 Adopting a wider stance increases the stability of a passive dynamic system, as well as increases the lateral clearance of the swing leg from the stance leg.According to the dynamic walking model, the mediolateral "rocking" of the legs can be also described by the inverted pendulum paradigm, and a wider stance would similarly require increased ground reaction force to redirect the COM, and thus increase the metabolic demand.For a narrower stance, due to insufficient clearance, hip torque may be increased to project the swing leg out laterally, leading to increased metabolic cost as well.