Impacto das propriedades físicas dos objetos nos movimentos de alcance em lactentes saudáveis de 4 a 6 meses de idade.

Abstract

Many studies have shown that object properties can lead to adjustments in reaching, but few have investigated the specific influence of object size and rigidity on reaching movements in young infants, especially by using kinematic analysis. Objective: Investigate the effect of object size and rigidity in the qualitative and quantitative variables of reaching in 4-to-6- month-old infants, and verify how such variables change over age. Methods: Nine healthy infants were placed in a baby chair reclined at 50º from the horizontal The infants were shown four objects of different size and rigidity large rigid (RG), small rigid (RP), large soft (MG), and small soft (MP) for a period of 1 minute each. Reaching movements were recorded by three digital cameras and the images were analyzed by using the 3D movement reconstruction. A total of 378 reaches were analyzed in relation to the following variables: qualitative proximal and distal adjustments, hand preference, and grasping of objects ; and quantitative straightness index, mean velocity, movement units, and adjustment time index. Results: There were many different changes in reaching over age. The answers to stimulation and the grasping of objects increased. Proximal adjustments were shown to be variable. Hand preference changed from the left to the right. The hand was predominantly half-opened in both the beginning and the end of the reaches; however, hand opening angle increased in the end of the reaches from the fifth to the sixth month. In all the ages, hand orientation was predominantly oblique when touching the object, and vertical when grasping it. As regards the quantitative variables, the straightness index and the mean velocity increased, and the movement units decreased. No significant changes were observed in the adjustment time index. Infants performed bimanual movements for the large rigid object, and unimanual movements for the small ones. The adjustments were similar in relation to the large soft object. Right-hand preference was higher for the small objects. In the beginning of the reaches, the hand was predominantly half-opened for all the objects; while in the end of the reaches, the hand was predominantly opened for the object large rigid. Hand orientation was predominantly oblique when touching the object, and vertical when grasping it, especially for the object large rigid. The soft objects were more frequently grasped than the rigid ones. The straightness index was higher for the large soft object, while the adjustment time index and the number of movement units were lower for this same object. The number of movement units increased for the small objects. No significant changes were verified in the mean velocity. Conclusion: It is possible to suggest that reaching movements are improved over age, thus indicating an increase in the infants performance during the studied period of life. Once the infants perceive the object properties, they adjust the essential variables of their movements by using their available motor abilities, thus suggesting the existence of a complex perceptionaction coupling.