The power expenditure in walking is presented as the sum of the work required for accelerating and decelerating the legs, plus the gravitational work associated to the lift of the trunk at each step. These two contributions are, respectively, inversely and directly proportional to pace length. As a consequence this model predicts an optimal pace, for a given speed, which corresponds to minimal power demand. This is in agreement with experiments on subjects walking on a treadmill. The model is primarily of pedagogical interest and its quantitative predictions not very accurate.