Flatfoot kinematics and coordination during bilateral heel rises: Effects of knee angle variation

Flatfoot alignment alters foot biomechanics and lower limb coordination; however, its impact on dynamic performance during bilateral heel rises under varying knee conditions remains unclear. This study compared heel-rise kinematics and intersegmental coordination in 12 flatfoot and 12 normal-foot participants performing ten bilateral heel rises at 60 BPM under randomized knee extended and knee flexed conditions. The three-dimensional kinematics were recorded using a 12-camera Vicon system. The ascending and descending phases were divided into the early, middle, and late periods. The maximum normalized heel height, vertical and anterior center-of-mass displacement, anterior knee translation, pelvic and foot kinematics, and shank-hindfoot coordination via modified vector coding were analyzed. Under knee extension, the flatfoot group showed greater vertical center-of-mass displacement (p = 0.044). With knee flexion, anterior knee translation was larger in subjects with flatfoot (p = 0.022), who also exhibited greater hallux extension during the middle to late periods (p < 0.05). In the early ascending period with knee flexion, normal-foot participants displayed more proximal-dominant in-phase shank-hindfoot coupling (47.5 % vs. 23.3 %, p = 0.032), whereas flatfoot participants showed increased distal-dominant in-phase coupling (34.1 % vs. 12.4 %, p = 0.013). Individuals with flatfoot adopt compensatory forward knee translation and enhanced hallux/hindfoot motion, especially under knee flexion, reflecting reduced foot stiffness and altered coordination. Incorporating knee-flexed assessments and coordination metrics into clinical heel-rise testing may improve the detection of foot stiffness deficits and guide personalized rehabilitation.