Linear dependence between step parameters across all conditions. (A) Mean step length, (B) mean step width, and (C) RMS step length variability are plotted against RMS step width variability for all conditions. Each symbol denotes a different condition, and each color denotes a different subject (N = 11). Dependence was tested using linear regression with a single slope, but with a separate offset for each subject; plot shows data with offsets averaged together. All step parameters were found to be linearly dependent, regardless of perturbation type (asterisk * denotes P < 0.05). (Note that step width variability is plotted on the horizontal axis to demonstrate linear dependence, but is not an independent variable.)