Approximate entropy used to assess sitting postural sway of infants with developmental delay

Abstract

Infant sitting postural sway provides a window into motor development at an early age. The approximate entropy, a measure of randomness, in the postural sway was used to assess developmental delay, as occurs in cerebral palsy. Parameters used for the calculation of approximate entropy were investigated, and approximate entropy of postural sway in early sitting was found to be lower for infants with developmental delay in the anterior-posterior axis, but not in the medial-lateral axis. Spectral analysis showed higher frequency features in the postural sway of early sitting of infants with typical development, suggesting a faster control mechanism is active in infants with typical development as compared to infants with delayed development, perhaps activated by near-fall events.

Figure 1

All data acquisition used the same force plate, which is built into the floor as is typical in a gait laboratory. a. single pendulum, b. double pendulum, c. infant sitting.

Figure 2

Plot of approximate entropy using “standard” parameters (m=2, r=0.2*std(Data), N=8.3 sec @240 Hz, lag=1) versus estimated signal-to-noise. Signal-to-noise is the ratio of the variances of the estimated signal and estimated noise (σ_s²/σ_n²).

Figure 3

Effect of R-parameter. Wilcoxon rank sum (Mann-Whitney U test) p values for comparison of approximate entropy(m=2,r,t=8.3 sec, lag) for single versus double pendulums (triangles), infants with cerebral palsy versus typical development in anterior-posterior axis (circles) and in medial-lateral axis (squares), plotted versus R value used in the calculation of approximate entropy. Similarity of points in comparison vectors r is determined by R*std(Data) in plots a and b, and by R*std(estimated noise) in plot c. Plot b is an expanded plot of the infant sitting data in a, for comparison with plot c using the same y axis scale as plot c, but note x axes differ between plots b and c.

Figure 4

Mean values of ApEn(m=1, r=std(Data), t=8.33sec@240Hz, lag=8) for postural sway of early and advanced infant sitting in the anterior-posterior(AP) axis (a) and in the medial-lateral(ML) axis (b). Groups were infants with typical development (TD) and infants with delayed development (DD). Error bars indicate +/− 1 std.

Figure 5

Spectral analysis of infant sitting postural sway in the anterior-posterior axis. Plotted in black is the average periodogram for all trials for infants with developmental delay, early sitting (a), advanced sitting (b), and infants with typical development, early sitting (c) and advanced sitting (d). To aid in visual comparison, plotted in grey on all four plots is the average periodogram for all trials of the developmentally delayed, early sitting. Artifacts seen at 30 Hz are due to electrical power distribution, and are not related to infant sitting.

Figure 6

Spectral analysis of infant sitting postural sway in the medial-lateral axis. Plotted in black is the average periodogram for all trials for infants with developmental delay, early sitting (a), advanced sitting (b), and infants with typical development, early sitting (c) and advanced sitting (d). To aid in visual comparison, plotted in grey on all four plots is the average periodogram for all trials of the developmentally delayed, early sitting. Artifacts seen at 30 Hz are due to electrical power distribution, and are not related to infant sitting.