The aim of this work was to propose developmental indexes relative to the control of balance and gravity forces, using force-plate data, for children in their first 5 years of independent walking. The first part of this paper is devoted to the definition of an index to quantify postural capacity during walking. Based on the assumption that the vertical acceleration of center of mass (CM) reflects the capacity of muscular forces between the head-arms-trunk and the stance leg segments to control the external forces, the value of the CM vertical acceleration at foot contact is proposed as a developmental index of the postural capacity of the child to control gravitational forces. This index was analyzed longitudinally in five children, over the course of eight experimental sessions. The children were examined during their first 5 years of independent walking (for a total of 457 step sequences). The covariation between the CM vertical acceleration at foot contact and the gait velocity was considered as a second index characterizing the development of coordination between the postural and dynamic requirements of body progression. From these indexes it was established that the postural capacity needed just to control balance with the leg muscles was not attained before 4-5 years of independent walking, i.e., at about 5-6 years of age.