Author(s): NAKATSUGAWA, T., YUYA, E., TAKEO, M., Institution: TOKYO INSTITUTE OF TECHNOLOGY, Country: JAPAN, Abstract-ID: 1556

INTRODUCTION:
Greater coupling angle variability (CAV) which quantifies the variability of coupling motion that occurs in two segments, was reported to predict the incidence of overuse injuries [1]. Coupling motion is considered to be influenced by the soft tissue stiffness around the segments [2]. However, it is unclear how foot stiffness affects the foot CAV during running. The purpose of this study was to compare the CAV of intra-foot segment during running between groups with high and low foot torsional stiffness.
METHODS:
Sixteen men running with mid- and forefoot strike participated in this study. Midfoot passive resistant torque and forefoot angle relative to rearfoot were measured simultaneously from maximum forefoot eversion to inversion. Foot torsional stiffness was calculated as the slope of the regression line in the range from 0° to 10° inversion of forefoot. Participants were divided into two groups based on foot torsional stiffness: high stiffness group (HSG; n = 8) and low stiffness group (LSG; n = 8). Participants ran barefoot at 3.3 m/s ± 10% on a 10 m runway, and we collected ground reaction force and kinematics of shank, rear-, mid-, and forefoot using a three-dimensional motion analysis system. CAV was calculated as the circular standard deviation of the coupling angle quantified with modified vector coding technique [3]. The mean CAV in each phase was evaluated by dividing the stance phase into early (0~33%), mid (34~67%), and late stance (68~100%). The following couplings were investigated: 1) frontal rearfoot vs. sagittal midfoot, 2) frontal rearfoot vs. frontal midfoot, 3) frontal midfoot vs. sagittal forefoot, and 4) frontal midfoot vs. frontal forefoot. Independent t-tests or Mann-Whitney U tests were used to compare the CAV of each phase in two groups (α=0.05).
RESULTS:
CAV of frontal midfoot vs. sagittal forefoot and frontal midfoot vs. frontal forefoot at mid-stance in LSG were significantly greater than in HSG. Additionally, CAV of frontal rearfoot vs. sagittal midfoot at late stance was also significantly greater for LSG compared to HSG.
CONCLUSION:
Greater CAV indicates that the coordinated motion of two segments for shock absorption and propulsive force transmission differs among trials. Passive stabilizers including ligaments and fascia contribute to the restraint of foot motion [4]. In addition, LSG showed significantly greater mid- and rearfoot frontal ROM than HSG during gait [5]. Foot torsional stiffness reflects mechanical passive properties and low torsional stiffness foot may not control segmental motion well, resulting in increased CAV due to disruption of coordination patterns. Soft tissue stiffness such as foot torsional stiffness is considered noteworthy for detailed gait analysis.

1. Desai, G.A. et al., J Sports Sci 39: 38-47, 2021
2. Pohl, M.B. et al., Gait Posture 25: 295-302, 2007
3. Needham, R. et al., J Biomech 47: 1020-6, 2014
4. De la Portilla, et al., J Biomech 84: 183-90, 2019
5. Magalhães, F. A. et al., J Biomech 119: 110328, 2021