The Influence of Different Footwear Insole Stiffness on Center of Pressure and Ankle Kinematics during Walking: A Case Report

During locomotion, the foot–ankle system plays an important role for forward progression of the body. The center of pressure (COP) is regarded as the point of the ground reaction force (GRF) vector acting on the foot surface during the stance phase. COP movement trajectory and velocity reflect the stance phase forward progression of the foot segment and the ankle joint motion characteristics. This study aimed to investigate different levels of footwear insole stiffness on COP forward velocity, GRF and ankle joint angles during walking stance phase. Two healthy subjects (one female, one male; age 26.5 ± 6.4 years, height 168.5 ± 2.1 cm, weight 64.9 ± 5.4 kg) participated in this study. Subjects were asked to walk along a 10 m walkway at two different speeds: self–selected normal (SSN) and self–selected fast (SSF). Within each walking speed, subjects were required to walk under two different insole stiffness conditions: (1) normal shoe insole (NSI) from the testing shoe (Nike Free RN Flyknit 2017) used in this study; (2) 1.6 mm thick carbon fiber insole (CFI) fitted within the testing shoe. Stiffer insole (CFI) significantly decreased peak ankle internal rotation angle (p = 0.001) and sagittal plane angle ROM (p = 0.022); additionally, CFI significantly increased peak ankle eversion angle compared to the NSI condition (p = 0.028). In conclusion, increasing footwear insole stiffness would alter stance phase ankle joint motion at SSF walking speed. Additionally, stiffer insoles may tend to decrease COP peak velocity at the initial heel strike and the terminal stance phase. Future research should investigate the combined effects of various insole properties on lower extremity system kinematic and kinetic patterns in various locomotion activities.