VISCOELASTIC PROPERTIES
OF BUTTOCK SOFT TISSUES WITH PRESSURE ULCER SUSCEPTIBILITY
Jue Wang, David M. Brienza, Patricia Karg, and Gina Bertocci,
Soft Tissue Mechanics Laboratory, School of Health and Rehabilitation Sciences
University of Pittsburgh, Pittsburgh, PA 15260
ABSTRACT
Preliminary experiments were performed on the intact buttock soft tissues of 5 subjects with
spinal cord injuries (SCI) and histories of pressure ulcers (PU). Compared to the experimental
data from 6 subjects withoutSCI, significant differences in the material properties, reduced
relaxation parameter,  (P<0.05) and elastic response, E0, (P<0.001) across the two groups
were observed. Further investigation in the SCI population between those with and without
pressure ulcer histories is recommended.
BACKGROUND
People with SCI who sit for long periods of time are at high risk of developing pressure ulcers
on the buttock soft tissues over the ischial tuberosities (IT). According to a report of the US
Regional SCI System, 8.0% of all ulcers in people with SCI occurring during initial
hospitalization occurred over an IT. A VA hospital care review in which patients were followed
for 4 years post injury found 23-28% of all ulcers occurred over an IT [1, 2].
RESEARCH QUESTIONS
Our long-term goal is to identify biomechanical properties that relate to risk for pressure ulcer
development. This study focuses on the comparison of biomechanical responses of buttock soft
tissues between people without SCI and people with SCI and a history of pressure ulcers.
METHOD
Five male subjects with spinal cord injuries (C4 to
T12) and the occurrence of a pressure ulcer history
within the past two years were recruited. None of the
subjects had had a PU on the test site. Six control
subjects without any neuromuscular disease were
recruited. Both groups’ age range was from 35 to 64
and body fat percentage was less than 30%.
A Computer Automated Seating System (CASS)
(11 by 12 sensor array) and related test protocol for in
vivo indentation tests was used for data collection [3,
4]. With the subject sitting on the CASS, the response
of his buttock tissues 4 cm in front of ischial tuberosity
was recorded. The indentation loading cycle consisted
Fig.1 The design of this study
of: indentation at 1mm/sec to 20% of initial
thickness of bulk tissue, 5 min. hold 5, and recovery. Tissue thickness was tracked using an
automated ultrasound tracking technique. The information record included the thickness of tissue
layers, force, and tilt angle of the test sensor. Fung's QLV theory was used to assess viscoelastic
parameters of buttock soft tissue.
P[u(t); t]=G(t)*Pe(u)
G(0)=1
(1)
Where, G(t) is reduced relaxation function that describes the stress response with time, and
Pe(u) is the elastic response function.
The Zheng’ equations [5] were used to define G(t) and Pe(u).
VISCOELASTIC PROPERTIES OF BUTTOCK SOFT TISSUES
G(t) = 1 -  + e-t
(2)
2ah
e
e
(3)
P (u) 
k(h,)E ()
(1 v 2 )
Ee (u) = E0 + E1(t)
(4)
where, a is the radius of the indenter, h is the original thickness of soft tissue, k is a scaling factor
that depends on the Poisson’s ratio v and the relative aspect ratio a/h(1-), and  is relative
indentation. k can be obtained from the table in Hayes’ paper in 1972. In this study, the poison’s
ratio is taken to be 0.45 [5] and the boundary condition was assumed as G(  )=G(300s).
The reduced relaxation parameters,  and the time constant, were determined by curve
fitting G(t) to experimental data of buttock soft tissue. The unrelaxed initial elastic indentation,
E0, and the unrelaxed nonlinear elastic modulus, E1 were determined from the elastic response.
Differences of in viscoelastic parameters between the subjects without SCI and the subjects with
pressure ulcer SCI was investigated.
RESULTS
Compared to the results of subjects without SCI, the G(t) curve from subjects with SCI
showed much less relaxation. And, the relaxation for the SCI group tended to be linear (Fig.2
(a)). The data from subjects without SCI showed clear reduced relaxation and fast viscous
phenomenon (Fig.2 (b)). The comparison of two typical elastic response curves between the
groups is displayed in Fig. 2 (c). A decreased amplitude of stress, Pe(), for the SCI subject
was observed. A non-linear elastic response phenomenon in a set of data from an SCI subject
(a)
(b)
(c)
(d)
Fig. 2 (a) A typical G(t) Curve from subject with SCI and pressure ulcer history; (b) A typical G(t)
Curve from subject without paraplegia; (c) Comparison of elastic response data across two
research groups. (d) A set of typical elastic response from disabled subject 3
Table 1 Comparison of QLV parameters between two groups
Results from subjects without paraplegia Results from subjects with pressure ulcer susceptibility
Parameter Range

Mean
Standard Deviation
Mean Square Error for
curve fitting

E0
 (
(N/mm2)
s)
0.068- 44.2 – 18 – 42
0.328
164.2
<0.039 <54.5 < 10
<5%
E1
(N/mm2)
- 93+46
< 65

 (s)
0.016- 25.3
0.130
–81.2
<0.104 <38.3
< 11 %
E0
(N/mm2)
-11.2 –
+9.63
< 7.5
E1
(N/mm2)
-18.5 - +395
< 41
was also observed (Fig. 2 (d)). This particular elastic response curve took a 4th order nonlinear shape. The comparison of QLV parameters is summarized in Table 1. Significant
differences exist for  (P<0.05) and for E0 (P < 0.001) between the two groups.
DISCUSSION
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VISCOELASTIC PROPERTIES OF BUTTOCK SOFT TISSUES
The CASS, data acquisition protocol, and QLV modeling have been successfully used to
analyze the difference in viscoelastic properties of buttock soft tissues between subjects with SCI
and a history of having had a pressure ulcer and subjects without SCI. A future investigation
should be conducted to compare the visoelastic properties of subjects with SCI with and without
histories of having had a pressure ulcer.
In addition, for most subjects, the elastic response modulus was linear. However, a higher
order (nonlinear) elastic response was observed in one subject in the SCI group. A higher body fat
percentage and greater fat tissue thickness may explain the difference in the elastic response in
this particular subject compared to the others. The physical interpretation of this high-order QLV
model parameter should be further studied and given definition.
REFERENCE
1. Young, J. S. et al. Pressure sores and the spinal cord injured. In J. S. Young, P. E. Burns,
A. M. Bowen, & R. McCutchen, (Eds), In spinal cord injury statistics----experience of
the regional spinal cord injury system. 1982, p. 95-121.
2. Lee, B. Y. et al. Surgiral management of pressure sores. Contemp. Orthop. 1982. No.5:
p.49-55
3. Brienza, D.M., et al., A system for the analysis of seat support surfaces using surface
shape control and simultaneous measurement of applied pressure. IEEE Trans. on Rehab.
Eng., 1996. 4(2): p. 103-13.
4. Wang, J., et al. Design of an ultrasound soft tissue characterization system for the
computer-aided seating system. in RESNA'96 annual conference. 1996. Salt lake city.
5. Zheng, Y.P. and A.F.T. Mak, Extraction of Quasi-Linear Viscoelastic parameters for
lower limb soft tissues from manual indentation experiment. J. Biomech. Eng., 1999.
121(June): p. 330-9.
ACKNOWLEDGMENTS
This study was funded Paralyzed Veterans of America, Spinal Cord Research Foundation,
and the NIDRR RERC on Wheeled Mobility grant # H133E990001.
Seating and Soft Tissue Biomechanics Laboratory, University of Pittsburgh, 5052 Forbes
Tower, Pittsburgh, PA 15260, Tel: 412-383-6726, Fax: 412-383-6597, juewang@pitt.edu
Alternative Text
Fig. 1. The design of this study
This is a diagram to show data acquisition and signal processing method. A load-indentation
test is applied on the tissues with high and with low pressure ulcer susceptibility. The tissues’
biomechanical responses over time are collected. Quasi-Linear Viscoelastic (QLV) model is
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VISCOELASTIC PROPERTIES OF BUTTOCK SOFT TISSUES
used to do curve filling into experimental data. The Characteristic parameters of soft tissues
are extracted from the QLV model. Comparing the QLV parameters from the two population
groups, we try to find the difference of OLV parameters between the two groups. The
characteristic values may be used to identify the subjects at high pressure ulcer risk.
Fig. 2 (a) A typical G(t) curve from subject with SCI and pressure ulcer history
This figure shows the relationship between reduced relaxation function, G(t), and time, t, of a
subject in the disabled group. The vertical coordinate is reduced relaxation function, G(t), with
scale from 0 to 1 and the horizontal coordinate represents time, t, with scale from 0 to 330s.
The G(t) curve includes three phases, indentation, hold 5 minutes, and recovery. G(0) was
normalized into 1. The loading response from subject with SCI and pressure ulcer history
shows that there is a slight reduce relaxation property with a linear attenuation during the hold
phase. No fast viscous attenuation phenomenon in hold phase was observed.
Fig. 2 (b) A typical G(t) Curve from subject without paraplegia
This figure shows the reduced relaxation function, G(t), property overtime of a subject in the
control group. The vertical coordinate is reduced relaxation function, G(t), with scale from 0
to 1 and the horizontal coordinate represents time, t, with scale from 0 to 330s. The G(t) curve
includes three phases, indentation, hold 5 minutes, and recovery. G(0) was normalized into 1.
The G(t) curve from subject without paraplegia shows that there is a steady and larger reduced
relaxation phenomenon and the fast viscous phenomenon was clearly shown during the hold
phase.
Fig. 2 (c) Comparison of typical elastic response properties between two groups
This figure exhibits the a typical relationship of stress, Pe(), in indentation phase, to relative
indentation, , across two groups. The vertical coordinate represents stress, Pe(), with the
range from 0 to 10N and the horizontal coordinate is relative indentation,  with the range
from 0 to 0.15. Comparing to the Pe(value from the control group, the Pe() of buttock soft
tissue of subject in the research group had a lower response velocity to indentation loading .
Fi.g 2 (d) A set of typical elastic response from disabled subject
This figure exhibits a set of elastic response curve from a subject in research group. The
vertical coordinate is stress, Pe(), with the scale from 0 to 4.5N and the horizontal coordinate
represents time, t, with the scale from 0 to 12s. All elastic response curves from three
indentation test trials exhibited the following pattern: upward first with angle about 45 degree,
flat, and upward again at the same degree angle.
4