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<H1><A NAME="SECTION00050000000000000000">Discussion</A></H1>
<P>
This quantitative study of stride variability and dynamics reveals
several interesting new findings: 1) Stride to stride variations in gait cycle
duration are significantly larger in healthy 3 and 4 year old
children compared to 6 and 7 year old children and in 6 and 7 year old
children compared to children ages 11 to 14. 2) The temporal structure
of gait fluctuations is not fully developed in 7 year old
children, while in older children (11 to 14 year olds), stride dynamics
approach the values observed in adults. 3) Different features of stride
dynamics do not develop at the same time (Table 4). Thus, while
visual observation might suggest that the stride dynamics of
children are not different from that of adults, quantitative
measurement of gait dynamics indicates that stride-to-stride control
of walking is not fully mature even in 7 year olds.
<P>
A number of similarities have been reported in the gait pattern of children and elderly adults (5,6,23). This finding may reflect a
reappearance of primitive reflexes or simply diminished control of
balance (23). The present study
demonstrates that parallels also exist with respect to stride dynamics.
In older adults and persons with neurological
impairment, alterations of stride dynamics have been observed (3,4,7,8,10,14).
However,
while the stride dynamics of young children share some characteristics
of the unstable dynamics of older persons and those with
neurological impairment, there appear to be important differences as
well. For example, the present findings suggest that the fractal scaling
index changes monotonically throughout the lifespan (highest in
children, lower in adults and lowest in the elderly and persons with
neurological disease). In contrast, stride variability likely changes in a
U-shaped fashion (high in children, lower in adults, and higher with
disease and perhaps also in very advanced age). Thus, from the
perspective of stride time dynamics, the changes
in gait of older persons do not simply reflect a return to
an immature gait pattern.
<P>
The alterations of the dynamics of the stride time in the younger
children may be due to a number of factors. The increased variability
may in part be related to decreased walking velocity and decreased
postural stability at lower speeds (23). However, while adjustment
for height minimized the effects of age on velocity, the age-related
differences in both the magnitude of the variability and in the
dynamics persisted after controlling for height. A number of factors
also suggest that the observed age-related changes in the temporal
organization of stride dynamics are most likely not simply
attributable to reduced height, gait speed, change in concentration
during the walk, or increased stride-to-stride variability
(unsteadiness). For example, fractal scaling indices were similar in
the 3 and 4 year old children compared with the 6 and 7 years old
children, despite significant differences in stride-to-stride
variability, velocity, and height. Age-related differences in stride dynamics were evident
in dynamical measures even after detrending to minimize the effects of
changes in speed or local average stride time. Moreover, an age-related effect
was observed in the ratio of spectral balance, a measure that was derived
independently of stride to stride variability and very low frequency
changes likely to be associated with change of speed or loss of concentration.
<P>
Future study of children walking at different speeds may help
elucidate the role of velocity on stride dynamics in children.
In addition, studies that include assessment of
motor control and balance as well as other aspects of the locomotor
control system
may also help clarify the role of
potential contributing factors to the development of mature
gait dynamics. Perhaps, differences in motor control development
account for some of the observed heterogeneity in stride dynamics
within each age group (e.g., Figure 1).
An intriguing possibility is that these dynamical measures
may provide a means of quantifying the stage of maturational development.
In any case, it seems that 1) stride time
dynamics most likely depend on some aspect of the neuromuscular control system
that is not merely related to walking velocity or gait variability,
and 2) the immature gait dynamics in children may reflect the subtle
ongoing development of more than one component of motor control.
The dynamical action theory of motor
control postulates that locomotor function can be viewed as
a complex system with multiple degrees of freedom whose
collective behavior is governed in part by the principle of
self-organization (13,23,27). Therefore, perhaps mature locomotion dynamics
emerge only once all of the interacting individual components are
fully developed. The change in scaling exponents with age,
a measure associated with a non-equilibrium dynamical
system with multiple-degrees-of-freedom (1,22), may reflect
this emergent behavior. Candidate elements that could affect
stride dynamics include
biomechanical and neural properties that are
known to mature only in older children (e.g., electromyogram recruitment patterns
are more variable in children under 7 years of age) (15,23).
Additional studies will be needed to explain these complex age-related
changes in the magnitude and temporal
structure of stride dynamics. Nonetheless, the present findings have
potentially
important implications for the understanding and modeling of the
integrative control of locomotor function and neural development. Further,
the results suggest the possibility that quantitative measures
of stride dynamics may be useful in augmenting the early detection
and classification of gait disorders in children.
<P>
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