Biomedical Engineering Reference
In-Depth Information
in physiological systems [
67
]. The characterization in the linear view is correct
and applicable only to simple systems, which in fact is not applicable to living
organisms due to its extraordinary complexity, with large numbers of structures and
processes [
68
].
In the postural system there are large nonlinearities, due to the elastic property
and the shortening of the muscles and the nonlinear control of the nervous system
[
69
]. Thus, the displacements of COM and its relation with the oscillation of COP
are good candidates to assess inter-segmental movements in the standing position.
Initially it was established that the existence of reflex mechanisms in the spinal cord
and brainstem, were responsible for maintaining the standing position, adjusting
the configuration and reflexes of postural control by disruption of rectification [
70
].
Recently an alternative hypothesis was developed, taking into account an argument
originally proposed by Morasso [
71
]. This argument suggests that the standing
position is no different from other forms of movement, because it requires planning
anticipation and monitoring of internal models, such as the movement of a human
limb in a controlled manner. However, these control mechanisms are not obvious
in the standing position because the body movements and muscle movements has
reduced amplitude.
The control of postural stability is usually associated with the process control of
a inverted pendulum [
72
]. Modelling postural stability as an inverted pendulum, it
is assumed that there is a rigid structure on top of both ankles and this structure
fluctuate around these rigid structures. However, the human body is a multi-
segmented structure capable of motion in all joints above the ankles [
53
]. This
model relates a variable that is controlled (COM) and a variable that controls
(COP), predicting that the difference between COP and COM is proportional to the
horizontal acceleration of COM but with a negative correlation, i.e., in the anterior-
posterior plane, when COP is ahead of the COM, the acceleration is posterior, when
the COP is behind the COM acceleration is anterior [
72
]. The same correlation was
observed when the COP and COM are in the medial-lateral plane. These findings
were validated experimentally by Winter and colleagues [
54
]. In standing position
when the feet are placed side by side, maintaining the COP in the limits of stability
depends of two different strategies [
73
]:
•
Ankle Strategy - in small perturbations, posture control is maintained through the
dorsal and plantar flexors of the foot, acting alone to maintain postural control in
the anterior-posterior plane.
•
Hip Strategy - in large postural disturbances, when the ankle strategy can not be
sufficient to maintain postural stability, the central nervous system acts with hip
strategy. In this case a hip flexion displaces the COM in the posterior direction
and a hip extension moves the COM in anterior direction [
74
].
With these strategies, it's intended to maintain the COM position within the
limits of the base support [
75
]. Clearly, the COP follows the COM displacement
and oscillates at his side to keep the COM in the desired position between the
2 ft. Therefore and because the COP oscillates from either side of the COM, its
displacement is always slightly larger than the COM [
54
]. On the other hand, during
an intermediate position of the feet 45
ı
(when in double support phase, during
