Step-down biomechanics in unilateral patellofemoral pain: a cross-sectional study of young females

BACKGROUND: Despite extensive research on biomechanical abnormalities in the affected lower limb of young women with patellofemoral pain (PFP) during step-down tasks, the presence of such abnormalities in the unaffected limb remains unclear. This study aims to fill the gap in existing literature by investigating bilateral biomechanics in this population, with the goal of providing an evidence base for targeted rehabilitation and preventive strategies. METHODS​​: In this cross-sectional study using purposive sampling, 16 young females with unilateral PFP and 16 healthy controls, all aged 18–30 years, underwent three-dimensional joints angles and torques assessments of the hip, knee, and ankle during step-down tasks. Three valid trials were collected per lower limb, with analyses focusing on the initial contact (IC) and peak vertical ground reaction force (vGRF) moments. RESULTS​​: No significant differences were found between the bilateral limbs of the CON group. At the IC moment, the unaffected limb of the PFP group showed greater hip adduction than the affected limb (P = 0.016). At the peak vGRF moment, the unaffected limb exhibited larger hip adduction and internal rotation angles (P = 0.021 and P = 0.044), along with greater hip abduction and knee valgus torques (P = 0.032 and P = 0.04). Compared to the CON group, the PFP group’s affected limb had a smaller hip flexion angle at IC (P = 0.021). Moreover, the unaffected limb of the PFP group showed smaller hip flexion and abduction angles (P = 0.016 and P = 0.003). At the peak vGRF moment, the affected limb of the PFP group exhibited smaller hip flexion and larger plantarflexion angles (P = 0.042 and P = 0.023). The unaffected limb in the PFP group had smaller hip flexion, abduction, and external rotation angles (P = 0.042, P = 0.005, and P = 0.042), and larger plantarflexion angles (P < 0.001). CONCLUSION​​: Young females with unilateral PFP exhibited compensatory biomechanical alterations in the unaffected limb. Prolonged overuse without timely intervention may increase joint stress and lead to long-term dysfunction. Early bilateral intervention such as hip abduction and external rotation muscle training is essential to prevent such complications. CLINICAL TRIAL NUMBER​​: Not applicable.