Blood Lactate’s Relationship with Step Kinematic Asymmetry, Stance-Phase Biomechanics, and Spring–Mass Model Variables at the Transition from Curve to Straight Sprinting in the 200 m Dash: A Cross-Sectional Study

Background: Curve sprinting in the 200 m dash entails distinct biomechanical requirements under strenuous anaerobic conditions that differ from sprinting on the straightway. This study aimed to examine possible relationships between anaerobic biomarkers (specifically, peak post-test blood lactate concentration; BLa) and step kinematic asymmetry, stance-phase biomechanics, and spring–mass model (SMM) variables at the curve-to-straight transition during a 200 m sprint. Methods: Sixteen adult male club- and national-level sprinters performed a maximal 200 m sprint test in this exploratory cross-sectional study. Kinematical data were recorded for the first step immediately after the geometric end of the curve, and peak BLa was measured at 3, 5, and 7 min post-test. Based on data normality, differences in inter-limb step parameters were examined using a paired t-test or Wilcoxon signed-rank test. Pearson’s and Kendall’s correlation coefficients evaluated the relationships between BLa and the biomechanical variables (significance level: a = 0.05). Results: Peak BLa was 16.16 ± 2.61 mmol/L and demonstrated no significant correlation (p > 0.05) with sprint time (24.89 ± 1.24 s). Only stride and step frequency during the outer-to-inner leg step was correlated (p < 0.05) with BLa. Furthermore, the step initiated by the outer leg had a larger length (p < 0.05) compared to the step length generated from the inner leg. Conclusions: These findings stem from the different roles between legs while handling the centripetal force requirements during curve sprinting. The outer leg generated a longer step to facilitate curve-to-straight transition, whereas the inner leg functioned primarily to stabilize and steer the body.