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Figure Legends

Figure 1: Representative walking time series of 4, 7 and 11 year old children. The stride-to-stride fluctuations are largest in the 4 year old and smallest in the 11 year old. The coefficient of variation, a measure of variability, was 8.4, 4.3 and 1.9 % in the 4, 7 and 11 year old, respectively.

Figure 2: Representative time series after detrending (for same datasets as shown in Figure 1). Even after detrending, which minimizes the effects of local changes in the mean value, stride-to-stride fluctuations in stride time are still largest in the 4 year old and smallest in the 11 year old. The standard deviation was 60, 27 and 20 msec for the time series of these 4, 7 and 11 year old subjects. (For graphical purposes, 2 off-scale data points with values between .2 and .3 sec are not shown for the 4 year old.)

Figure 3: Stride time variability as a function of age. Shown is the coefficient of variation (CV) calculated during the 30 stride sub-section of each subject's time series with the lowest variability. Even during this period of relatively steady walking, gait variability decreases with age. The inset shows the data points replotted on log-log axes. The slope of the best fit line is close to -1.0, indicating that CV decreases inversely with age (tex2html_wrap_inline446). Note how the stride time variability observed in the oldest children approaches that of healthy adults (12) (error bars correspond to the mean tex2html_wrap_inline288 the standard error for young adults).

Figure 4: Representative results of spectral analysis (for datasets shown in Figure 1). The time series were normalized so that the total power is the same in each of the spectra. Note the subtle decrease in low frequency power and increase in high frequency power with age. The ratio of low (0.05 to 0.25 stridetex2html_wrap_inline306) to high frequency power (0.25--0.50 stridestex2html_wrap_inline306) was 9.0, 4.6 and 1.5 for the 4, 7 and 11 year old, respectively.

Figure 5: Ratio of power in the relatively low (0.1 to 0.2 stridetex2html_wrap_inline306) to relatively high (0.3 to 0.4 stridetex2html_wrap_inline306) frequency bands decreases with age. Note that this index of stride dynamics excludes highest and lowest frequencies. In the oldest children, the ratio approaches that of healthy adults (error bars correspond to the mean tex2html_wrap_inline288 the standard error for young adults). See also Table 3.

Figure 6: Fractal scaling index, tex2html_wrap_inline314, decreases with age. This finding suggests an age-related change in stride-to-stride dynamics over the range of 10 to 20 strides. The scaling exponent shown here was determined after 2nd order detrending of each window of observation. Note how tex2html_wrap_inline314 observed in the oldest children approaches that of healthy adults (error bars correspond to the mean tex2html_wrap_inline288 the standard error for young adults).

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tex2html_wrap_inline468Note that unlike variability measures, temporal structure measures are sensitive to the order of the data points in the time series.

Table 2: Stride Time Variability
3 & 4 Year Olds 6 & 7 Year Olds 11--14 Year Olds
SD (msec) Original Time Series 55 tex2html_wrap_inline288 5tex2html_wrap_inline472 31 tex2html_wrap_inline288 2 23 tex2html_wrap_inline288 1tex2html_wrap_inline468
After Detrending 48 tex2html_wrap_inline288 4tex2html_wrap_inline472 28 tex2html_wrap_inline288 1 22 tex2html_wrap_inline288 1tex2html_wrap_inline468,tex2html_wrap_inline490
1st 30 Strides 47 tex2html_wrap_inline288 5tex2html_wrap_inline472 25 tex2html_wrap_inline288 1 17 tex2html_wrap_inline288 1tex2html_wrap_inline468,tex2html_wrap_inline490
CV (%) Original Time Series 6.1 tex2html_wrap_inline288 0.5tex2html_wrap_inline472 3.3 tex2html_wrap_inline288 0.2 2.1 tex2html_wrap_inline288 0.1tex2html_wrap_inline512tex2html_wrap_inline490
Lowest 30 stride segment 3.1 tex2html_wrap_inline288 0.2tex2html_wrap_inline472 1.8 tex2html_wrap_inline288 0.1 1.2 tex2html_wrap_inline288 0.1tex2html_wrap_inline512tex2html_wrap_inline490
1st 30 Strides 5.1 tex2html_wrap_inline288 0.5tex2html_wrap_inline472 2.6 tex2html_wrap_inline288 0.1 1.6 tex2html_wrap_inline288 0.1tex2html_wrap_inline512tex2html_wrap_inline490

SD: standard deviation; CV: coefficient of variation. tex2html_wrap_inline468 and tex2html_wrap_inline472 denote p < .005 and p < .0001, respectively, compared to the 6 and 7 year olds. tex2html_wrap_inline490 denotes p < .0001 comparing the oldest and youngest groups. Kruskal-Wallis tests detected significant differences among the three groups for all measures (p < .0001).

Table 3: Spectral Analysis
3 & 4 Year Olds 6 & 7 Year Olds 11--14 Year Olds
% High Frequency Power (0.25--0.5 stridetex2html_wrap_inline306) 0.054 tex2html_wrap_inline288 0.010 0.064 tex2html_wrap_inline288 0.012 0.100 tex2html_wrap_inline288 0.022
    Low/High Ratio 6.8 tex2html_wrap_inline288 1.2 4.1 tex2html_wrap_inline288 0.5 2.3 tex2html_wrap_inline288 0.5tex2html_wrap_inline568
    Low/High Ratio after Detrending 0.30 tex2html_wrap_inline288 0.02tex2html_wrap_inline468 0.20 tex2html_wrap_inline288 0.03 0.15 tex2html_wrap_inline288 0.03tex2html_wrap_inline490
% High Frequency Power (0.3--0.4 stridetex2html_wrap_inline306) 0.022 tex2html_wrap_inline288 0.005 0.023 tex2html_wrap_inline288 0.004 0.038 tex2html_wrap_inline288 0.009
    Low/High Ratio 6.8 tex2html_wrap_inline288 1.1 4.5 tex2html_wrap_inline288 0.7 2.3 tex2html_wrap_inline288 0.3tex2html_wrap_inline568
    Low/High Ratio after Detrending 0.41 tex2html_wrap_inline288 0.06 0.29 tex2html_wrap_inline288 0.04 0.18 tex2html_wrap_inline288 0.02tex2html_wrap_inline490

tex2html_wrap_inline468 denotes p < .05 compared to the 6 and 7 year olds. tex2html_wrap_inline490 denotes p < .005 comparing the oldest and youngest groups. Kruskal-Wallis tests detected significant differences among the three groups for all measures except high frequency power.

Table 4: Effects of Age on Stride Time Dynamicstex2html_wrap_inline468
3 & 4 Year Olds 6 & 7 Year Olds 11--14 Year Olds
Variability tex2html_wrap_inline614tex2html_wrap_inline614 tex2html_wrap_inline614 --
Low/High Frequency Power tex2html_wrap_inline614tex2html_wrap_inline614 tex2html_wrap_inline614 --
Autocorrelation Decay Time tex2html_wrap_inline626 -- --
Fractal Scaling Exponent tex2html_wrap_inline614 tex2html_wrap_inline614 --

tex2html_wrap_inline468Compared to the oldest children in whom stride time dynamics are the most adult-like. Note how different aspects of the temporal structure of the stride dynamics tend to mature at different ages. (The low/high ratio was not statistically different in the two youngest groups, however, this representation reflects the observed trend toward a decreased ratio in the 6 and 7 years olds compared to the 3 and 4 year olds.)

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