The relationship between the anteroposterior and mediolateral margins of stability in able-bodied human walking

•Frontal and sagittal plane margins of stability may covary.•This negative covariation may become critical during step lengthening.•We found covariation between margins of stability during normal walking.•There was no covariation during step lengthening.•Humans may increase step width to prevent critical lateral instability. Control of dynamic balance in human walking is essential to remain stable and can be parameterized by the margins of stability. While frontal and sagittal plane margins of stability are often studied in parallel, they may covary, where increased stability in one plane could lead to decreased stability in the other. Hypothetically, this negative covariation may lead to critically low lateral stability during step lengthening. Is there a relationship between frontal and sagittal plane margins of stability in able-bodied humans, during normal walking and imposed step lengthening? Fifteen able-bodied adults walked on an instrumented treadmill in a normal walking and a step lengthening condition. During step lengthening, stepping targets were projected onto the treadmill in front of the participant to impose longer step lengths. Covariation between frontal and sagittal plane margins of stability was assessed with linear mixed-effects models for normal walking and step lengthening separately. We found a negative covariation between frontal and sagittal plane margins of stability during normal walking, but not during step lengthening. These results indicate that while a decrease in anterior instability may lead to a decrease in lateral stability during normal walking, able-bodied humans can prevent lateral instability due to this covariation in critical situations, such as step lengthening. These findings improve our understanding of adaptive dynamic balance control during walking in able-bodied humans and may be utilized in further research on gait stability in pathological and aging populations.