AN ONTOGENIC MODEL FOR THE SENSORIMOTOR ORGANIZATION OF BALANCE CONTROL IN HUMANS

On the basis of a review of the literature including the authors' own experimental studies, a model for the ontogenesis of balance control in children was developed. This qualitative ontogenetic model has to do with the equilibrium strategies built up by children in situations which are difficult but within the scope of their abilities. The model involves two functional principles. First: The stable reference frame on which the balance control is based can be either the support on which the subject is standing or the vertical gravity. When the frame of reference is the support surface, balance control is temporally organized either from the feet to the head (posture) or from the hip to the head (locomotion) (ascending organization). When the frame of reference is the vertical gravity, balance control is temporally organized from the head to the feet (descending organization). Secondly: Children gradually become able to master the various degrees of freedom which have to be controlled simultaneously during movement. For example, the head can be stabilized either on the trunk with the neck structures blocked (the en bloc mode of operation) or in space with the neck structures loose (the articulated model. Four main periods can be said to occur during the human life span. The first extends from birth up to the acquisition of the upright stance. This period is characterized by the development of postural responses along a cephalocaudal gradient. This chronological cephalocaudal progression with age of the ability to control several body segments may correspond to a descending temporal organization of unperturbed postural control, associated with an articulated operation of the head-trunk unit. The second period takes place from the acquisition of the upright stance up to around the age of 6. During this period, our experimental results are consistent with an ascending organization of balance control, from the feet to the head in postural stance and from the hips to the head in locomotion. This ascending organization is associated with an en bloc mode of head-trunk operation, which serves to minimize the degrees of freedom. The third period begins at around the age of 7 and continues up to an upper age-limit which is as yet unknown. It is characterized by the return to an articulated mode of head-trunk operation, whereby the head stabilization necessary for the descending temporal organization of balance control is ensured. Lastly, the fourth period, which is reached during adulthood, combines the main features of the third period with a new skill involving the articulated operation of the head-trunk unit along with a selective control of the degrees of freedom at the neck level.