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In-Depth Information
A centipede was happy quite,
Untilafroginfun
Said, “Pray, which leg comes after which?”
This raised her mind to such a pitch,
She lay distracted in the ditch
Considering how to run.
Walking is one of those things that is learned in early childhood, but is done later
with little or no thought, day in and day out. We walk confidently with a smooth pattern
of strides and on flat level ground there is no visible variation in gait. This appar-
ent lack of variability is remarkable considering that walking is created by the loss
of balance, as pointed out by Leonardo da Vinci in his treatise on painting. Da Vinci
considered walking to be a sequence of fallings, so it should come as no surprise that
there is variability in this sequence of falling intervals, even if such variability is usually
masked.
The regular gait cycle, so apparent in everyday experience, is no more regular than
“normal sinus rhythm” or breathing. The subtle variability in the stride characteris-
tics of normal locomotion was first discovered by the nineteenth-century experimenter
Vierordt [
85
], but his findings were not followed up for over a century. The random
variability he observed was so small that the biomechanical community has histori-
cally considered these fluctuations to be biological noise. In practice this means that
the fluctuations in gait were thought to contain no information about the underlying
motor-control process. The follow-up experiment to quantify the degree of irregularity
in walking was finally done in the middle of the last decade by Hausdorff
et al
.[
28
].
Additional experiments and analyses were subsequently done by West and Griffin [
98
],
which both verified and extended the earlier results.
Walking is a complex process, since the locomotor system synthesizes inputs from the
motor cortex, the basal ganglia and the cerebellum, as well as feedback from vestibular,
visual and proprioceptive sources. The remarkable feature of the output of this complex
web is that the stride pattern is stable in healthy individuals, but the duration of the
gait cycle is not fixed. As with normal sinus rhythm, for which the interval between
successive beats changes, the time interval for a gait cycle fluctuates in an erratic way
from step to step. The gait studies carried out to date concur that the fluctuations in the
stride-interval time series exhibit long-time inverse power-law correlations indicating
that the phenomenon of walking is a self-similar fractal activity; see West [
100
]fora
review.
Walking consists in a sequence of steps and the corresponding time series is made up
of the time intervals for these steps. The stride-interval time series for a typical subject is
shown in Figure
2.24
, where it is seen that the variation in time interval is on the order of
3%-4%, indicating that the stride pattern is very stable. The stride-interval time series
is referred to as stride-rate variability (SRV) for consistency with the other two time
series we have discussed. It was the stability of SRV that historically led investigators
to decide that they could not go far wrong by assuming that the stride interval is constant
and that the fluctuations are merely biological noise.
