Longitudinal study of unimanual actions and grasping forces during infancy.

Learning to grasp an object is an important milestone in neurological development during infancy. Several studies report development of reaching in infants but less attention is devoted to reaching with successful grasping and to development of grip force. This study investigates, in the first weeks of life, the development of palmar grasp both for assessment of unimanual/bimanual grasping actions in centrally and laterally placed objects and for measurement of exerted power grip force. We longitudinally examined ten infants from 18 weeks to 41 weeks, at 2-weeks intervals, with a toy placed in three positions (body midline, ipsilateral and contralateral shoulder). Our main aim was to study this development through an ecological approach. This was possible thanks to the use of the "biomechatronic gym", a new platform purposively developed for measuring reaching and grasping forces. These longitudinal trials showed a good level of acceptance and involvement by infants of the entire system. The results demonstrated a significant increase in unimanual power grip force between the 18th and 30th week followed by a flat period until the 41st week; we also ascertained an early tendency to play bimanually with centrally and laterally placed objects with a subsequent increase in all ages of unimanual successful power grasping both for central task and midline crossing. These developmental changes may be, in part, related to organism constraints such as maturation of the corticospinal tract.