Accurate quantification of soft tissue properties, specifically the stress relaxation behavior of viscoelastic tissues such as plantar tissue, requires precise testing under physiologically relevant loading. However, limitations of testing equipment often result in target strain errors that can contribute to large stress errors and confound comparative results to an unknown extent. Previous investigations have modeled this artifact, but they have been unable to obtain empirical data to validate their models. Moreover, there are no studies that address this issue for plantar tissue. The purpose of this research was to directly measure the difference in peak force for a series of small target strain errors within the range of our typical stress relaxation experiments for the subcutaneous plantar soft tissue. Five plantar tissue specimens were tested to seven incremental target strain error levels of −0.9%, −0.6%, −0.3%, 0.0%, 0.3%, 0.6%, and 0.9%, so as to undershoot and overshoot the target displacement in 0.3% increments. The imposed strain errors were accurately attained using a special compensation feature of our materials testing software that can drive the actuator to within 0% $(1−2 μm)$ of the target level for cyclic tests. Since stress relaxation tests are not cyclic, we emulated the ramp portion of our stress relaxation tests with 5 Hz triangle waves. The average total stress variation for all specimens was $25±5%$, with the highest and lowest stresses corresponding to the largest and smallest strain errors of 0.9% and −0.9%, respectively. A strain overshoot of 0.3%, the target strain error observed in our typical stress relaxation experiments, corresponded to an average stress overshoot of $3±1%$. Plantar tissue in compression is sensitive to small target strain errors that can result in stress errors that are several fold larger. The extent to which the overshoot may affect the peak stress will likely differ in magnitude for other soft tissues and loading modes.

1.
Centers for Disease Control and Prevention, National Diabetes Fact Sheet: General Information and National Estimates on Diabetes in the United States, 2007, Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, 2008.
2.
van Schie
,
C. H. M.
, 2005, “
A Review of the Biomechanics of the Diabetic Foot
,”
Int. J. Low. Extrem. Wounds
,
4
(
3
), pp.
160
170
.
3.
Ledoux
,
W. R.
, and
Blevins
,
J. J.
, 2007, “
The Compressive Material Properties of the Plantar Soft Tissue
,”
J. Biomech.
0021-9290,
40
(
13
), pp.
2975
2981
.
4.
Miller-Young
,
J. E.
,
Duncan
,
N. A.
, and
Baroud
,
G.
, 2002, “
Material Properties of the Human Calcaneal Fat Pad in Compression: Experiment and Theory
,”
J. Biomech.
0021-9290,
35
(
12
), pp.
1523
1531
.
5.
Fung
,
Y. C.
, 1993, “
Biomechanics: Mechanical Properties of Living Tissues
,”
Bioviscoelastic Solids
,
Springer-Verlag
,
New York
.
6.
Dortmans
,
L. J.
,
Sauren
,
A. A.
, and
Rousseau
,
E. P.
, 1984, “
Parameter Estimation Using the Quasi-Linear Viscoelastic Model Proposed by Fung
,”
J. Biomech. Eng.
0148-0731,
106
(
3
), pp.
198
203
.
7.
Kwan
,
M. K.
,
Lin
,
T. H.
, and
Woo
,
S. L.
, 1993, “
On the Viscoelastic Properties of the Anteromedial Bundle of the Anterior Cruciate Ligament
,”
J. Biomech.
0021-9290,
26
(
4–5
), pp.
447
452
.
8.
Funk
,
J. R.
,
Hall
,
G. W.
,
Crandall
,
J. R.
, and
Pilkey
,
W. D.
, 2000, “
Linear and Quasi-Linear Viscoelastic Characterization of Ankle Ligaments
,”
J. Biomech. Eng.
0148-0731,
122
(
1
), pp.
15
22
.
9.
Gimbel
,
J. A.
,
Sarver
,
J. J.
, and
Soslowsky
,
L. J.
, 2004, “
The Effect of Overshooting the Target Strain on Estimating Viscoelastic Properties From Stress Relaxation Experiments
,”
J. Biomech. Eng.
0148-0731,
126
(
6
), pp.
844
848
.
10.
Abramowitch
,
S. D.
, and
Woo
,
S. L.
, 2004, “
An Improved Method to Analyze the Stress Relaxation of Ligaments Following a Finite Ramp Time Based on the Quasi-Linear Viscoelastic Theory
,”
J. Biomech. Eng.
0148-0731,
126
(
1
), pp.
92
97
.
11.
Pai
,
S.
, and
Ledoux
,
W. R.
, 2009,
Structural Properties of Diabetic and Normal Plantar Soft Tissue
,
State College
,
PA
.
12.
Ledoux
,
W. R.
, and
Hillstrom
,
H. J.
, 2002, “
The Distributed Plantar Vertical Force of Neutrally Aligned and Pes Planus Feet
,”
Gait and Posture
0966-6362,
15
(
1
), pp.
1
9
.
13.
Gooding
,
G. A.
,
Stess
,
R. M.
,
Graf
,
P. M.
,
Moss
,
K. M.
,
Louie
,
K. S.
, and
Grunfeld
,
C.
, 1986, “
Sonography of the Sole of the Foot. Evidence for Loss of Foot Pad Thickness in Diabetes and Its Relationship to Ulceration of the Foot
,”
0020-9996,
21
(
1
), pp.
45
48
.
14.
Cheng
,
S.
,
Clarke
,
E. C.
, and
Bilston
,
L. E.
, 2009, “
The Effects of Preconditioning Strain on Measured Tissue Properties
,”
J. Biomech.
0021-9290,
42
(
9
), pp.
1360
1362
.