Biomechanical effects of 3D-printed metamaterial foot orthoses with anti-pronatory graded stiffness on lower limb kinematics and kinetics in healthy adults: A within subjects immediate effects cross over study

Excessive or prolonged foot pronation is associated with lower limb overuse injuries. Foot orthoses incorporating anti-pronatory design features are frequently used to reduce excessive pronation and related tissue stresses. This study will examine the biomechanical effects of non-customised 3D-printed metamaterial orthoses incorporating antipronatory stiffness designs and compare with neutral orthoses and orthoses with typical antipronatory design features in healthy adults. This study includes four interventional and two control arms. As a randomised cross-over study, each participant will experience each intervention and control arm. Participants will be invited to attend one appointment. The appointment is anticipated to take approximately 90 minutes. At the appointment, following screening for inclusion/exclusion criteria and provision of informed consent, participants will complete walking trials while wearing standardised footwear and orthotic conditions presented in a randomised order. During each condition, plantar pressures, lower-limb kinematics, and temporospatial gait parameters will be recorded. Rest periods will be provided between conditions to minimise fatigue. Intervention arm 1: Ethylene-Vinyl Acetate (EVA) foam orthosis with medial heel skive. A prefabricated foot orthosis based on the insole from Comparator Arm 1, modified with a 4 mm medial heel skive. This represents a traditional anti-pronatory orthotic design commonly used in podiatric practice. This will be manufactured from EVA with medium hardness. Intervention Arm 2: 3D-printed metamaterial insole with uniform stiffness. A 3D-printed metamaterial foot orthosis/insole with uniform stiffness across the device, equivalent to approximately medium EVA hardness. The geometry of this will be matched to that of Comparator Arm 1. Intervention Arm 3: 3D-printed metamaterial insole with moderate medial stiffness gradient: a 3D-printed metamaterial foot orthosis/insole designed with high medial rearfoot stiffness and medium lateral rearfoot stiffness to provide an anti-pronatory graded stiffness pattern. The medial rearfoot region will be equivalent to EVA with high hardness, while the lateral rearfoot region will be equivalent EVA with medium hardness. The geometry of this will be matched to that of Comparator Arm 1. Intervention Arm 4: 3D-printed metamaterial insole with high medial stiffness gradient: a 3D-printed metamaterial foot orthosis/insole designed with high medial rearfoot stiffness and low lateral rearfoot stiffness. The medial rearfoot region will be equivalent to EVA with high hardness, while the lateral rearfoot region will be equivalent EVA with low hardness. The geometry of this will be matched to that of Comparator Arm 1. Each orthotic condition will be fitted to standardised footwear in a randomised order. Participants will be blinded to the orthotic condition being tested. Following a familiarisation period, participants will walk at a self-selected comfortable speed while in-shoe plantar pressures are recorded using the PEDAR-X system and lower-limb kinematics and temporospatial gait parameters are measured using the Zebris FDM-T gait analysis system. For each orthotic condition, at least twelve valid PEDAR steps and five Zebris gait cycles will be collected. Rest periods of 2 to 3 minutes will be provided between study arms to minimise fatigue. As this is an immediate effects study, participants will not wear the orthoses outside the laboratory and no assessment of adherence will be performed. Should any intervention be discontinued before completion of testing (for example, due to discomfort reported by the participant), this will be recorded and reported. All orthotic devices will be fitted by members of the research team at a QUT research laboratory, under the supervision of a registered podiatrist.