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Purpose: Modifying foot progression angle (FPA), the angle between the line from the heel to the second metatarsal head and the line of progression, can reduce peak knee adduction moment (pKAM). However, determining the optimal FPA that minimizes pKAM without inducing unnatural walking patterns can be challenging. This study investigated the FPA-pKAM relationship using a robotic stepping trainer to assess the feasibility of determining the optimal FPA based on this relationship. In addition, it examined knee moments during stepping with three different FPAs, as stepping is a recommended exercise for knee osteoarthritis rehabilitation.
Methods: Twenty-six asymptomatic individuals stepped on a robotic stepping trainer, which measured six-axis footplate-reaction forces/torques and three-dimensional ankle kinematics to determine external knee moments. The robot rotated the footplates slowly (~0.5 deg·s -1 ) between 10°-toe-out and 10°-toe-in while participants stepped continuously, unaware of the footplate rotations. The slope of pKAM-FPA relationship during continuous stepping was determined. Peak three-dimensional knee moments were compared between the 10°-toe-in, 0°-FPA, and 10°-toe-out FPAs with repeated-measures ANOVA. Multiple linear regression determined the covariates that predicted pKAM during stepping.
Results: Eighteen participants had lower pKAM and KAM impulse with 10°-toe-in than 10°-toe-out ( P < 0.001) and 0°-FPA ( P < 0.001 and P = 0.008, respectively; called toe-in responders). Conversely, eight participants reduced pKAM and KAM impulse with 10°-toe-out compared with 0°-FPA ( P < 0.001, P = 0.017) and 10°-toe-in ( P = 0.026, P = 0.004; called toe-out responders). A linear pKAM-FPA relationship was determined for each individual, and its slope (the pKAM rate with FPA) was positive for toe-in responders ( P < 0.01) and negative for toe-out responders ( P = 0.02). Regression analysis revealed that smaller pKAM with toe-in, in toe-in responders, was explained by increased tibia medial tilt, tibia internal rotation, footplate-reaction lateral force, footplate-reaction anterior force, and decreased footplate-reaction internal rotation torque.
Conclusions: Individuals may exhibit different responses to FPA modification during stepping. The slope and intercept of the linear pKAM-FPA relationship can be determined for individual subjects. This allows for a targeted pKAM reduction through guided FPA positioning and potentially offers subject-specific precision knee osteoarthritis rehabilitation.
