From structure to function: Biomechanical markers of symptomatic flatfoot during running and a single leg drop and...

Static classification of flatfoot fails to explain why some individuals develop symptoms while others do not. Although altered foot kinematics have been documented, few studies directly compare symptomatic and asymptomatic individuals, and none have examined multi-segment joint kinetics during sport activities. This study aimed to examine multi-segment biomechanical and intrinsic foot muscle differences between symptomatic and asymptomatic individuals with flexible flatfoot during running and a single-leg drop-hop task, and to explore their clinical relevance.

Seventeen symptomatic and seventeen asymptomatic individuals with flexible flatfoot were matched for age, sex, and BMI. Multi-segment foot kinematics and kinetics were captured during running and a single-leg drop-hop. Morphology and strength of intrinsic and extrinsic foot muscles were assessed using ultrasound. Group differences were evaluated using a linear mixed-effects model.

Symptomatic individuals exhibited altered frontal plane ankle kinematics and reduced range of motion in the sagittal plane, particularly during the drop-hop task. Kinetic deficits included reduced power generation and absorption at the ankle and Chopart joints, and decreased MTP-I power absorption during running. Significant reductions in quadratus plantae and flexor digitorum longus muscle morphology were observed in the symptomatic group compared to the asymptomatic group. No differences were detected at the Lisfranc joint.

Symptomatic flexible flatfoot presents with task- and joint-specific functional deficits in both biomechanics and foot muscle morphology. Our findings support a shift from structural assessment toward performance-based clinical evaluation. Sport-specific rehabilitation should address neuromuscular control, joint power, and foot core function in dynamic contexts.