Although both groups made progress on the GMFM, the COP measures indicated an advantage for the group with perceptual-motor intervention. The COP measures appear sensitive for assessment of infant posture control and quantifying intervention response.

Infants born at full term exhibited COP displacements in the caudal-cephalic direction that were smaller in amplitude, but may be considered more complex or less predictable, than those of infants born preterm. One explanation is that infants born preterm exhibited more stereotypic patterns of movement, resulting in large, but repetitive, COP excursions. A combination of linear and nonlinear measures may provide insight into the control of posture of young infants.

Objective To establish the test-retest reliability of linear and nonlinear measures, including intra- and inter- session reliability, when used to analyze the center of pressure (COP) time series during the development of infant sitting postural control in infants with or at risk for cerebral palsy (CP). Design Longitudinal study. Setting University hospital laboratory. Participants Infants with or at risk for CP (N=18; mean age at entry in the study ± SD, 13.1.7 ± 3.6mo). Interventions Not applicable. Main Outcome Measures Infant sitting COP data was recorded for 3 trials at each session (2 sessions for each month within 1 week) for 4 consecutive months. The linear COP parameters of root mean square and range of sway for both the anterior-posterior and the medial-lateral directions, and sway path, were calculated. In addition, the nonlinear parameters of approximate entropy, Lyapunov exponent (LyE), and correlation dimension for both directions were also calculated. Intra-session and inter-session reliability was computed by the intraclass correlation coefficient (ICC). Results Regarding nonlinear measures, LyE showed high intra-session and inter-session ICC values in comparison to all other parameters evaluated. Intra-session and inter-session reliability increased overall in the last 2 months of the data collections and as sitting posture improved. Conclusions Our results suggested that the methodology presented is reliable way of examining the development of sitting postural control in infants with or at risk for CP, and the reliability results generally parallels values found in sitting postural behavior in typical infants. Therefore, this methodology may be helpful in examining efficacy of therapy protocols directed at advancing sitting postural control in infants with motor developmental delays.

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