The Effects of Carbon Insoles on Vertical Leg Stiffness and Reactive Strength as Indicators of Sprint Performance

Background: Sprinting is the peak expression of running performance and different strength and physical characteristics play roles in the expression of sprint speed. Leg stiffness and reactive strength index (RSI), which measures the strength of stretch-shortening cycle, are two major factors on rate of force development and performance. It is well known that carbon fiber insoles optimize energy return while minimizing energy loss. Objectives: The purpose of this study was: (1) to investigate the effects of a carbon fiber insole on the expression of vertical leg stiffness (kvert) and RSI during 20-yard sprint and drop jump; and (2) to examine the effects of the carbon insoles on sprint kinetics and kinematics. Methods: Using a randomized crossover design, fifteen participants performed a drop jump and a 20-yard sprint in two shoe conditions (carbon, traditional insoles) to measure RSI, Kvert, peak vGRF, ground contact time (GCT), speed, knee angle at contact, and knee angle at toe-off. Results: Significant differences between conditions for the performance variables occurred only in the drop jump (kvert, p = 0.023; peak vGRF, p = 0.001). Conclusions: Further research is needed to examine sprint kinetics and kinematics with varying insole stiffness at maximal velocity.

This study investigates the impact of carbon insoles on leg stiffness and reactive strength during sprinting.