Mechanical testing methods for determining stiffness of ankle-foot orthoses: A scoping review

Stiffness of ankle-foot orthoses (AFOs) has been assessed using many methods; however, results are unstandardized due to varied testing conditions such as range of motion and speed. Therefore, the aim of this scoping review was to provide a state-of-the-space review of the studies that investigate AFO ankle stiffness in the sagittal plane and then provide insight into the details of the developed devices and protocols. Six electronic databases (MEDLINE, SCOPUS, Web of Science, CINAHL, EMBASE, and IEEE Xplore) were searched for studies from when indexing began to April 2024. Articles of any design that developed or used testing devices and/or protocols to assess AFO ankle stiffness in the sagittal plane were eligible. Seventy-two articles were included in this review, of which 51 developed mechanical testing devices/protocols to quantify AFO stiffness while 21 articles referenced existing methods. The most common apparatuses used in the primary studies were the universal testing machine and customized manual machines. Other methods included specialty automated devices, manual devices in a gait analysis laboratory, hanging weights, universal testing machine combined with motion capture, muscle training machine, and 6-axis robot arm. Ankle angle of the AFO was the most common controlled variable, followed by loading force. This scoping review comprehensively outlines the variety of devices and protocols used to assess AFO ankle stiffness in the sagittal plane. Many researchers chose methods that leveraged their available resources, demonstrating a variety of approaches. For future studies, we recommend that parameters chosen are justified, reliability studies are undertaken, and AFO design is adequately reported.