 The relationship between step width and energetic cost for the step-to-step transitions can be described as formula_15, where formula_13 is the rate of metabolic cost, formula_7 is the mechanical work rate, and formula_18 is the step width.Step width variability is associated with the active control of the nervous system for walking stability, and is reduced in the presence of external stabilizers while increased with exposure to balance perturbations.Step length variability is similarly attributed due to a coupling that exists between medio-lateral and fore-aft motion in walking, but to a diminished degree.