Biomechanical response to altered footwear longitudinal bending stiffness in the early acceleration phase of sprinting

Longitudinal bending stiffness (LBS) of footwear has been shown to affect performance in jumping and sprinting tasks. A detailed description of the mechanisms underlying these performance alterations is lacking in the literature at the moment.Therefore, the purpose of this study is to describe why performance in a linear acceleration task is affected by LBS. Fifteen male athletes were analysed using full-body motion analysis combined with ground reaction force (GRF) measurements during the first step of a full effort 5 m sprint in a low stiffness baseline (BL), medium stiffness (MS) and high stiffness
(HS) condition. A significant reduction in acceleration performance (¡6.3%) was found in the HS condition compared to BL. Changes in acceleration performance in MS and HS were related to altered contact times, ground force application and
overall body orientation, but not to alterations in energy absorption at the metatarsal phalangeal (MTP) joint. A gearing function of LBS was evident from increased MTP and ankle joint GRF lever arms, which might offer a potential to
improve the effectiveness of horizontal force application. Nonetheless, athletes in this study were not using this potential to improve acceleration performance, possibly due to missing strength capacities. The results of this study indicate that high LBS might lead to reduced acceleration performance in athletes lacking the capacities to make use of the gearing function of footwear LBS. Footwear studies need to address the interrelationship between LBS, individual strength capacities, average ground force application and its effectiveness during acceleration tasks in the future.