RESUMEN
This study aimed 1) to assess the test-retest reliability of dynamic postural stability index (DPSI) assessments using a ski-specific jump protocol that consists of single-leg landings on a three-dimensional force plate after forward-performed double-leg drop jumps from a box over a hurdle (DJSLLs), 2) to provide reference values for female and male youth competitive alpine skiers; 3) to explore their changes in DPSI over 3 years during adolescence; and 4) to investigate potential associations of DPSI with age and biological maturation. Using three-dimensional force plates, 16 healthy subjects were tested on the same day (test-retest reliability experiment; five test-retest assessments of right leg landings), and 76 youth skiers aged 13-15 years were tested 3 times within 2 years (main experiment; average of two trials per leg each time). The test-retest reliability experiment revealed an ICC(3,1) and 95% CI of 0.86 [0.74, 0.94] for absolute DPSI assessment. The within-subject SEM of absolute DPSI was 16.30 N [13.66 N, 20.65 N], and the standardized typical error was moderate (0.39 [0.33, 0.50]). Both absolute and relative DPSI values were comparable between male and female youth competitive alpine skiers. The mean absolute DPSI in year 1 (195.7 ± 40.9 N), year 2 (196.5 ± 38.9 N) and year 3 (211.5 ± 41.3 N) continuously increased (i.e., worsened) (p < 0.001). Mean relative, i.e. body weight force normalized, DPSI values significantly decreased, i.e., improved, from year 1 to 2 (0.42 ± 0.01 vs. 0.36 ± 0.004; p < 0.001) and year 1 to 3 (0.42 ± 0.01 vs. 0.36 ± 0.01; p < 0.001). Absolute DPSI correlated with age and biological maturation, while no such correlations were found for relative DPSI values. Our findings suggest that DPSI is a reliable and sensitive measure of dynamic postural control during DJSLLs and that relative DPSI improves annually in competitive youth skiers when accounting for body weight. Future work should consider biological maturation testing during the growth spurt, and normalizing to body weight force could be a possible solution.
RESUMEN
OBJECTIVES: (1) To study the reliability of quantifying rear-chain stabilization capacity during deadbug bridging (DBB), (2) to provide reference values for competitive alpine skiers, and (3) to study associations with age, anthropometrics, maturation, skiing performance and back overuse complaints. DESIGN: Cross-sectional. SETTING: Biomechanical field experiment including questionnaires. PARTICIPANTS: 12 healthy subjects (reliability experiment); 133 skiers of the U16 category and 38 of the elite category (main experiment). MAIN OUTCOME MEASURES: DBB performance was quantified using 3D motion capture as the maximum amplitude of the relative vertical displacement of two pelvis markers (DBBdisplacement). Additionally, in U16 skiers, age, anthropometrics, maturation, skiing performance, and back overuse complaints were assessed. RESULTS: The reliability experiment revealed an ICC(3,1) and 95% CI of 0.81 [0.61, 0.93]. Within-subject SEM was 3.89 mm [3.16 mm, 5.12 mm]. Depending on sex and category, medians of DBBdisplacement in skiers ranged between 29 mm and 45 mm. DBBdisplacement differed between elite and U16 skiers (p < 0.001), but not between sexes. In U16 skiers, DBBdisplacement was independent of age, anthropometrics, and biological maturation, however, associated with skiing performance and back overuse complaints (p < 0.05). CONCLUSION: The proposed approach may be considered an adequate method to quantify athletes' rear-chain stabilization capacity.