METHODS FOR SITTING POSTURE EVALUATION: STATIC POSTURE AND
APPLICATIONS
S.Scena, R.Steindler
Department of Mechanics and Aeronautics, University of Rome ‘‘La Sapienza’’
Via Eudossiana 18, 00184 Roma, Italy
[email protected], [email protected]
ABSTRACT
Pressure sores (PS), are frequent in subjects with spinal cord injuries, and to minimize PS these subjects must get used to
change frequently their posture; useful informations to this aim may come testing healthy subjects and studying their kinematic
behaviour during long sitting posture. Two different investigation systems have been developed to this aim in the Department
of Mechanics and Aeronautics (DMA) of Rome University “La Sapienza”: the two systems, a pressure map sensor arranged on
a chair and a potentiometer-based device to measure trunk rotations at the basis of the spine are shown in this paper and their
outputs are compared and correlated. The behaviour of ten healthy subjects is then evaluated during one hour sitting posture
for preliminary considerations: there is a postural change every 7.7 ± 6.7 minutes in the frontal plane and every 5.7 ± 2.7
minutes in the sagittal plane; pelvis movements can be resumed by Centre of Pressure (COP) displacements and the largest
values of these displacements are 5.2 ± 2.4 cm in the frontal plane and 3.0 ± 1.4 cm in the sagittal plane; the largest rotations
in the two planes are respectively 8.4 ± 0.7° and 20.7 ± 12.6°. The results of the study are discussed and satisfactory
compared with literature results.
Introduction
Pressure sores and consequent decubitus ulcers are frequent in subjects with spinal cord injuries and thus wheelchair
bounded [1]. The main reason of PS is pressure on muscular tissues; generally the largest pressures are near the bone
protuberances [2]. PS treating is based on pressure reduction on shear and friction stress reduction, on attention to bacterial
contamination on nutritional deficit correction. Some studies [3] take care of the fact that the muscular tissues are more
stressed from mechanical loads than bound tissues.
The knowledge about strains and stresses of buttock soft tissues during sitting posture can be obtained in several ways. A
model of interactions between the chair and the body has been developed [4]. Force and momentum variations on the spine
due to trunk rotation during sitting posture have been measured [5]. Strain and stresses of fat tissues have been calculated by
means of a “riverse engineering” method [6]. A finite element model (FEM), of unstrained buttocks has been developed using
magnetic resonance images: these images, acquired in the coronal plane, have been compared with the images acquired
during the posture and used as boundary conditions in the FEM: the data analysis has shown that the tissue highest tensions
are in the gluteus muscles (32 ± 9kPa) , not in the fat tissues (18 ± 4kPa) or in the skin near body-chair interface.
From a clinical point of view, the most significant method for PS prevention, is that of training patients to change their posture
frequently [7], for instance with the help of arms. There are no many data about the correct periods for these manoeuvres, and
so useful inputs can be obtained studying the spontaneous kinematic behaviour of healthy subjects during long length sitting
posture. Interesting data have been obtained in Tel Aviv University from ten healthy subjects (five males and five females, age
28 ± 3 years) sitting on a wheelchair during 90 minutes [7]: the rotations of the trunk, the model of which is a double pendulum
in the frontal plane and a simple pendulum in the sagittal plane have been observed: markers for video motion analysis by
cameras have been applied on the subject trunk and shoulders to measure the rotations in the frontal plane, while the
rotations in the sagittal plane have been measured by a potentiometer attached between the trunk and the thigh.
The same study has been made in the DMA laboratory using two different devices, a pressure map sensor, arranged on a
chair, and a potentiometer-based device: the acquisition of the pelvis p-maps makes it possible to study the characteristics of
the posture and of the displacements of the tested subjects, and makes also possible to measure pelvis contact area and
maximum and mean pressures; the potentiometer-based device measures the rotations of the trunk in the frontal and in the
sagittal plane at the basis of the spine.
Methods
Pressure map sensor has 4096 sensing elements resulting from the cross-over of 64x64 conductive strips [9]: the conductive
strips (rows and columns, 0.25 cm width) have been electroplated on the inner side of two plastic sheets (kapton), and a
piezoresistive sheet (velostat) is inserted between the conductive strips; the sensor works with contact resistance variation
with pressure variation, the resolution is 0.5 cm, the saturation limit is about 400÷500 Pa; there is some drift, but time delay is
practically negligible and so the sensor is apt for dynamic applications as posture marked by body sway both when standing
and when sitting [10][11]; a proper electronic device [12], is used to scan the sensing elements row to row, column to column,
the p-map sensor has been stuck to a plastic plate 5 mm thick and then arranged on a chair (fig. 1a); so pelvis p-maps can be
acquired and pelvis displacements can be observed: particularly relevant is the lifting (partial or complete) of one or both
buttocks; from each acquired map it is possible to calculate the COP coordinates and to plot the trends in the frontal and in the
sagittal planes.
The potentiometer-based device is formed by two angular potentiometers that measure trunk rotations at the spine basis: the
first potentiometer (fig. 1b) measures the rotations in the frontal plan; it is housed in a square aluminium alloy bearing, which
can move in the two directions of the frontal plane thanks to two vertical slides and to one horizontal slide, all in carbon steel,
and so designed as to avoid bends which may give rise to system seizure or to slow down its motion. The horizontal slide,
hexagonal section, makes possible forward bearing rotations: the rotations in the sagittal plane are measured by the second
potentiometer (fig. 1c). The system vertical position can be adjusted by two springs on the vertical slides, then fixed by two
stops. A rectangular plate (aluminium alloy), is connected to the bearing; this plate is integral with the first potentiometer: a
couple of belts, starting from the rectangular plate, connect the whole device to the back of the tested subject; one belt is
knotted round the stomach, the other round the thorax; the device takes the place of the back of the chair on which the
pressure map sensor is put.
(a)
(b)
(c)
Fig. 1 .- The chair with the measurement devices (a); the two views of the back (b), (c)
The first aim of this paper is that of correlating the outputs of the two measuring devices to better investigate the sitting
posture: 10 subjects, DMA students, have been tested on the chair of fig. 1; all the subjects were healthy, without neurological
or physical problems, and gave their approval before the test. Thanks to the belts, the subject back was firmed to the
potentiometer device; afterwards, all the subjects moved their trunk according to an established protocol. The first part of the
protocol consisted in trunk displacements in the frontal plane, the second part in displacements in the sagittal plane; the total
length of the test was 250 s, the sampling frequency 8 Hz. During each test the pelvis pressure maps were acquired, the COP
trends plotted and compared with the potentiometer outputs, so making it possible to correlate the trunk rotations with the COP
displacements.
The second aim of this paper, is a preliminary study of the spontaneous behaviour of healthy subjects during long length sitting
posture. 10 DMA students else were asked to sit on the chair for one hour after their approval; this hour was spent looking at a
movie: so the movements of the tested subjects were completely spontaneous.
Results
With regard to the first aim of this paper, we try to establish a linear relation between COP displacements, as coming from the
acquired pressure maps, and the trunk rotations as measured by the two potentiometers; so we write the following relations:
COPx = Kθθ + COPx ,mean
(1)
COPy = Kϕϕ + COPy ,mean
(2)
and we look for their validity. In (1) and (2) COPx and COPy are the COP coordinates in medial-lateral displacements (frontal
plane) and in anterior-posterior displacements (sagittal plane), θ is the trunk rotation in the frontal plane, φ is the trunk rotation
in the sagittal plane and COPx,mean e COPy,mean are test references corresponding to the upright position.
20
18
COPx [cm]
16
14
. . Misure
. Measures
__ Sigmoidal Fitting
Sigmoide
__ tangent in inflexion
12
Retta
point
Interpolante
10
8
6
-30
-20
-10
0
10
20
30
Rotazione
Medio Laterali
Rotations
[°] [°]
Fig. 2 – Data acquired during medial-lateral rotations
18
16
COPy [cm]
14
12
. . . Measures
Misure
10
__Retta
Linear Fitting
interpolante
8
6
4
-40
-30
-20
-10
0
10
20
30
[°]
RotazioniRotations
Antero-Posteriori
[°]
Fig. 3 – Data acquired during anterior-posterior rotations
40
Trunk rotation in the frontal plane does not show a linear trend (fig.2), and so we refer to a sigmoid curve:
COPx (θ ) = COPx ,min +
L
1 + ae −bθ
,
(3)
where a and b are the fitting unknown parameters, while
L = COPx ,max − COPx ,min ,
(4)
is the amplitude of the COP medial-lateral excursion.
Kθ coefficient in (1) can be assumed as the derivative of (3) inflexion point; so:
Kθ =
bL
;
4
(5)
the intersections of the sigmoid tangent in the inflexion point with the sigmoid asymptotes can be considered the limits of the
linear relation (1) between COPx and θ; in each test a and b have been calculated by the fitting, and so the approximating
sigmoid function has been found; it has then be possible to calculate Kθ, its reciprocal Kθ -1 and the limits of the linear trend.
Trunk rotation in the sagittal plane shows a linear trend (fig.3), and so we refer immediately to a linear fitting for all values of
COPy and φ; this means that there are no limits for (2) validity.
The obtained data give Kθ = 35.8 ± 8.4 cm/rad (a t distribution with p>0.05 has been used); so (from rad to °), Kθ-1 = 1.8±0.4
°/cm. In the sagittal plane the data give Kφ = 6.1±1.4 cm/rad, and so Kφ-1=10.5±2.5°/cm. With regard to the linearity limits, the
result is: θmax=13.1±3.0° and θmin=-17.5±3.8° ; in security, relation (1) may be considered valid when trunk rotations don’t
exceed ± 10 ÷ 12°, while trunk rotations may reach ±25°. Relation (2), on the opposite, is valid for any rotation. The precision
of the adopted fittings comes from the Pearson coefficients which are 0.93 ± 0.03 for the sigmoid function in the frontal plane
and 0.87 ±0.07 for the straight line in the sagittal plane.
Rotations [°]
The COP trends, with regard to their mean values and divided by their own K may be placed on the rotation trends as shown
in figg. 4 and 5 show.
θ
COPx/Kθ
Fig. 4 – Comparison of COPx/Kθ and θ
40
30
10
Rotazioni [°]
Rotations [°]
20
Misurate
Φ
0
Calcolate
COPy/Kφ
da COP
-10
-20
-30
-40
0
10
20
30
40
50
60
70
80
90
100
tempo [s]
time [s]
Fig. 5 – Comparison of COPy/Kφ and φ
With regard to the second aim of the paper, the long length sitting posture tests took place. From each acquired pressure map
(fig. 6) it is possible to calculate COP coordinates and to plot their trends in the frontal and in the sagittal planes (fig. 7). First of
all it can be observed that there are continuous displacements about 1mm length (microdisplacements corresponding to small
random pelvis sways), and more significant displacements about 1 cm length (macrodisplacements corresponding to changes
of posture). The attention was thus given to the macrodisplacements during which the examination of the pressure maps
shows partial or complete lifting of one or both buttocks; so, taking care of literature [13], we have examined the medial-lateral
and the anterior-posterior COP displacements equal or larger than 1 cm. The subjects change their posture each 7.7±6.7
minutes in the frontal plane and each 5.7±2.7 minutes in the sagittal plane. Moreover during several tests a forward slipping
has been observed. The largest displacements (both for time length and amplitude), take place about 40 minutes after the
beginning of the test, i.e. at the 66% of the test. For each tested subject the amplitude of the largest displacements in both the
directions has been calculated: these data are very important to know how the injured patients must behave when they change
their posture. The maximum amplitude of the medial-lateral COP displacements is 5.2±2.4 cm, while the maximum amplitude
of the anterior-posterior displacements is 4.0±1.4 cm.
Fig. 6 – A typical pelvis pressure map
time [h.min]
Fig. 7 –COPx and COPy trends during a long length test
We have then examined the trunk rotations at the basis of the spine (fig. 8); taking care of the previous results we have
considered rotations in the frontal plane equal or larger than 1,8° and rotations in the sagittal plane equal or larger than 10,5°.
The rotation frequencies in both planes are practically the same of COP displacements frequencies. The maximum rotations
are 8.4±0.7° in the frontal plane, and 20.7±12.6° in the sagittal plane.
Discussion
The study of the correlations between the pelvis movements and the trunk rotations at the spine basis, and the preliminary
analysis of long time sitting posture have shown some significant facts. In the frontal plane the COP displacements don’t go
over precise values, i.e. there are asymptotic limits while trunk rotations at the spine basis go on increasing: in fact, when one
buttock lifts, the COP position does not change: it keeps fixed in the other buttock, even if the trunk goes on rotating. On the
opposite, in the sagittal plane the COP goes on with its displacement while the trunk rotates. To conclude, the displacements
in the frontal plane correspond to the prevalent posture on one buttock (particularly, the right one), while the displacements in
the sagittal plane correspond most of all to forward trunk rotations.
During the long length tests, the ratios maximum rotations/maximum COP displacements are some smaller than those
calculated during the correlation tests; we think that during the correlation tests the subjects pay particular attention to the
trunk rotations, while during the long length tests what above does not happen; this probably means that during the correlation
tests the trunk is like a rigid body more than during long length tests; however the values of the two ratios are comparable.
Our results satisfying agree with literature results, particularly with Tel Aviv University results both for displacement
frequencies and for the largest rotation amplitudes: in Tel Aviv tested, the subjects have changed their posture every 9.1±6.5
minutes in the frontal plane and every 6±2 minutes in the sagittal plane; the maximum frontal sways at the basis and at the half
of the spine have been 15±7° and 14±7°, the shoulder rotation has been 8±4º, while the maximum sagittal sways have been a
10.3±7°.
So, while interesting correlations have been established between pelvis displacements and trunk rotations, preliminary one
hour sitting posture tests give reliable suggestions about frequency and characteristics of posture changes for injured patients
in order to avoid pressure sores. However, it must be pointed out that longer tests are necessary, further parameters must be
studied (mean displacements and rotations mean and maximum pressures) and further measurements must be made (the
translations at the basis of the spine, the loads on chair arms), for the best knowledge of the investigated phenomenon and to
establish the interventions on the injured subjects.
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