INTRODUCTION Ankle-foot orthoses (AFOs) are commonly prescribed to treat a range of lower-limb deficits by providing ankle support during walking. Current prescription standards provide general guidelines for choosing between AFO types but are limited in terms of guiding specific design parameter choices. AFO design parameters; e.g., trimlines, thickness, and material type, affect the AFO’s torsional stiffness (Major, 2004). In order to prescribe the best designs for their patients, clinicians need to know if and how stiffness affects patient outcomes. Therefore, the purpose of this literature review was to determine the impact of AFO torsional stiffness on walking performance. METHOD A literature search was conducted during February 2018 using three databases (Pubmed, Engineering Village, and Web of Science), PRISMA guidelines (Moher, 2009), and the following search terms: (ankle-foot orthosis OR AFO OR ankle foot orthosis) AND (stiffness OR resistance OR compliance OR rigidity OR flexibility OR energy storage OR energy return) AND (gait OR walking OR outcomes OR performance). Potentially relevant articles were screened against the following exclusion criteria: (1) no experimental subject data was presented; (2) the study did not include an AFO/AFO-like device; (3) no comparisons were drawn across stiffness conditions or only one stiffness was tested for each subject; (4) the intervention applied powered assistance and not passive resistance at the ankle; (5) AFO stiffness varied (was not constant) within a single stiffness condition; and (6) the study did not include a walking activity. The remaining articles were further analyzed, and their quality was assessed by three independent reviewers using a 10-point modified PEDro scale (Tyson, 2013). Each was scored by reviewer consensus. RESULTS After applying the exclusion criteria, 26 of 287 potential articles were included. A majority of the studies had low to moderate quality scores (i.e., below 7/10); the highest rating received was 8/10. Only one study ensured subjects were blinded to the test conditions, only three included sufficient participant eligibility criteria, and 11 papers failed to perform between-group statistical analyses. The included papers tested a variety of stiffnesses (0.02 to 8.17 Nm/ deg), covered several populations (healthy, post-stroke, cerebral palsy, Charcot-Marie-Tooth, and lower-limb salvage and trauma), and reported a variety of outcome measures. Nineteen of 26 studies measured stiffness experimentally. Measurement methods differed between studies. Ankle kinematics parameters were the most frequently reported and consistently affected measures. Greater stiffnesses generally resulted in decreased peak ankle plantar flexion, dorsiflexion, and range of motion, as well as increased initial contact ankle angle. At the knee, increased stiffness caused reduced peak knee extension during stance, increased peak knee flexion during stance, and increased knee flexion at initial contact. There was low evidence for the effect of stiffness on hip or pelvis kinematics. Stiffness did not affect hip kinetics, and there was low evidence for its effects on ankle and knee kinetics, muscle activity, metabolics, ground reaction force pattern, and gait spatiotemporal parameters. There were no generalizable trends for the impact of stiffness on user preference. DISCUSSION Drawing comparisons between the studies proved difficult due to differences in stiffness reporting and measurement, and the low number of participants for each subject population. Others have also highlighted the lack of established standards for reporting AFO design variables (Malas, 2011) and patient deficits (Chisholm, 2012). Nonetheless, stiffness consistently affected distal joint kinematics. CONCLUSION AFO torsional stiffness is a key factor influencing ankle movement. Clear reporting standards for AFO design parameters, as well as additional higher quality research is needed with larger sample sizes and different clinical populations to ascertain the true effect of stiffness on gait. [ABSTRACT FROM AUTHOR]