Design and analysis of sprint footwear to investigate the effects of longitudinal bending stiffness on sprinting performance

There is evidence to suggest that the bending stiffness of footwear can be adapted to influence sprinting performance. In addition, it has been suggested that to achieve maximal performance, the mechanical properties of this footwear needs customising to an individual athlete. Due to a lack of detailed biomechanical data, the influence of longitudinal bending stiffness on the dynamics of the lower extremity during sprint running remains largely unexplained and is subject to considerable speculation. Thus, the aim of this work is to develop functional sprint footwear in a range of different longitudinal bending stiffnesses in order to explore the effects on measures of sprinting performance and lower extremity dynamics. Novel mechanical test procedures were developed and benchmark properties of current commercial sprint spikes were ascertained. Bending stiffness data showed considerable variability amongst those sprint spikes aimed at athletes of a higher competitive standard, which indicates that there is no consensus regarding optimum stiffness. A kinematic analysis of barefoot and shod sprinting was undertaken to investigate the influence of sprint footwear on lower extremity kinematics. Medial and lateral sagittal plane data were collected at the start and in the acceleration (10 m) and maximal speed (50 m) phases of a 100 m distance. Metatarsophalangeal joint (MPJ) angular range and velocity were significantly reduced in sprint spikes compared to barefoot conditions and the magnitude of the controlling affect was larger at 10 m compared to 50 m. Selective laser sintering of nylon was used to produce a number of sprint shoe sole units each of different thickness. These were attached to standard uppers to produce a range of longitudinal bending stiffnesses encompassing those already commercially available. The influence of shoe stiffness on sprinting perfonnance was assessed using specific jump metrics that were selected for use based on their high correlations with sprinting perfonnance during starting and maximal speed sprinting. Results indicated that sprint shoe longitudinal bending stiffness influenced the dynamics of the lower extremity during squat and bounce drop jumps. The relationship between maximal perfonnance and shoe stiffness was specific to the jump metric; best performance was achieved in intermediate stiffness shoes for the squat jumps and high stiffness for bounce drop jumps. Six bespoke pairs of sprint shoes with bending stiffness spanning and exceeding that of current commercial sprint spikes were developed. Results showed that MPJ and ankle joint dynamics were affected by longitudinal bending stiffness during squat and bounce drop jumps. Angular velocities of the MP and ankle joints were significantly reduced with increasing longitudinal bending stiffness. For the squat jump, ankle joint moments increased with shoe longitudinal bending stiffness and reached an individually optimal level within the stiffness range. This was also the case for ankle joint power and mechanical energy. The bounce drop jump saw mechanical energy generation at the MPJ increase with shoe longitudinal bending stiffness. Different levels of longitudinal bending stiffness were required for maximal performance in each jump type. This infers that sprint shoe bending stiffness requirements may vary according to the phase of the race. Furthermore, individual responses to different stiffnesses highlighted the importance of personalising mechanical properties to the requirements of a particular athlete for maximal performance. This research has focused on the use of discrete jump metrics to assess performance and therefore future work should aim to investigate the implications of different stiffness conditions using measures of actual sprinting. Also, further detailed musculoskeletal explorations are required in order to fully understand the precise mechanism by which longitudinal bending stiffness influences performance.