Exploring performance-related inter-limb asymmetry thresholds in speed skating: a CART analysis approach

Long-term specialized training can lead to increased inter-limb asymmetry in speed skaters. While excessive inter-limb asymmetry may impair athletic performance, its impact on different skating phases and the corresponding thresholds remain unclear. The objective of this study was to investigate the relationship between lower limb strength asymmetry and split skating times, and to identify corresponding asymmetry thresholds using a data-driven Classification and Regression Tree (CART) analysis. Thirty-nine adolescent male speed skaters (age: 16.85 ± 0.81 years, height: 177.45 ± 4.26 cm, body mass: 66.03 ± 5.60 kg) underwent inter-limb asymmetry assessments based on single-leg vertical drop jump (SVDJ) and single-leg lateral squat jump (SLSJ) tasks. Skating performance was evaluated via a 500 m time trial, where split times for the start (20 m), straight (100 m), and curve phases (110-200 m), as well as the total time were recorded. Results showed that SVDJ jump height asymmetry explained 19.70% of the variance in 20 m start time (F(1, 37) = 9.07, p = 0.005). Moreover, in conjunction with SLSJ performance asymmetry, it explained 20.95%-24.65% of the variance in 100 m straight time (Best λ = 0.04, root mean squared error (RMSE) = 0.27) and 500 m total time (Best λ = 0.12, RMSE = 1.07). The CART analysis identified asymmetry in SVDJ jump height and SLSJ take-off velocity as the optimal variables (defined by the lowest RMSE) for distinguishing skating performance, with thresholds of 12.07% (RMSE = 0.15) and 10.26% (RMSE = 0.49), respectively. Independent samples t-tests revealed significant differences in split skating time between the high- and low-asymmetry groups defined by these thresholds (p < 0.01, Hedges' g = 1.10-1.44). Exceeding asymmetry thresholds for SVDJ jump height and SLSJ take-off velocity may negatively affect start, straight and 500 m total skating times in adolescent male speed skaters. This inter-limb asymmetry threshold classification method may serve as a reference for performance evaluation in other sports.