] Kinematics (impact) Velocity SD [mm/s] Acceleration SD [m/s2] Face angle SD [°] Pressure No yips Yips One arm No yips Yips Sensory trick No yips Yips Context change No yips Yips No yips 1.4 ± 0.5 1.4 ± 0.5 2.8 ± 0.6 3.0 ± 1.4 1.3 ± 0.4 1.5 ± 0.5 3.0 ± 0.6 3.0 ± 1.8 1.6 ± 0.4 1.6 ± 0.5 2.7 ± 0.6 4.0 ± 1.4 1.4 ± 0.4 1.7 ± 0.6 2.8 ± 0.7 4.1 ± 2.3 3.9 ± 1.5 3.4 ± 1.7 2.8 ± 1.3 2.9 ± 1.4 3.1 ± 1.5 3.7 ± 1.8 5.69 ± 4.43 78.3 ± 7.7 6.45 ± 4.55 88.9 ± 11.3 4.00 ± 2.75 80.5 ± 9.8 7 .08 ± 6.18 91.3 ± 11.8 7.21 ± 6.18 76.9 ± 8.5 6.98 ± 5.54 86.5 ± 11.0 6.20 ± 6.29 78.2 ± 9.1 6.07 ± 4.00 87.8 ± 11.1 6.85 ± 6.13 78.1 ± 8.8 5.39 ± 4.47 86.8 ± 10.8 1.44 ± 1.23 1.00 ± 0.45 13.8 ± 11.0 9.6 ± 3.8 4.4 ± 2.3 12.0 ± 4.2 12.8 ± 4.5 9.5 ± 4.6 1.31 ± 1.03 1.07 ± 0.49 9.6 ± 7.3 8.3 ± 3.4 4.0 ± 2.3 12.8 ± 6.7 12.2 ± 4.6 7.8 ± 3.1 1.24 ± 0.94 0.96 ± 0.51 13.3 ± 10.4 10.2 ± 4.8 5.8 ± 3.9 15.6 ± 13.9 17.3 ± 12.2 12.4 ± 11.5 1.20 ± 0.94 1.15 ± 0.64 9.1 ± 7.4 8.3 ± 3.3 4.1 ± 2.6 13.7 ± 8.2 12.7 ± 5.2 8.5 ± 3.8 1.11 ± 1.05 1.01 ± 0.51 15.0 ± 12.2 14.2 ± 4.6 5.4 ± 3.0 5.7 ± 3.0 7.0 ± 4.7 4.5 ± 2.6 0.93 ± 0.51 1.04 ± 0.28 11.3 ± 6.8 13.3 ± 5.2 4.6 ± 2.4 4.3 ± 2.3 6.6 ± 8.9 2.5 ± 1.4 1.22 ± 1.13 1.00 ± 0.51 15.0 ± 14.7 10.4 ± 6.3 5.8 ± 3.9 12.8 ± 7.5 13.3 ± 7.6 9.1 ± 5.7 1.22 ± 0.89 1.16 ± 0.57 9.8 ± 6.7 8.7 ± 3.1 3.9 ± 2.2 13.7 ± 7.7 12.2 ± 4.2 8.4 ± 3.4 1.31 ± 0.93 1.03 ± 0.47 16.0 ± 12.8 11.8 ± 5.6 5.1 ± 3.0 15.0 ± 7.7 14.7 ± 6.1 11.1 ± 6.1 1.26 ± 1.00 1.22 ± 0.64 12.9 ± 9.0 11.8 ± 5.0 4.6 ± 2.9 17.0 ± 10.2 14.9 ± 6.4 10.6 ± 4.5 68.3 ± 32.1 2.00 ± 1.31 1.35 ± 1.00 65.8 ± 18.1 1.29 ± 0.76 0.87 ± 0.30 75.6 ± 28.5 1.89 ± 1.05 1.14 ± 0.46 62.2 ± 16.2 1.21 ± 0.95 1.00 ± 0.26 144.0 ± 81.9 3.76 ± 2.54 5.45 ± 4.12 66.9 ± 23.0 1.44 ± 0.86 1.51 ± 0.45 73.1 ± 28.6 1.96 ± 1.17 1.26 ± 0.58 60.3 ± 14.6 1.37 ± 1.06 0.86 ± 0.25 135.4 ± 43.7 4.15 ± 2.09 1.14 ± 0.38 125.5 ± 36.0 3.96 ± 2.08 1.03 ± 0.30 M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 Control Note. BB: biceps brachii; bpm: beats per minute; ECG: electrocardiography; ECR: extensor carpi radialis; EMG: electrocardiography; FCR: ﬂexor carpi radialis; HR: heart rate; LF/HF: low frequency/high frequency; MVC: maximum voluntary contraction; WAI-S: German State Competition Anxiety Inventory. 1279 1280 M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 Table 5 F values and effect sizes of multivariate and univariate statistics. Source F df, df error gp 2 Manipulation check of pressure condition Group Conditiona Somatic anxiety Concern Optimism Conditiona Group 4.32* 2.74 5.54* 7.49** 1.92 0.11 3, 3, 1, 1, 1, 3, .265 .186 .127 .165 .048 .009 Stress level and state anxiety in all conditions Group LF/HF ratio Heart rate Anxiety Thermometer Conditionb LF/HF ratio Heart rate Anxiety Thermometer Conditionb Group 3.37* 0.11 10.06** 0.04 7.70*** 0.89 18.10*** 8.65*** 1.66 3, 36 1, 38 1, 38 1, 38 12, 456 4, 152 2.957, 112.359 4, 152 12, 456 .219 .003 .209 .001 .168 .023 .324 .185 .042 Putting performance Group Conditionb Holed putts Distance Conditionb Group Holed putts Distance 0.82 6.19*** 6.58*** 6.66** 5.74*** 4.49** 8.29*** 2, 37 8, 304 4, 152 2.437, 92.589 8, 304 4, 152 2.437, 92.589 .043 .140 .148 .149 .131 .106 .179 Muscle activity Group Conditionb FCR right ECR right BB right FCR left ECR left BB left Conditionb Group 1.00 6.02*** 2.46 20.94*** 1.19 19.70*** 14.42*** 21.12*** 0.76 6, 33 24, 600 3.029, 115.119 2.565, 97.456 2.260, 85.894 2.302, 87.488 2.578, 97.979 1.786, 67.886 24, 600 .154 .194 .061 .355 .030 .341 .275 .357 .030 Cocontraction Group Conditionb Cocontraction right Cocontraction left Conditionb Group 0.64 2.40* 4.16* 0.87 0.47 2, 37 8, 304 2.554, 97.051 2.104, 79.945 8, 304 .033 .059 .099 .022 .012 Kinematics Group Velocity SD Acceleration SD Face angle SD Rotation SD Conditionb Velocity SD Acceleration SD Face angle SD Rotation SD Conditionb Group Velocity SD Acceleration SD Face angle SD Rotation SD 6.37** 8.53** 8.18** 18.07*** 24.89*** 15.68*** 35.79*** 28.43*** 28.49*** 34.47*** 3.14** 9.91*** 4.21* 16.61*** 22.54*** 4, 35 1, 38 1, 38 1, 38 1, 38 16, 608 2.151, 81.745 2.290, 87.035 1.091, 41.463 1.034, 39.274 16, 608 2.151, 81.745 2.290, 87.035 1.091, 41.463 1.034, 39.274 .421 .180 .177 .322 .396 .290 .485 .428 .429 .476 .103 .207 .100 .304 .372 36 36 38 38 38 36 Note. BB: biceps brachii; ECR: extensor carpi radialis; FCR: ﬂexor carpi radialis; LF/HF:low frequency/high frequency. a Control and pressure condition. b Control, pressure, one-arm, sensory-trick, and context-change condition. * p < .05. ** p < .01. *** p < .001. M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 1281 Table 6 F values and effect sizes of planned simple contrasts. F gp 2 Stress level and state anxiety in all conditions Heart rate Condition Pressure Condition One arm 24.74*** 27.91*** .394 .423 Anxiety Thermometer Condition Condition Pressure One arm 22.84*** 9.17** .375 .194 One arm Context change One arm 9.70** 11.29** 8.13** .203 .229 .176 One arm One arm 9.64** 15.05*** .202 .284 One arm Context change 84.80*** 22.74*** .691 .374 One arm Context change 96.02*** 13.00** .716 .255 One arm Context change 24.98*** 10.78** .397 .221 One arm Context change 109.36*** 12.03** .742 .240 One arm 7.474** .164 One arm Context change One arm 21.94*** 95.25*** 20.79*** .366 .715 .354 One arm Context change One arm 9.48** 69.54*** 6.66* .200 .647 .149 One arm One arm 33.44*** 17.92*** .468 .320 One arm One arm Context change 37.05*** 23.23*** 7.05* .494 .379 .156 Source Putting performance Holed putts Condition Condition Condition Group Distance of missed putts Condition Condition Group Muscle activity ECR right Condition Condition FCR left Condition Condition ECR left Condition Condition BB left Condition Condition Cocontraction Cocontraction right Condition Kinematics Velocity SD Condition Condition Condition Group Acceleration SD Condition Condition Condition Group Face angle SD Condition Condition Group Rotation SD Condition Condition Group Condition Group Condition vs. Control Note. df, df error = 1, 38. BB: biceps brachii; ECR: extensor carpi radialis; FCR: ﬂexor carpi radialis. p < .05. p < .01. *** p < .001. * ** The scores on the Anxiety Thermometer were signiﬁcantly higher in the pressure (M = 4.0, SE = 0.26) and the one arm putting (M = 3.6, SE = 0.30) conditions than in the control condition (M = 3.0, SE = 0.26). 70 140 60 120 50 100 40 80 30 60 20 40 10 20 0 Distance of Missed Putt [cm] M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 Holed Putts [%] 1282 Holed - Yips Holed - No Yips Distance - Yips Distance - No Yips 0 Control Pressure One Arm Sensory Trick Context Change Putting Condition Fig. 1. Putting performance of the two groups in the different putting conditions. 3.2.3. Putting performance A 2 (Group) 5 (Condition) repeated-measures MANOVA with number of holed putts and distance of missed putts from hole as dependent variables showed no main effect of group but a signiﬁcant main effect of condition as well as a signiﬁcant interaction effect. A more detailed inspection of the variables uncovered that these signiﬁcant effects can be seen in both the holed putts and the distance parameter. The participants holed substantially fewer putts in the one arm (M = 45.7, SE = 3.7) and context change (M = 45.7, SE = 2.4) conditions than in the control condition (M = 57.5, SE = 2.4). They also missed the hole with a larger distance in the one-arm condition (M = 29.1, SE = 3.1) compared to the control condition (M = 20.3, SE = 2.1). The yips group holed signiﬁcantly fewer putts compared to the nonaffected group in the one-arm condition and missed the hole with a larger distance at the same time (Fig. 1). 3.2.4. Electromyography A 2 (Group) 5 (Condition) repeated-measures MANOVA with relative RMSs of six muscles as dependent variables indicated a signiﬁcant main effect of condition only. Univariate tests showed a highly signiﬁcant effect of condition in all tested muscles of the left arm and in the right ECR. The participants had lower muscle activity in all tested muscles of the left arm in the one-arm condition compared to the control condition. In contrast, they had higher muscle activity in these muscles in the context change condition. Higher muscle activity was also observed in the right ECR in the one arm (M = 13.8, SE = 0.77) and the context change (M = 11.8, SE = 0.84) conditions than in the control condition (M = 9.0, SE = 0.57). A 2 (Group) 5 (Condition) repeated-measures MANOVA with the indices for cocontraction for the left and right arm signalled only a main effect of condition. The cocontraction index of the right arm was substantially lower in the one-arm condition (M = 1.02, SE = 0.13) than in the control condition (M = 1.4, SE = 0.18). 3.2.5. Kinematics A 2 (Group) 5 (Condition) repeated-measures MANOVA with the standard deviations of face angle, rotation, velocity, acceleration as dependent variables exhibited signiﬁcant main effects of group and condition, and an interaction effect. All four kinematic parameters showed their contribution to these effects on the univariate level. Yips-affected participants had across all putting conditions higher values than nonaffected participants. The increase in inconsistency was larger for yips-affected participants than for nonaffected participants when the one-arm condition was compared to the control condition. Fig. 2 shows descriptive statistics for impact rotation standard deviation. Whereas the 1283 M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 Impact Rotation SD [ /s] 160 140 120 100 80 60 Yips 40 No Yips 20 0 Control Pressure One Arm Sensory Trick Context Change Putting Condition Fig. 2. Inconsistency in rotation during impact. inconsistency in rotation decreased from the control to the context change condition for affected participants, the opposite was apparent for the nonaffected participants. Considering all participants, higher values of all four kinematic parameters occurred in the one-arm condition than in the control condition. They also showed higher inconsistency in impact velocity and acceleration in the context change condition. 4. Discussion The goal of this study was to ﬁnd sensitive methods to detect the yips independent of its aetiology. A range of different methods, which can be classiﬁed as psychometric, behavioral, and physiological measurements, were applied to ﬁnd differences between yips-affected and nonaffected golfers. The online survey asked for self-reports, neurological symptoms, and psychometric measures via standardised questionnaires. First, without the grouping criterion based on the one-arm putting test before the actual experiment, 15% of the yips-affected golfers would have been wrongly assigned to the nonaffected group. Results of previous studies using only self-reports for yips grouping might be subsequently interpreted with caution (Adler et al., 2005; McDaniel et al., 1989; Sachdev, 1992; Smith et al., 2000; Stinear et al., 2006). There are several reasons why some yips-affected golfers might not be aware of being affected: The symptoms can be subtle, or these golfers might have poor body perception. Alternatively, these golfers might not know much about the yips and explain their putting problems by just bad putting technique or low skill ability. The classiﬁcation of the yips as a focal dystonia was not supported by this study; the neurological items in the online survey, including the ADDS, questions about other neurological disorders, and family history, did not indicate a neurological basis of the yips in our sample. The sensory trick putting condition did not show any effect. Originally, it was assumed that the sensory trick would cause a temporal absence of the yips symptoms and that cocontractions would appear in yips-affected golfers as typical signs of focal dystonia (Albanese & Lalli, 2009). We consequently assume that our sample consisted mostly of golfers affected with the yips of no neurological origin. A more detailed neurological examination might have uncovered some cases. Psychometric measures that characterize yips-affected golfers and separate them from the nonaffected golfers were not found in this study. The two groups did not differ in anxiety, perfectionism, movement or decision reinvestment, somatic complaints, or stress-coping strategies, in contrast to results of previous studies (McDaniel et al., 1989; Rotheram et al., 2007). Many reasons for ﬁnding no results in the present study are conceivable: First, our sample probably differs from those in previous studies because of different yips grouping criteria. Second, various yips aetiologies in the sample might confound the results. Third, psychometric measures could be not sensitive enough to diagnose the yips. 1284 M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 In the putting experiment, participants had to putt in a control, pressure, one-arm, sensory-trick, and context-change condition. The pressure manipulation was effective according to the Anxiety Thermometer and heart rate and tendentially according to the WAI-S, but not in regards to the indicator of mental stress, the LF/HF ratio. Typical pressure effects such as detriments in putting performance, higher muscle activation, and changes in kinematics were observed (Cooke, Kavussanu, McIntyre, & Ring, 2010). The feedback interviews suggested that the applied stressors might have confounded themselves. For instance, the monotonous booing of the audience might have distracted the participants from the monetary incentive. This effect could be similar to music, which was previously used as an intervention to reduce self-awareness in choking-susceptible athletes (Mesagno et al., 2009). To interpret the results appropriately, it is also important to know the general differences between the putting conditions and the control condition. For instance, the one-arm condition was characterized by lower putting performance, more inconsistent execution, lower muscle activity of the left arm, lower heart rate, and higher state anxiety as measured by the Anxiety Thermometer. Furthermore, activation of the right ECR was higher, leading to a lower cocontraction index. It approached the value of an equal activation of the ﬂexors and extensors. The sensory-trick condition did not differ from the control condition in any estimated parameter. The context-change condition was described by lower putting performance, measured by the number of holed putts, and higher inconsistency in speed-related kinematic parameters. Future investigations using these conditions should be aware of these differences. We searched for sensitive methods for distinguishing the yips-affected from the nonaffected group. Both performance parameters, that is, number of holed putts and distance of missed putts to the hole, and all four kinematic parameters could separate the groups in the one-arm putting condition. Smith et al. (2000) also found this putting performance effect, but when the groups putted normally with both arms. Kinematic parameters such as rotation SD and face angle SD at impact have been previously reported to be sensitive methods for differentiating between yips-affected and nonaffected golfers (Marquardt & Fischer, 2008). In contrast to results in this study, impact velocity SD and acceleration SD were also able to separate the groups, indicating that the yips affects both the control of direction and the control of speed. Moreover, in contrast to the nonaffected golfers, yips-affected golfers showed an increase in stability with respect to the impact rotation when putting with the unihockey racket. This might be an indicator of the yips being very task speciﬁc and disappearing when changes are made to the putting context (Marquardt, 2009). Against expectations, none of the other parameters and conditions could distinguish between yips-affected golfers and nonaffected golfers, possibly because they did not have this feature. As an alternative explanation, the participants might have been able to compensate for the yips in the other putting conditions with the nonaffected arm, the use of their individual putter, and their preferred putting style. Muscle activity and the cocontraction ratio did not distinguish the groups, contradicting previous studies that detected greater forearm muscle activity either in general (Smith et al., 2000) or in the forearm extensors of the nondominant putting arm (Stinear et al., 2006). The appearance of cocontraction has been shown for a few yips-affected golfers before (Adler et al., 2005, 2011) but could not been conﬁrmed in this study, possibly because different ways of estimating cocontraction were used. In the present study the ratio of the ﬂexor and extensor forearm activity was used. Previous studies (Adler et al., 2005, 2011) deﬁned a cocontraction as the appearance of phasic peaks of extensor and ﬂexor forearm muscles within 50 ms, estimated via a subjective and uneconomic procedure. Subjective anxiety rating (Anxiety Thermometer) and stress level parameters (HR, LF/HF ratio) also could not distinguish the two groups in any putting condition. Instead, HR was lower for the yips group across all conditions, probably because more members in the yips group regularly took beta blocker medication, which lowers blood pressure. The present study has some limitations that should be considered in future investigations. Although the grouping criterion was not based on self-reports as it was in previous studies, the chosen criterion relying on a pre-test was not an optimal solution. The ﬁrst author was trained to judge obvious twists and jerks before the ball was hit during the execution of a one-handed putt. Future investigations should implement an even more objective criterion such as relying on kinematic data to improve the accuracy of the separation between yips-affected and nonaffected golfers. Moreover, letting the participants putt with their usual putter and grip style could have helped them to compensate M.K. Klämpﬂ et al. / Human Movement Science 32 (2013) 1270–1287 1285 for the yips symptoms. The sample size was limited to 40 participants for practical reasons, which might have led to lower power of the psychometric measures. Future studies on psychometric measures of yips-affected golfers are encouraged to use a larger sample size. Although the questionnaires were randomly presented in the online survey, the long period needed to ﬁll in the survey could have resulted in fatigue, which could have confounded the scores of the participants in the psychometric measures. Finally, future studies explaining the yips by the reinvestment theory could only include skilled participants, who could suffer from the detrimental effect of the attempt to consciously control the own movement. 5. Conclusions Many different methods have been used to detect the yips in golf. Estimation of putting performance and putter kinematics while the golfer putts with the dominant arm have been found to be sensitive to detect the yips. Future investigations of the yips should use such tests as a foundation to increase the validity of the yips diagnosis before actual manipulation of the golfers, such as with sensory tricks or intervention. Progress in understanding the yips phenomenon relies on such a standardized screening test. As a next step, future studies should focus on ﬁnding a way to distinguish yipsaffected golfers by their aetiologies to be able to subsequently test aetiology-dependent interventions. 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