Our study is the first comprehensive report on mild-yips golfers where sensitive movement-related measurements were utilized to reveal features of a movement disorder. Specifically we found that in mild yips (i) golfers experience reasonable amounts of stress that may contribute to a state of underperformance overlapping with their movement instabilities; (ii) for putting shots, whereas motion-tracking readily captures fine motor changes in movement trajectories, features of co-contraction imbalance on sEMG recordings may not be particularly evident; (iii) finally, the downswing is particularly affected, and the ensuing perturbations in muscle activity share dystonic features that are consistently identified as abnormal muscle synergy patterns.
Models that explain sports related anxiety conceptualize that the cognitive self-evaluation and stress response if left unchecked, result in increased muscle tension, loss of focus and a range of other physiological behavioral changes24. As a consequence, depending on the individual’s own threshold of sense of anxiety, the performance-anxiety loop can either streamline their quality of shots or can potentially debilitate the task25. The golfers in our study were not of anxious type as revealed by TAIS scores but experienced a certain degree of competition stress as seen from SCA test. Though we believe this to be normal stress responses during gameplay, the subjective feedbacks given during the experiment suggest otherwise (Table-1). Qualitatively, there appeared to be less disagreement among our golfers that anxiety was perhaps not the only factor contributing to their performance deficit. While any involvement of movement deficits due to dystonia at this stage is purely speculative, it is important to consider prior studies which have reported higher muscle activations and grip force impacting stroke play kinematics irrespective of levels of situation induced anxiety6,26,27.
Professional golfers frequently spend considerable time in perfecting the putting stroke28. To perform a smooth shot, expert golfers recommend that the start of downswing phase of the club to be dictated by gravity, then eventually adjusting the hand torque at ball impact. Of significance is the angular velocity of the putter club-head and the hand torque model which advocates minimizing the hand torque from the start of downswing to allow a less variable velocity at ball impact, making the putting shots more consistent and accurate29. As an outcome measure of motion patterns, we chose the putter-club angular velocity during the entire swing and found that it was largely inconsistent during the downswing phase for yips-shots. This result reflects the temporal difference between normal and yips-shots and hence indicates the change in the uniformity or regularity of the shots. We therefore interpret that the inconsistencies seen for yips-shots is a miscommunication in the co-contracting forearm muscles during such ‘fine adjustments’ that may have prevented an ideal trajectory anticipated by the golfers.
Our initial screening of sEMG differences in multiple muscle pairs between normal and yips-shots was mostly inconclusive (Supplementary Fig. 2). Adler et. al. reported that abnormal co-contraction patterns were observed in wrist flexors and extensors in the downswing phase in yips affected golfers9. Co-contractions are essential to maintain the joint position balance and in high-precision shots like putting, any discrepancy that results in an erratic trajectory may not necessarily imply muscle dysfunction30. Therefore, in low-force tasks like putting, due to the trial by trial sEMG variability, we abstained from over-reporting the effect of co-contractions as manifestations of yips in our participants.
This leads us to the next point in using synergy analysis to identify features of focal dystonia. In maintaining biomechanically constrained joint balance, we used muscle synergies to identify patterns of muscle activity that achieve multi-joint coordination. We observed that the muscles of the elbow-wrist joint required 3 synergies to provide the necessary balance, direction and speed to perform the putting stroke. The apparently high number of synergies for a putting stroke documented here is a response for a low force isometric task which necessitates precise movement control31.
The spatial synergy weights (W) represent the muscle activations during a specific time of interest, here, the entire course of the downswing. In maintaining downswing balance, we observed that the variability in W’s was generally seen to be constrained to similar spatial patterns for normal and yips-shots. This appears to be an expected outcome since expert golfers minimize movement at the wrists by locking them in position, control positional parameters by spatially scaling downswing times and orient club head to avoid change in trajectories32. Furthermore, the extracted W’s illustrate functional groupings and due to their anatomical proximity or effect of crosstalk, we speculate that this may have contributed to the similarity in spatial synergies.
Neural coefficients (C) are believed to represent neural commands from specific synergies that influences the W’s modulating it over time33. The observed differences in C’s in a subset of golfers signify an altered phasic muscle synergy activity in yips-shots than in normal conditions. These were uniquely defined for each golfer suggesting an individual-specific relationship in muscle activations from higher centers. Muscle activations which occur in a multi-dimension space, require a coordinative input in the form of neural information to exclude and select appropriate motor patterns to harmonize movement. This harmony is achieved by spinal pre-motor neurons which dynamically adjust activations from inhibitory and excitatory pre-motor neurons in conjunction with higher centers like sensorimotor cortex, basal ganglia and cerebellum19,34,35. With long years of practice and repeated use, these pre-motor neurons evolve to reduce variability and strengthen access to a specific synergy necessary for motor control36. Yips-shots are an extreme example of this creation of ‘specific synergy’ due to a highly sensitive pool of pre-motor neurons eventually leading to abnormal sensory integration37, impaired cortico-motor information processing or maladaptive plasticity22. In actively adjusting putting trajectory, these golfers were unable to maintain their co-contraction stability due to an abnormal synergy representation. The manifestations seen here of yips-shots are therefore an amplification of altered dynamic phasic activity that dystonia is a part of.
A crucial limitation of our study was the evaluation of golfers with respect to their subjective responses. Yips is known to be associated with strong subjective priors1 and therefore we placed substantial importance to participants’ responses which would imitate dystonic findings. Since the presented cohort were formally undiagnosed with the yips, we speculated this approach to be reasonable. A future goal will be to replicate these findings in clinically/neurologically established cases of golfers’ dystonia. With respect to putting strategies, every golfer plays with a certain degree of uniqueness and this subtle but diverse behavior in motion capture and sEMG led us to focus on a case-by-case basis. We also observed that testing in laboratory environments often does not bring out the same level of anxiety experienced by players as ‘sinking the putt’ in the green. Our goal was not to create a high-stress environment for the golfer but rather identify features of muscle and kinematic imbalance under any possible yips-like condition. For this reason, we were careful to interpret our findings on muscle synergies which were based on changes in unidirectional downswing movement. Detailed modeling using joint kinematics along with truncal muscle synergy estimation for putting shots could be beneficial to address in the future. Furthermore, it would be advantageous if a standardized anxiety test was specifically tailored to yips since it the first-line assessment for any yips affected athlete.
In our formulation, we focused on identifying features of dystonia via movement analysis though other crucial variables may also be at play. Using demographic variables such as golfing experience, duration of yips symptoms, practice rounds per year, along with results from anxiety scores, motion-capture and synergy analysis, we categorized the participant groups into 2 types, using an unsupervised cluster analysis algorithm (Fig. 4 and Supplementary Table-3). The basis for this classification system comes from a frequently documented ‘continuum’ model suggested by Smith et.al. with Type-1 (dystonia) and Type-2 (choking) yips4. Non-movement associated variables could help classify the golfers better, although our focus rested mainly on motion-capture and muscle synergies to identify the problem. Future studies will need a systematic evaluation of these effects.
Diagnosis of yips is fraught with difficulties mainly due to limited research, scant literature and incongruity within the target populations. Still, we were able to highlight and unravel abnormal kinematics and synergy patterns that influence motor behavior among golfers irrespective of their subjective feeling of yips. Future work will need to address the link between spinal and central causes of yips, their mechanisms and how interventions could rehabilitate these golfers using behavioral therapy, swing dynamics or ‘normalize’ the faulty synergies leading to an improvement in their performance.