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<H3><A NAME="SECTION00040100000000000000">Stride Time Variability Measures</A></H3>





  
  

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<P>





  
  

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Representative examples of the effects of age on the stride time





  
  

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fluctuations are shown in Figure 1.  The stride-to-stride variability





  
  

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is largest in the four year old, lower in the seven year old, 





  
  

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and smaller still in the eleven year old child. As summarized in Table 2,





  
  

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there was a highly significant effect of age on variability 





  
  

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(p &lt; .0001).  Both the standard deviation and coefficient of





  
  

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variation (CV) were significantly larger in the 3 and 4 year olds





  
  

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compared to the 6 and 7 year olds (p &lt; .0001). In addition, these





  
  

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measures were significantly larger in the 6 and 7 year olds compared





  
  

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to the 11 to 14 year old children (p &lt; .005).  Of note, the





  
  

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stride-to-stride variability of the 11 to 14 year old children was





  
  

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closest to the values obtained  in healthy, young adults (CV





  
  

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= 1.3 <IMG WIDTH=12 HEIGHT=27 ALIGN=MIDDLE ALT="tex2html_wrap_inline288" SRC="img6.png"> 0.1 % in the young adults and 2.1 <IMG WIDTH=12 HEIGHT=27 ALIGN=MIDDLE ALT="tex2html_wrap_inline288" SRC="img6.png"> 0.1 % in





  
  

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the 11 to 14 year olds).





  
  

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<P>





  
  

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In the representative examples shown in Figure 1, the local average of the stride time of the oldest child is relatively constant throughout the walk.  In





  
  

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contrast, for the two younger children, the local average appears to





  
  

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change from time to time.   Therefore, we next





  
  

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addressed two questions: 1) Is the increased variability in the





  
  

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younger children simply due to fatigue during this walk? 2) Is this





  
  

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increased variability due to a change in rate during the walk (e.g.,





  
  

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long-term slowing down or speeding up), and not indicative of





  
  

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short-term, stride-to-stride unsteadiness per se?





  
  

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<P>





  
  

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To evaluate these questions, we detrended each time series to minimize





  
  

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the effects of any local changes in average stride.  Figure 2 shows





  
  

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the results for the times series shown in Figure 1.  Even after





  
  

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detrending, variability is largest for the four year old child and





  
  

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smallest for the oldest child.  This inverse relationship between





  
  

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variability and age after detrending was found in general for all subjects as





  
  

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well.  The standard deviation of the detrended time series, a measure





  
  

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of the dispersion or variability, was significantly larger in the 3





  
  

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and 4 year olds compared to the 6 and 7 year old (p &lt; .0001) and in





  
  

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the 6 and 7 year olds compared to the oldest children (p = .004).





  
  

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<P>





  
  

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As a further test of these findings, we analyzed





  
  

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sub-sections of each subject's time series to find the 30 consecutive





  
  

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strides with the lowest CV. (A data analysis window was moved forward 5 strides at a





  
  

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time across the time series and in each window the CV was





  
  

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calculated). Variability during this segment should be largely





  
  

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independent of a subject's speeding up or slowing down during the





  
  

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trial and reflects the ``best-effort'' of the neuromuscular control





  
  

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system.  For the data shown in Figures 1 and 2, the CV calculated in





  
  

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this manner was 3.8, 1.9 and 1.1 % for the 4, 7 and 11 year old,





  
  

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respectively.  Figure 3 shows the results of this lowest variability





  
  

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time segment for all subjects.  Even during a relatively short time





  
  

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period, the fluctuations from one stride to the next were significantly





  
  

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increased in the 3 and 4 year olds compared to the 6 and 7 year olds





  
  

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(p &lt; .0001) and in the 6 and 7 year olds compared to the oldest





  
  

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children (p &lt; .0001). In fact, the CV of each of the oldest children





  
  

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was lower than that of all of the 3 and 4 year old children.





  
  

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<P>





  
  

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Finally, to confirm that the increased variability in the younger





  
  

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children was not simply due to fatigue or a change of speed during the





  
  

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walk, we studied the variability of only the first 30 strides. As was the





  
  

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case for the entire walk, both the standard deviation and coefficient





  
  

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of variation were significantly larger in the 3 and 4 year olds





  
  

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compared to the 6 and 7 year olds (p &lt; .0001) and in the 6 and 7





  
  

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year olds compared to the oldest children (p &lt; .0003) (Table 2).





  
  

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<P>





  
  

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