An investigation of the reproducibility of a self-selected natural feet position when standing: Implications for the assessment of upright standing posture

Background: Photogrammetry is often used to evaluate standing static postural alignment. Patients are often instructed to self-select a natural feet position but it's unclear whether this position can be consistently replicated across repeated assessments. Objective: To determine whether people can replicate a self-selected natural feet position in upright standing across three sessions on different days. Design: Between days test-retest reliability. Setting: University laboratory. Methods: Three variables - Base of Support (BoS), Foot Width (FW), Feet Opening Angle (FOA) - were measured from foot tracings of 150 participants (18-30 years) using established procedures. BoS data were assessed for systematic bias (Analysis of Variance), and absolute (Coefficient of Variation - CV%) and relative (Intraclass Correlation Coefficient - ICC) reliability. Results: There was systematic bias in the BoS data across the three testing sessions. The CV% for the BoS data was 15.2%. The ICC (95% CI) for the BoS data was 0.84 (0.79-0.87). There were moderate-large correlations between the BoS and both FOA and FW respectively within each session. Conclusion: If clinicians want to allow patients to use their self-selected natural feet position for repeated photogrammetric assessment of their static postural alignment it would be better to standardise the position of the feet, for example, by creating a tracing of a patient's self-selected natural feet position.