Intersession reliability of center of pressure measurement during bipedal standing with different foot placement angles

Introduction: The foot placement is a determinant of the base of support and influences standing balance. The reliability of postural stability tests with different foot placement angles is unclear. Research question: To determine and compare the intra- and inter-day reliability of the center of pressure-based postural stability while standing with different foot placement angles. Method: Twenty-five healthy adults (16 females and 9 males; age: 29 +/- 6 years) completed 70 s trials of eyes open and eyes closed stability tests with 0 degrees, 15 degrees, 30 degrees, and 45 degrees angles between the feet while standing on a forceplate in three sessions: two sessions were in the same day, and the third session was one-week apart. The repeatability of measurements was tested using analysis of variance, interclass correlation, and standard error of measurements. Result: Throughout the three study sessions, there was no difference in postural stability while participants stood with different foot placement angles. The interclass correlation scores ranged from 0.71 to 0.96, the standard error of measurements ranged from 2.1% to 12.9%, and no significant systematic changes (p < 0.05) occurred between the testing sessions for any foot placements. Standing with a 45 degrees angle between the feet with closed eyes showed higher reliability values than other conditions. The intra-day reliability scores were greater than inter-day reliability. Discussion: The relative reliability of postural stability could be impacted by foot placement angles, which might alter ankle mobility and base of support dimensions. The advantages of larger foot placement angles on improving the relative reliability of postural stability could be better demonstrated in healthy people under challenging conditions such as standing with closed eyes.