Skipping is a rhythmic form of bipedal locomotion that combines the step and hop. Skipping can be performed bilaterally (alternating lead foot) or unilaterally (continual lead foot). Unlike walking or running, skipping is an asymmetrical movement in which successive footfalls are not evenly spaced in time The gait is unique in that it has the sustained flight phase found in running and the double support phase found in walking. Skipping is most commonly used by children of around 4.5 years of age. It is unclear why the gait is adopted in early years. Skipping is 150% more metabolically demanding than running performed at the same speed.