Introduction Sprint performance is a priority for coaches and athletes. Several kinematic variables, including horizontal touchdown distance (HTD) and inter-knee touchdown distance (IKTD), are targeted by coaches to increase top sprinting speed. However, the results of past research are conflicting, potentially due to the use of experimental inter-athlete study designs where it is not possible to establish cause-effect relationships. Methods In this study, we used a predictive simulation approach to assess cause-effect relationships between HTD and IKTD and sprinting speed. We scaled a three-dimensional musculoskeletal model to match the anthropometry of an international caliber male sprinter and generated predictive simulations of a single symmetric step of top-speed sprinting using a direct collocation optimal control framework. We first used our simulation framework to establish the model`s top speed with minimal constraints on touchdown kinematics (the optimal simulation). Then, in additional simulations, we enforced specific HTD or IKTD values (±2, 4, and 6 cm compared with optimal). Results The model achieved a top speed of 11.85 m·s-1 in the optimal simulation. Shortening HTD by 6 cm reduced speed by 7.3%, whereas lengthening HTD by 6 cm had a smaller impact on speed, with a 1.6% reduction. Speed in the simulation was insensitive to the IKTD changes we tested. Conclusions The results of our simulations indicate that there is an optimal HTD to maximize sprinting speed, providing support for coaches and athletes to adjust this technique variable. Conversely, our results do not provide evidence to support utilizing IKTD as a key technique variable for speed enhancement. We share the simulation framework so researchers can explore the effects of additional modifications on sprinting performance (https://github.com/nicos1993/Pred_Sim_Sprinting).
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