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.
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.
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.
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.