Biomedical Engineering Reference
In-Depth Information
The evaluation of this GRF, permit to calculate the moments of force by inverse
dynamics (mechanic of rigid body methodology), and study the behavior of the
system in relation to a motor task [
47
]. Winter defined the COP [
48
] as the point of
application of the vertical ground reaction force. Represents the weighted average
of the pressures on the surface and is considered a major kinetic variable in the study
of postural stability. The COP determination using a single force plate, results in a
two-dimensional diagram (spatial and temporal), which results from the vector sum
of the anterior-posterior and medial-lateral components of force [
49
]. The center
of mass (COM) is considered the major kinematic variable in the study of postural
stability, and is defined as the point representing the total body mass in a global
reference system. In the human body, the COM coincides with the center of gravity
(COG), but is independent of any gravitational field [
50
]. The COM is determined
by the weighted average COM of each body segment, in a space defined by a three
dimensional referential.
The standing position is not a static position, presents oscillations that allow
in indirect way describing the stiffness and postural control [
11
]. In standing
position, humans have a postural sway with an average frequency of 0.27-0.45 Hz
[
51
], which is related to the constant change of neural inputs, primarily originated
from the spinal cord level [
48
,
52
]. These small oscillations are associated with
angular changes of about 1.0
ı
-1.5
ı
of the ankle, knee and hip [
53
], which causes
horizontal displacement of COM nearly about 4-28 mm [
54
]. To ensure that the
COM remains in the ideal position, the COP has to suffer larger displacements than
COM. Considering the COP behaviour a consequence from the individual control
of postural stability, accessing the variability of this variable during the standing
position or in gait, we can access the strategies used in postural control in both
planes (sagittal and frontal) [
55
], with the advantage of being immediately, non
invasive and easily acquired. When assessing the standing position we obtained the
“stabilograms”, which is the projection of the COP trajectory in a two-dimensional
space [
56
]. It was demonstrated in multiple sclerosis patients, at an early stage
disease, when there are no major functional changes, the static tests are less sensitive
[
57
]. Moreover the dynamic tests, usually through balance tasks to achieve, do not
reproduce the postural changes during walking, so it is essential to perform an
evaluation based on this functional activity.
2.3
Plantar Pressure
One method to study the COP in standing position or during gait is using the plantar
pressure plate [
58
]. The information obtained from this system is very important
because it allows to verify the relationship between plantar pressure and body
posture, as well as defining the COP variability [
59
]. This evaluation was recognized
as an important factor to access gait disturbances in neurological diseases. The
multidimensional and complex nature of postural stability, prevents this type of
information to be obtained with a single test [
16
]. The matrix of plantar pressure
