Simulated leg-length discrepancy: its effect on mean center-of-pressure position and postural sway.

We hypothesized that leg-length discrepancies of as little as 1cm would induce a significant postural shift and increase the extent of postural sway. We had 14 normal volunteers stand on a force platform with their feet in a standard position. Center-of-pressure data were recorded at 100Hz for 20 seconds while the subjects stood barefoot with no lifts or (in random order) with lifts of 1, 2, 3, and 4cm under their left and right feet. From these data we derived the mean center-of-pressure position and the extent of postural sway. Lifts of as little as 1cm shifted the mean center-of-pressure toward the longer leg to a statistically significant extent (p less than 0.001), the mean difference compared with the barefoot condition being 6.1% of the distance between the feet; increasing the discrepancy did not proportionately increase the effect. The postural sway (total travel of the center-of-pressure) in a mediolateral direction increased significantly with a 1cm discrepancy (p less than 0.01), and continued to increase in proportion to the magnitude of the discrepancy. There were no effects on anteroposterior position or sway and no influence of left-right dominance. These results support our hypothesis that a leg-length discrepancy of as little as 1cm may be biomechanically important.