The Postural Dynamics of Sitting Infants

Kathleen O'Brien, Purdue University

Abstract

When infants first begin to sit independently, they are highly unstable and unable to maintain upright sitting posture for more than a few seconds. Over the course of 3 months, the sitting ability of infants drastically improves. To investigate the mechanisms controlling the development of sitting posture a one degree-of-freedom inverted pendulum model was adapted from previously validated adult models for quiet standing. Passive muscle properties were modeled with a stiffness and damping term, while active neurological control was modeled with a proportional-integral-derivative (PID) controller. The control mechanisms were subsequently investigated by observing qualitative behavior with and without the passive and active mechanisms. Additionally, individual model parameters were systematically varied in order to observe and measure the qualitative behavior of the model. The findings revealed that it is likely that the first stages of sitting are reliant upon passive control and active control develops as infants become more adaptable to their environment and their own movements. Experimental data was then collected on 8 infant participants in both stage 2 and stage 3 of sitting. The range, standard deviation, and root mean square velocity were calculated from the center of pressure (COP) data. A frequency analysis was also performed to reveal the more complex dynamics of posture. Stage 2 sitters exhibited larger sway metrics and a higher median frequency. Power at higher frequencies suggests an increased magnitude and rate of sway within the stage 2 sitters. The reduced metrics found in stage 3 sitters suggest that experienced sitters are able to sit adaptively and actively incorporate sensory feedback into their postural control strategy. These results support the hypothesis developed in Study 1 that early sitting may heavily rely on the passive mechanism as the infant learns to integrate sensory feedback.

Degree

M.S.

Advisors

Claxton, Purdue University.

Subject Area

Kinesiology

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