The effect of rotational speed on ankle-foot orthosis properties

Ankle-foot orthoses (AFOs) are devices that support ankle motion. An AFO's sagittal plane rotational stiffness can affect gait kinematics. Because AFOs are often made from viscoelastic materials, their properties may vary at different walking speeds. The influence of rotational speed on AFO properties has not been thoroughly investigated. Therefore, the purpose of this study was to determine the impact of rotational speed on AFO stiffness about the ankle. We tested a sample of one thermoplastic off-the-shelf AFO and two 3-D printed carbon fiber enforced nylon AFOs. Each AFO's dynamic resistance torque was measured as it was flexed at five speeds (5-100 °/s) using a custom-built measurement apparatus. We compared loading stiffness, neutral angle, and energy dissipation parameters for each AFO across speeds. Parameter values were generally greater at higher speeds. These effects were statistically significant for all AFOs (p≤0.002). However, differences in AFO stiffness and neutral angle across speeds were quite small (<0.6 Nm/° and <2.2 °). Changes in the thermoplastic AFO's stiffness were lower than the minimum detectable difference. Energy dissipation, as indicated by hysteresis area, increased by up to 6.3 J (about 250%) at the highest speed. This demonstrates that AFO flexion speed can influence the properties of different AFOs over the range typically achieved in human walking. Future work should assess whether the observed small variations of stiffness and neutral angle have a clinically meaningful impact on user performance, as well as explore effects of angular speed on a variety of AFO materials and designs.