Abstract: BACKGROUND: Currently, research both domestically and internationally on patellofemoral pain syndrome has explored the kinematics and dynamics during daily activities such as stair ascent and descent, and walking. However, there is a lack of studies examining the lower limb biomechanical characteristics of young female patients with patellofemoral pain syndrome in different squatting conditions.
OBJECTIVE: To investigate the lower limb biomechanical characteristics among young female patients with patellofemoral pain syndrome in different functional states of the subtalar joint, providing theoretical support for the clinical treatment of various types of patellofemoral pain syndrome. 
METHODS: A total of 33 participants were included in this study. There were 10 subjects in the healthy control group (group C). The other 27 subjects with patellofemoral pain syndrome were divided into two groups according to the foot posture index: 14 subjects in the normal subtalar joint group (group A, foot posture index 0-6 points) and 13 subjects in the abnormal subtalar joint group (group B, foot posture index 7-12 points). The biomechanical indices of the subjects in each group were collected and compared when they walked on stairs at normal speed. The kinematic indices included the three-dimensional joint angles of the hip and knee and the sagittal plane joint angles of the ankle at the initial contact moment and the moment of maximum knee flexion angle during the stance period. The dynamic indices included the three-dimensional joint torques of the hip and knee and the sagittal plane joint torques of the ankle at the moment of maximum knee flexion angle during the stance period. The surface electromyography indices included the root mean square amplitudes of the vastus medialis, vastus lateralis, rectus femoris, semitendinosus and semimembranosus, biceps femoris, and gluteus medius in the pre-activation stage and the buffering stage. 
RESULTS AND CONCLUSION: (1) At the initial ground contact moment, group A exhibited a greater knee flexion angle (P < 0.05), greater hip external rotation angle (P < 0.01), and smaller knee external rotation angle (P < 0.01) compared to group B. Compared to group C, group A showed a greater knee flexion angle and smaller hip flexion angle (both P < 0.01). Group B demonstrated a greater knee external rotation angle and smaller hip external rotation angle and hip flexion angle (all P < 0.01) compared to group C. (2) At the moment of maximum knee flexion, group A had a smaller knee valgus angle (P < 0.05), smaller knee external rotation angle (P < 0.05), and greater knee flexion angle (P < 0.01) compared to group B. Compared to group C, group A showed a smaller knee valgus angle (P < 0.05), smaller hip flexion angle (P < 0.01), and smaller hip external rotation angle (P < 0.05). Group B had a smaller knee flexion angle, hip flexion angle, hip external rotation angle, and greater knee external rotation angle (all P < 0.01) compared to group C. Additionally, group A exhibited a greater hip internal rotation moment (P < 0.05) and plantarflexion moment (P < 0.01) compared to group C. (3) At normal speed during the staircase buffering phase, group C showed higher activation levels than group A in the vastus lateralis (P < 0.05), vastus medialis (P < 0.01), gluteus medius (P < 0.01), and biceps femoris (P < 0.05). Group C also had higher activation levels than group B in the vastus medialis (P < 0.01), gluteus medius (P < 0.01), and biceps femoris (P < 0.05). Additionally, group A showed higher activation in the semitendinosus and semimembranosus muscles compared to group B (P < 0.05). (4) These findings indicate that young female patients with patellofemoral pain syndrome have stiffer hip and knee joint buffering while descending stairs, potentially compensated by the ankle joint. Low muscle activation levels contribute to patellofemoral pain, with those having normal subtalar joints but experiencing pain showing the lowest and most abnormal activation. ③ Abnormal biomechanics in the normal subtalar joint group are mainly due to insufficient hip and knee flexion. Abnormal biomechanics in the abnormal subtalar joint group are mainly due to excessive subtalar joint pronation.

中国组织工程研究杂志出版内容重点：人工关节；骨植入物；脊柱；骨折；内固定；数字化骨科；组织工程

Key words: subtalar joint, patellofemoral pain syndrome, kinematics, kinetics, surface electromyography, daily living activities, female