 To maintain steady walking gait requires the sum of these work terms, or the net work, to be equal to zero since any changes in system energy would impart different COM velocities between the start and end of transition.While the ideal net mechanical work of this system is zero, muscle contractions are necessary to generate the ground reaction forces, implying that the metabolic energy expended is nonzero.The massless leg assumption in the inverted pendulum theory omits the amount of work required to swing the contralateral leg during single support.