Sex and training differences in kinematic determinants of run-to-sprint acceleration

Maximising velocity during the run-to-sprint transition is important for performance in many sporting activities; however, the kinematic mechanisms underpinning this run-to-sprint transition remain poorly understood. This study aimed to identify kinematic features associated with greater acceleration during the run-to-sprint transition and determine differences attributable to sex and training status. Healthy adults (28 males; 11 females) completed 30 m running trials. From a steady velocity (∼4 m·s⁻¹), participants accelerated in 5 of 20 block randomised trials. Spatiotemporal variables (mean step velocity, length, duration, cadence) and trunk and lower-limb kinematics (angles and excursions) were determined using markerless 3D motion capture for approach steps and transition steps (1-4). ANOVA (2 factor repeated measures) identified variables demonstrating significant step-to-step changes (within and between group differences), Pearson correlations (r ≥ 0.5), and Principal Component Analysis were used to guide a stepwise multiple linear regression. Change in acceleration was greatest at step 3. Three variables predicted greatest change in acceleration (r² = 0.79, p < 0.001): greater thorax flexion angle at mid swing (step 2), less knee flexion excursion during loading and shorter step length (step 3). Greater forward trunk lean occurred earlier for trained compared to untrained males, and males had greater trunk lean compared to females. Early trunk-knee adjustments appear important for maximising acceleration during the run-to-sprint transition, characterised by greater forward trunk lean, reduced step length, and stiffer knee mechanics. These findings may inform training and rehabilitation strategies aimed at improving rapid acceleration performance.