Influence of body orientation on kinematic parameters of infant?s reaching

Raquel de Paula Carvalho
Department of Physiotherapy, Neuropediatric Section, Univer

Eloisa Tudella
Department of Physiotherapy, Neuropediatric Section, University Federal of S?o Carlos

Geert J. P. Savelsbergh
Institute for Fundamental and Clinical Human Movement Sciences, Faculty of Human Movement Sciences, Vrije University of Amsterdam

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     Last modified: February 22, 2007

Abstract
The aim of this longitudinal study was to verify the effect of seated and supine positions on spatio-temporal parameters of reaching in 4-6-month-old infants. The main attribution is the supine position demands higher muscular torque at the beginning of reach, as the weight force vector position is farther from the longitudinal shoulder axis. While in seated position, the weight force vector position is closed to the longitudinal shoulder axis. However, it is not clear if seated position can improve reaching movement in young infants in relation to kinematic parameters of reaching. After approval of Research Ethics Committee of University Federal of Sao Carlos (protocol no. 092/2002), four healthy infants were selected from Health Centers after evaluating their medical records. The infants were observed during reaching trials at 4, 5 and 6-months-old. After positioned in a baby chair, a toy was offered at the shoulders height, and arm?s length. After each reach, the toy was taken away carefully and present again for a 2-min period. The evaluation was performed in the supine (0o) and seated (70o) positions, at pseudorandom sequence. The whole experimental phase was filmed using three digital cameras (60Hz). A total of 235 reaches were analyzed by using the 3D movement reconstruction (Dvideow), and the dependent variables were calculated using Matlab program. Chi-square, Mann-Whitney and Kruskal-Wallis tests were applied for variables: frequency, duration (s), straightness index, deceleration time (s), and mean velocity (m/s). The significance level used was p<0.017, which was corrected for the number of comparisons. Our results showed that frequency of reaching (X2(2)=12.209; p=0.002) and straightness index (H(2)=17.492; p<0.01) increased over age. The frequency of reaches was higher (X2(1)=7.896; p=0.005) in seated than supine position. Significant differences between the positions were observed at 4 months, when the duration (U(1)=179.5; p=0.001) and deceleration time (U(1)=214.5; p=0.009) decreased in the seated position. There were no significant differences at 5 and 6 months. After experience and improvement of motor abilities to control their arms, infants can solve the problems related to instability of arms in supine position. Thus, the results suggest that young infants are able to change kinematical parameters of reaching to adapt themselves to intrinsic and extrinsic constraints, e.g. age and position.
Keywords: Reaching; kinematics; body orientation.
Funding acknowledgements: Capes.