| Summary: | Grinding set-up in America's Cup sailing provides the power behind tacking and gybing, where the yacht crosses the wind to change direction. Grinding is also used for trimming the sails, which changes the angle on which the yacht is headed. This study provided a descriptive biomechanical overview of grinding on an America's Cup class yacht, and experimentally evaluated the influence of technique instruction on backward grinding performance. Inter-subject differences in body position (technique) throughout the grinding cycle, the ability to alter grinding technique within an eight-day technique intervention period, and the effect of technique on grinding performance as determined by power output were assessed. The quasi-experimental design, in which each of eleven Team New Zealand America's Cup grinders served as their own control, assessed four trials of backward grinding at baseline and post-biomechanical technique intervention testing sessions. Each trial was a maximal effort performed against a high load (250 W) and sustained over a period of eight seconds. Sagittal plane video was used to analyse joint kinematics (elbow, shoulder, trunk, hip, knee, ankle angles and joint centre positions) and to calculate the centre of body mass relative to the grinder pedestal. Height, weight, and limb lengths were obtained from each grinder using the ISAK protocol. Current backward grinding technique employed by the majority of grinders did not optimally use biomechanical principles. Recommendations for improvement were specific to each individual but focused on lowering trunk position and distancing the trunk from the grinding pedestal. Real-time visual feedback was provided to the grinder operators with the main focus being the position of their hip joint (viewed in the sagittal plane), and lowering the shoulder to be vertically level with the apex of the grinding handle cycle. During the intervention the grinders were given added correctional instruction relating to their body position according to perceived technique requirements. Recommendations were based on biomechanical principles regarding body position, and how body position could be altered to optimise the contribution of body weight and force production by the muscles of the upper limb in order to improve the torque applied to the handles. Altering grinding technique according to biomechanical principles produced 4.7% (p = 0.012) greater power during five seconds of grinding performance. Muscular strength, when measured using a 1RM bench pull (116.4 ± 9.8 to 117.3 ± 10.3), was unaffected by the intervention program, thus not contributing to the increased power output observed during grinding. Moderate changes to body position were observed after the eight-day intervention. Forward lean of the trunk decreased from 25° to 17° (p = 0.028) due to a lower hipy position (-0.09 m to -0.16 m below hub, p = 0.019). The more vertical trunk alignment resulted in the shoulderx position being further from the hub (0.33 m to 0.41 m, p = 0.013), producing a greater line of pull due to a more efficient shoulder vector angle (47° to 36°, p = 0.009). Variability (standard deviation and confidence intervals) decreased in all but four kinematic measures (which exhibited no change) indicating improved consistency in grinding technique. Regression analysis indicated the best predictors for high-load backward grinding performance were COMx position relative to the grinding pedestal and maximal strength. Changes in COMx position explained 40% (p = 0.166) of the variation in grinding performance, while maximal strength showed a relationship of 0.23% (p = 0.144) increase in performance per kilogram of bench pull 1RM. A one standard deviation difference in maximal strength altered the effect of COMx position by 0.26% per centimetre (p = 0.008). Weaker predictive factors were body weight, standing height, and pull angle, while brachial index did not appear to have any substantial influence on backward grinding performance. For future research greater subject numbers should enable more conclusive findings, especially in terms of the technique mechanisms and their relative levels of influence on performance.
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