Biomechanical effects of varying arch support hardness in foot orthosis for adults with flexible flatfoot: A...

Foot orthosis hardness governed the trade-off between medial arch correction, plantar pressure redistribution, and load transfer along the lower limb, yet the relationship between hardness and biomechanical improvement in flexible flatfoot remains unclear.

To investigate how different arch support hardness levels affect biomechanics and muscle forces in adults with flexible flatfoot, and to determine how these effects vary with hardness.

Twenty women aged 18-40 years with flexible flatfoot (navicular drop > 10 mm, arch height index < 0.31), all asymptomatic, with no prior foot orthosis use and no recent lower-limb injury, were recruited from campus. Participants walked at a self-selected speed under four conditions: no arch support, arch support with shore C 30, shore C 50, and shore C 70 hardness. Research integrated bayesian statistical analysis and musculoskeletal simulations. Biomechanical data were collected via Vicon motion capture system and Kistler force plate and analyzed using OpenSim and JASP.

Foot orthosis improved rearfoot biomechanics: they reduced frontal plane range of motion, moment, and joint reaction forces at the ankle, subtalar, and midtarsal joint, and decreased tibialis posterior peak muscle force; however, these benefits plateaued as hardness increased. Harder foot orthosis (shore C 70) increased knee coronal range of motion and moment, knee sagittal peak joint reaction force, and vastus medialis peak muscle force, and shifted loading distally, increasing metatarsophalangeal joint range of motion, moment, and joint force.

Increasing hardness can, within limits, enhance rearfoot control and reduce tibialis posterior force, but progressively increases loading at the knee and forefoot. The prescription of arch support hardness should balance corrective benefits against the risk of knee and metatarsophalangeal joint loading.