Measures of frontal plane stability during treadmill and overground walking.

Given the consequences of falling to the side by older adults, attention has focused on identifying variables associated with changes in lateral stability and fall risk. Step-width (SW) and step-width variability (SWV) have traditionally been associated with such changes. Recently the "margin of stability" (MOS) has been adopted for describing dynamic stability. Although these measures may be influenced by the conditions during which locomotion occurs, only one published within-subject study has compared SW (but not SWV or MOS) during overground and treadmill walking. Therefore, we compared SW, SWV and minimum MOS (MOS(min)) in 10 healthy young subjects walking at self-selected speeds, both overground and on a treadmill. We found SW was significantly larger (p=0.001), and SWV significantly smaller (p=0.001) during treadmill walking, and that these changes were meaningfully correlated between tasks. In contrast, MOS(min) was insensitive to treadmill versus overground walking. This suggested first, that SW and SWV only partially reflect frontal plane stability, and second, that the goal of the central nervous system may be to maintain a constant MOS(min) regardless of task.