Infants born preterm exhibit different patterns of center-of-pressure movement than infants born at full term

Abstract

Background: Infants born preterm are at risk for developmental impairments related to postural control.

Objective: The purpose of this study was to determine whether infants born preterm and infants born at full term differed in postural control at 1 to 3 weeks after term age.

Design: This study included 17 infants born preterm (mean gestational age=31.9 weeks, range=25.0-34.6) and 15 infants born at full term (mean gestational age=38.9 weeks, range=37.3-40.6). All infants were without diagnosed neurological or genetic conditions.

Measurement: Center-of-pressure (COP) data were recorded at 5 Hz while each infant was positioned supine on a pressure-sensitive mat in an alert behavioral state. Root mean square (RMS) displacement and approximate entropy (ApEn) were used to describe the COP movement variability in the time series. Differences between groups were identified using independent t tests.

Results: The COP time series were found to be deterministic, suggesting order in the time series. Infants born preterm exhibited significantly larger RMS values in the caudal-cephalic direction than infants born at full term (1.11 and 0.83 cm, respectively; t=-2.6, df=30, P=.01). However, infants born at full term had significantly larger ApEn values in the caudal-cephalic direction (1.19 and 1.11, respectively; t=2.4, df=30, P=.02). The 2 groups did not differ in RMS or ApEn values in the medial-lateral direction or the resultant.

Conclusions: 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.

Figure 1.

Example of pressure mapping method and pictorial display of pressure distribution: (A) a preterm infant positioned supine on the pressure-sensitive mat while awake and active, (B) a screen shot from the data collection software representing one frame of pressure data (in millimeters of mercury) and the center-of-pressure trace for the preceding 10 seconds.

Figure 2.

A model of supine postural control. The photographs in this figure depict an infant positioned supine on the pressure-sensitive mat while awake and active. The arrow represents the location of the center of pressure (COP) for the infant's position. As the infant moves from (A) a position of antigravity flexion to (B) a position of extremity and trunk extension, the COP shifts. The continuous evaluation of these shifts in COP provides a summary measure of the infant's postural control.

Figure 3.

Group differences in center-of-pressure (COP) movement variability: (A) group differences in root mean square (RMS) of the COP in caudal-cephalic (cc) and medial-lateral (ml) directions and the resultant, (B) group differences in approximate entropy (ApEn) calculated from the COP time series in the caudal-cephalic and medial-lateral directions and the resultant. Asterisks represent statistically significant differences (P<.05). Error bars represent the 95% confidence intervals.