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Citation: Brandl, A.; Schleip, R. The
In Vitro and Vivo Validation of a New
Ultrasound Method to Quantify
Thoracolumbar Fascia Deformation. J.
Clin. Med. 2025,14, 1736. https://
doi.org/10.3390/jcm14051736
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Article
The In Vitro and Vivo Validation of a New Ultrasound Method to
Quantify Thoracolumbar Fascia Deformation
Andreas Brandl 1, 2, * and Robert Schleip 1,3
1Conservative and Rehabilitative Orthopedics, TUM School of Medicine and Health, Technical University of
Munich, 80333 Munich, Germany; robert.schleip@tum.de
2School of Osteopathy, College Sutherland, 22769 Hamburg, Germany
3Department for Medical Professions, Diploma Hochschule, 37242 Bad Sooden-Allendorf, Germany
*Correspondence: andreas-rudi.brandl@tum.de
Abstract: Background: A new method for quantifying thoracolumbar fascia deformation
(TLFD) and its shear capacity has been introduced, and its reliability for discriminating
patients with low back pain (LBP) from healthy controls has been demonstrated in a
recent paper. The aim of this study was to investigate the method in terms of criterion
validity. Methods: First, the concurrent validity of the TLFD ultrasound measurement
method (TLFD_US) was tested
in vitro
, using a custom-made tissue sliding device that
mimics tissue shearing and generates ground truth data. Second, ultrasound images and
videos of TLFD were acquired from 10 acute LBP patients and 10 healthy controls by
a blinded assessor.
In vivo
, the concurrent validity of TLFD_US and speckle tracking
analysis was then tested. Third, the contribution of the surrounding tissue layers of the
erector spinae muscle and dermis to TLFD was calculated using multiple linear regression.
Results: The
in vitro
concurrent validity between TLFD_US and ground truth was excellent
(ICC = 0.99; p< 0.001).
In vivo
, the concurrent validity between TLFD_US and speckle
tracking analysis was large (r = 0.701; p< 0.001). Multiple linear regression revealed a large
effect regarding the relationship between dermis shear and TLFD (R
2
= 0.353; p= 0.01).
Conclusions: TLFD_US
showed excellent criterion validity. Its suitability for capturing
morphological parameters of the thoracolumbar fascia is further reinforced.
Keywords: thoracolumbar fascia; deformation; ultrasound; validity; acute low back pain
1. Introduction
One of the leading causes of disability-related loss of life years worldwide is low
back pain (LBP). It therefore represents a significant global economic burden on healthcare
systems [
1
]. While LBP-related pain syndromes are estimated to cost USD 135 billion in the
United States [
2
], in Germany, where the study was conducted, they are the most common
condition, with a 14% share of disability-related loss of life years [
3
]. In addition, the cure
rate for acute LBP (aLBP) is only one third. The remainder, i.e., the majority of all aLBP
patients, suffer new episodes of pain within the next year [4].
Besides some risk factors such as older age, higher BMI and greater physical exer-
tion or in particular previous phases of aLBP [
5
], there is increasing evidence that the
thoracolumbar fascia (TLF) may play a contributory role in the development of low back
pain [
6
–
9
]. The TLF is an aponeurotic, diamond-shaped structure that covers the paraspinal
muscles and separates them from the abdominal muscles with a layer. The most posterior
layer is dominated by the insertion of the latissimus dorsi muscle [
10
]. Its rich innerva-
tion, involving in particular type A and C nociceptive nerve fibers, makes it susceptible
J. Clin. Med. 2025,14, 1736 https://doi.org/10.3390/jcm14051736
J. Clin. Med. 2025,14, 1736 2 of 13
to pathophysiological changes [
8
,
11
]. In recent years, a number of potential causes of
nociceptor sensitization have been identified: the loss of elasticity and lubricity [
6
], micro-
injuries [
12
], swelling due to overuse [
13
], and even indications that psychosocial factors
may considerably reduce the fascial ability to shear or its stiffness [14–16].
In view of these TLF-LBP relationships, ultrasound (US) methods have been developed
to quantify the sliding and deformation properties of the TLF [
6
,
17
–
20
]. Two generally
different approaches have been developed for these methods. One is the use of US mea-
surements for landmarks of prominent anatomical structures [
19
,
21
], and the other is the
use of automated methods based on speckle tracking and cross-correlation functions [
6
,
17
].
We have recently introduced a new, landmark-based easy-to-use method for diagnosis and
treatment control in daily practice. Its intra- and inter-rater reliability in quantifying TLF
deformation (TLFD) was therefore demonstrated in a comprehensive study [
21
]. For a
detailed theoretical framework of the method, its intra- and inter-rater reliability, and its
clinical relevance, please see Brandl et al. [
21
]. However, to date, it is neither clear that the
absolute distance value of the TLFD US measurement (TLFD_US) corresponds to the real
distance nor in which tissue layers (dermis and erector spinae muscle) the deformation
occurs and in what proportion.
This work follows on from our previous reliability study, in which we described in
more detail the rationale for a new ultrasound measurement method for assessing TLF
properties and outlined its relevance for clinical use and further practical application. The
aim of the present study was to investigate the criterion validity of the TLFD_US method
in terms of a comparison between the measurements within the ultrasound images and
real-world data. Therefore, the concurrent validity of the ground truth data
in vitro
and
the speckle tracking analysis
in vivo
should be evaluated. In addition, the sliding of the
TLF in other tissue layers should be determined as a secondary objective with the speckle
tracking analysis that defines the TLFD.
2. Materials and Methods
The work presented here was a criterion validity study and part of a larger project
investigating the neuromotor associations of the TLF. The project was prospectively reg-
istered with the German Registry for Clinical Trials (DRKS00027074) and was reviewed
and approved for
in vivo
validation with human participants by the ethics committee of
the Diploma University of Applied Science (No. 1014/2021). This study was conducted in
accordance with the Declaration of Helsinki and written informed consent was obtained
from the participants.
2.1. Ultrasound Measurement of the Thoracolumbar Fascia Deformation
The TLFD_US using the junction between the latissimus dorsi muscle and the TLF as
an anatomical landmark has already been described by Brandl et al. [
21
–
23
]. A detailed
description of the ultrasound examination protocol can be found at https://dx.doi.org/10
.17504/protocols.io.eq2lyjbmwlx9/v1, accessed on 4 February 2025.
Briefly, each participant sat on a treatment table and flexed their trunk to an approxi-
mate flexion angle of 60 degrees (starting position) as determined by the examiner, and here,
a static US image was acquired (Clarius L15 HD3, 5–15 MHz Linear Transducer, Vancouver,
BC, Canada; Figure 1A,B). Participants then extended their trunk to 0 degrees, which was
the ending position of the trunk extension task (TET), and a static, secondary US image
was acquired again (Figure 1C,D). The measurement was the distance between the junction
of the latissimus dorsi muscle with the TLF and an artificial reference created by reflective
tape on the skin shown in the images (Figure 1B,D). The difference between the distances
of the starting and the ending position represents the deformation of the TLF (Figure 1B,D).
J. Clin. Med. 2025,14, 1736 3 of 13
J. Clin. Med. 2025, 14, 1736 3 of 14
the distances of the starting and the ending position represents the deformation of the TLF
(Figure 1B,D).
Figure 1. The measurement procedure. (A) The flexion phase of the trunk. (B) The measurement
time point at the starting position. (C) The fully extended position of the trunk extension task. (D)
The measurement time point at the ending position. The left white cross on the measurement line
marks the LD/TLF junction. The white cross on the right marks the center of the artificial reference
created by reflective tape on the skin. TLF, thoracolumbar fascia; LD, latissimus dorsi muscle.
In a comprehensive recent reliability study by our research group, intra-rater relia-
bility was rated as excellent, with an ICC of 0.92 and a minimal detectable change of 5.54
mm (p < 0.001). The inter-rater reliability was slightly worse, with a good ICC of 0.78 and
a minimal detectable change of 8.70 mm (p < 0.001). Furthermore, a cut-off point of 6 mm
was determined to differentiate LBP patients from healthy individuals, meaning that a
TLFD of less than 6 mm identified 100% of LBP patients (sensitivity) and more than 6 mm
identified 93.75% of healthy individuals (specificity) [21].
2.2. In Vitro Concurrent Validity with Ground Truth
To generate ground truth data for the subsequent validation of the TLFD_US, a cus-
tom-built tissue sliding device was constructed. A polyurethane gel pad that mimics the
erector spinae muscle is aached to a plateau mounted on a linear actuator (SFU1605-100
mm, Shandong Sair Mechanical Guide Co., Ltd., Gaotang, China) and driven by a high-
precision stepper motor (NEMA17, ACT Motor GmbH, Bremen, Germany). This
Figure 1. The measurement procedure. (A) The flexion phase of the trunk. (B) The measurement time
point at the starting position. (C) The fully extended position of the trunk extension task.
(D) The
measurement time point at the ending position. The left white cross on the measurement line marks
the LD/TLF junction. The white cross on the right marks the center of the artificial reference created
by reflective tape on the skin. TLF, thoracolumbar fascia; LD, latissimus dorsi muscle.
In a comprehensive recent reliability study by our research group, intra-rater reliability
was rated as excellent, with an ICC of 0.92 and a minimal detectable change of 5.54 mm
(p< 0.001). The inter-rater reliability was slightly worse, with a good ICC of 0.78 and a
minimal detectable change of 8.70 mm (p< 0.001). Furthermore, a cut-off point of 6 mm
was determined to differentiate LBP patients from healthy individuals, meaning that a
TLFD of less than 6 mm identified 100% of LBP patients (sensitivity) and more than 6 mm
identified 93.75% of healthy individuals (specificity) [21].
2.2. In Vitro Concurrent Validity with Ground Truth
To generate ground truth data for the subsequent validation of the TLFD_US, a custom-
built tissue sliding device was constructed. A polyurethane gel pad that mimics the erector
spinae muscle is attached to a plateau mounted on a linear actuator (SFU1605-100 mm,
Shandong Sair Mechanical Guide Co., Ltd., Gaotang, China) and driven by a high-precision
stepper motor (NEMA17, ACT Motor GmbH, Bremen, Germany). This configuration
allows the gel pad to move laterally under another gel pad that mimics the skin and
SAT (Figure 2).
J. Clin. Med. 2025,14, 1736 4 of 13
J. Clin. Med. 2025, 14, 1736 4 of 14
configuration allows the gel pad to move laterally under another gel pad that mimics the
skin and SAT (Figure 2).
The gel pads (21 cm × 31 cm; Technogel GmbH, Berlingrode, Germany), which rep-
resent a two-layer phantom model of the lumbar tissues with its typical thickness and
stiffness, were manufactured on the basis of properties described in the literature. For a
detailed description of the development, properties and application, see Bartsch et al. [24]
and Brandl et al. [25].
Figure 2. The tissue sliding device. The device consists of a computer-controlled linear drive with a
ball screw on which the imitation erector spinae gel pad is connected to the overlying layers in a
freely movable manner. The ultrasound transducer, which is aligned at right angles using a spirit
level, is permanently installed with a holder. The transducer and each gel pad are separated by
ultrasound gel. A detailed device description with additional video material and the generation of
ground truth data is publicly available in the Zenodo repository: hps://zenodo.org/rec-
ords/11402043, accessed on 4 February 2025.
Between the gel pads, ultrasound gel (DocCheck Ultrasound Gel, Gello GmbH Gel-
technik, Ahaus-Wüllen, Germany) for reducing friction and enabling smooth gliding was
aached. At each gel pad, a strain gauge sensor (ZD10-100, Shenzhen Lanqi Technology
Co., Ltd., Shenzhen, China) was aached to measure any shear deformation during their
relative movement. Equipped with a digital caliper, the stepper motor maintained con-
stant speeds with a positioning accuracy of ± 1 µm. This ensured the reliable documenta-
tion of the displacement of each gel pad relative to the other. No shear deformation was
detected within the gel pads when determining the ground truth data (<0.1 mm). The
measurement uncertainty for the absolute distance between the gel pads was ±2 µm, k =
2 (95% confidence interval), in accordance with the “Guide to the expression of uncer-
tainty in measurement” [26].
With this seing, 36 distances between 3 and 20 mm with a constant shear rate of 3.15
mm/s were recorded. In addition, 10 different velocities from 2.41 mm/s to 5.36 mm/s were
recorded for a distance of 20 mm. These distances and speeds correspond to the typical
TLFD and trunk extension velocities of 200 LBP patients and healthy individuals that we
previously assessed [27]. The complete ground truth dataset and epidemiologic data as
well as a detailed description of the device including the design description are available
at hps://zenodo.org/records/11402043, accessed on 4 February 2025.
Figure 2. The tissue sliding device. The device consists of a computer-controlled linear drive with a
ball screw on which the imitation erector spinae gel pad is connected to the overlying layers in a freely
movable manner. The ultrasound transducer, which is aligned at right angles using a spirit level, is
permanently installed with a holder. The transducer and each gel pad are separated by ultrasound
gel. A detailed device description with additional video material and the generation of ground truth
data is publicly available in the Zenodo repository: https://zenodo.org/records/11402043, accessed
on 4 February 2025.
The gel pads (21 cm
×
31 cm; Technogel GmbH, Berlingrode, Germany), which
represent a two-layer phantom model of the lumbar tissues with its typical thickness and
stiffness, were manufactured on the basis of properties described in the literature. For a
detailed description of the development, properties and application, see Bartsch et al. [
24
]
and Brandl et al. [25].
Between the gel pads, ultrasound gel (DocCheck Ultrasound Gel, Gello GmbH Gel-
technik, Ahaus-Wüllen, Germany) for reducing friction and enabling smooth gliding was
attached. At each gel pad, a strain gauge sensor (ZD10-100, Shenzhen Lanqi Technology
Co., Ltd., Shenzhen, China) was attached to measure any shear deformation during their
relative movement. Equipped with a digital caliper, the stepper motor maintained constant
speeds with a positioning accuracy of
±
1
µ
m. This ensured the reliable documentation of
the displacement of each gel pad relative to the other. No shear deformation was detected
within the gel pads when determining the ground truth data (<0.1 mm). The measure-
ment uncertainty for the absolute distance between the gel pads was
±
2
µ
m, k = 2 (95%
confidence interval), in accordance with the “Guide to the expression of uncertainty in
measurement” [26].
With this setting, 36 distances between 3 and 20 mm with a constant shear rate of
3.15 mm/s
were recorded. In addition, 10 different velocities from 2.41 mm/s to 5.36 mm/s
were recorded for a distance of 20 mm. These distances and speeds correspond to the
typical TLFD and trunk extension velocities of 200 LBP patients and healthy individuals
that we previously assessed [
27
]. The complete ground truth dataset and epidemiologic
data as well as a detailed description of the device including the design description are
available at https://zenodo.org/records/11402043, accessed on 4 February 2025.
The TLFD_US of the distances corresponding to each ground truth was performed
with a reference marker in each phantom tissue layer clearly visible in the US image
(Figure 3). Images were acquired simultaneously with the generation of ground truth data
for all 36 distances and 10 velocities using the tissue sliding device with an attached US
J. Clin. Med. 2025,14, 1736 5 of 13
transducer by a blinded examiner with a total of 10 years of practice in US examinations.
Subsequently, the measured distances were compared with the respective real value by an
evaluator who was also blinded.
J. Clin. Med. 2025, 14, 1736 5 of 14
The TLFD_US of the distances corresponding to each ground truth was performed
with a reference marker in each phantom tissue layer clearly visible in the US image (Fig-
ure 3). Images were acquired simultaneously with the generation of ground truth data for
all 36 distances and 10 velocities using the tissue sliding device with an aached US trans-
ducer by a blinded examiner with a total of 10 years of practice in US examinations. Sub-
sequently, the measured distances were compared with the respective real value by an
evaluator who was also blinded.
Figure 3. Two-layer phantom tissue ultrasound imaging. Skin, SAT, TLF: The first gel pad, which
was aached to the tissue sliding device, represents the cutis, the subcutaneous adipose tissue and
the thoracolumbar fascia. The second gel pad represents the erector spinae muscle with its marker
displaced laterally by the linear actuator (yellow dashed circle). The other marker served as a refer-
ence (yellow dashed line). A detailed description of the imaging procedure with additional video
material and the generation of ground truth data is publicly available in the Zenodo repository:
hps://zenodo.org/records/11402043, accessed on 4 February 2025.
2.3. In Vivo Concurrent Validity with Speckle Tracking Analysis
To assess the concurrent validity of the manual TLFD_US and the automated speckle
tracking analysis commonly used in research, 10 aLBP patients and 10 healthy individuals
were studied. The reliability of speckle tracking methods for fascial shearing was previ-
ously tested by Langevin et al. (ICC = 0.98) [6] and Tomita et al. (ICC = 0.95) [17] for the
TLF and by Ellis for the tibial nerve (ICC = 0.75) [28].
We calculated the sample size of 20 participants based on the least acceptable corre-
lation to define concurrent validity between the methods of r = 0.60 [29], a significance
level of 0.05, and a statistical power of 0.80 [30]. Inclusion criteria for the aLBP group ac-
cording to the European guidelines for the treatment of aLBP (<6-week pain duration)
were a visual analog scale score higher than 3 and an Oswestry disability score higher
than 10. Healthy individuals had no pain episodes or physician visits for LBP in the last 5
years. Exclusion criteria were age under 18 or over 60 years, operations or scars in the area
of the TLF, skin changes (e.g., urticaria or neurodermatitis), medications that affect blood
circulation or act as muscle relaxants, and rheumatic diseases.
The investigator, who was blinded to the group membership, measured the TLFD
sonographically from each participant as described in Section 2.1. A US video of the entire
trunk extension task was then recorded from the TLF (for more details on this procedure,
see Brandl et al. [23]). The videos were then tracked in post-analysis using Kinovea (ver-
sion 0.9.5; Kinovea open source project, www.kinovea.org). The tracking pixels from the
videos were tested for several scenarios, and Kinovea was recommended as a valid and
Figure 3. Two-layer phantom tissue ultrasound imaging. Skin, SAT, TLF: The first gel pad, which
was attached to the tissue sliding device, represents the cutis, the subcutaneous adipose tissue and
the thoracolumbar fascia. The second gel pad represents the erector spinae muscle with its marker
displaced laterally by the linear actuator (yellow dashed circle). The other marker served as a
reference (yellow dashed line). A detailed description of the imaging procedure with additional
video material and the generation of ground truth data is publicly available in the Zenodo repository:
https://zenodo.org/records/11402043, accessed on 4 February 2025.
2.3. In Vivo Concurrent Validity with Speckle Tracking Analysis
To assess the concurrent validity of the manual TLFD_US and the automated speckle
tracking analysis commonly used in research, 10 aLBP patients and 10 healthy individuals
were studied. The reliability of speckle tracking methods for fascial shearing was previously
tested by Langevin et al. (ICC = 0.98) [
6
] and Tomita et al. (ICC = 0.95) [
17
] for the TLF and
by Ellis for the tibial nerve (ICC = 0.75) [28].
We calculated the sample size of 20 participants based on the least acceptable correla-
tion to define concurrent validity between the methods of r = 0.60 [
29
], a significance level
of 0.05, and a statistical power of 0.80 [
30
]. Inclusion criteria for the aLBP group according
to the European guidelines for the treatment of aLBP (<6-week pain duration) were a visual
analog scale score higher than 3 and an Oswestry disability score higher than 10. Healthy
individuals had no pain episodes or physician visits for LBP in the last 5 years. Exclusion
criteria were age under 18 or over 60 years, operations or scars in the area of the TLF, skin
changes (e.g., urticaria or neurodermatitis), medications that affect blood circulation or act
as muscle relaxants, and rheumatic diseases.
The investigator, who was blinded to the group membership, measured the TLFD
sonographically from each participant as described in Section 2.1. A US video of the entire
trunk extension task was then recorded from the TLF (for more details on this procedure,
see Brandl et al. [
23
]). The videos were then tracked in post-analysis using Kinovea
(
version 0.9.5;
Kinovea open source project, www.kinovea.org). The tracking pixels from
the videos were tested for several scenarios, and Kinovea was recommended as a valid
and reliable tool [
31
,
32
]. The individual speckle tracking of pixels was applied within a
4×7 mm
rectangle representing the region of interest as described by
Rodriguez et al. [33].
The analysis involved the determination of lateral tissue displacement from the starting to
the ending position (Figure 1). The measured pixels were converted to millimeters using
the scale bar in the US video and the Kinovea calibration function [
31
]. The procedure was
J. Clin. Med. 2025,14, 1736 6 of 13
performed simultaneously by the program for the different lumbar tissue layers (dermis,
SAT, TLF, and erector spinae) and the artificial reference in the US video that we had
previously defined (Figure 4).
J. Clin. Med. 2025, 14, 1736 6 of 14
reliable tool [31,32]. The individual speckle tracking of pixels was applied within a 4 × 7
mm rectangle representing the region of interest as described by Rodriguez et al. [33]. The
analysis involved the determination of lateral tissue displacement from the starting to the
ending position (Figure 1). The measured pixels were converted to millimeters using the
scale bar in the US video and the Kinovea calibration function [31]. The procedure was
performed simultaneously by the program for the different lumbar tissue layers (dermis,
SAT, TLF, and erector spinae) and the artificial reference in the US video that we had
previously defined (Figure 4).
Figure 4. The speckle tracking analysis of lumbar tissue. The respective pixels of the regions of in-
terest (4 × 7 mm rectangles) are tracked in an ultrasound video showing the trunk extension task.
The displacement of DER, TLF, and ES from the Ref was post-analyzed using Kinovea software,
version 0.9.5. Ref, artificial reference of reflective tape on the skin; DER, dermis; TLF, thoracolumbar
fascia; ES, erector spinae muscle; TLFD, deformation of the TLF.
2.4. Statistical Analysis
All data met the criteria for parametric testing. Descriptive statistics are reported as
the mean, standard deviation (SD), minimum, maximum and 95% confidence interval
(95% CI).
The intraclass correlation coefficient (ICC) between the in vitro TLFD_US and the
distance ground truth was calculated using a 2-way random effects model (absolute agree-
ment, ICC(2,k), multiple raters, k = 2). The resulting values from the ICC calculation were
categorized as “poor” (<0.50), “moderate” (0.50 to 0.75), “good” (0.75 to 0.90), and “excel-
lent” (>0.90) according to Koo and Li [34]. A Bland–Altman plot was created to provide
further visual information on the limits of agreement between the manual TLFD_US and
the real values. The agreement between TLFD_US and the speed ground truth was pre-
sented in a Bland–Altman difference quantile–quantile plot (Q-Q) in addition to the de-
scriptive evaluation.
The Pearson’s product moment correlation coefficient was calculated between in vivo
TLFD_US and speckle tracking analysis and interpreted according to Cohen [35] as
”small” (0.1 to 0.3), “medium” (0.3 to 0.5) or “large” (0.5 to 1.0) correlations.
A multiple linear regression model with the speckle tracking analysis of TLF as the
dependent variable and dermis as well as the erector spinae muscle sliding as independ-
ent variables was performed to predict TLFD adhesion properties to adjacent tissue layers.
Effect sizes (adjusted R2) were interpreted according to Cohen as “small” (0.01–0.08), “me-
dium” (0.09–0.24), and “large” (>0.25) [35].
The significance level was set at p = 0.05. Analyses were performed using Jamovi 2.3
(The jamovi project, hps://www.jamovi.org, accessed on 4 February 2025).
Figure 4. The speckle tracking analysis of lumbar tissue. The respective pixels of the regions of
interest (4
×
7 mm rectangles) are tracked in an ultrasound video showing the trunk extension task.
The displacement of DER, TLF, and ES from the Ref was post-analyzed using Kinovea software,
version 0.9.5. Ref, artificial reference of reflective tape on the skin; DER, dermis; TLF, thoracolumbar
fascia; ES, erector spinae muscle; TLFD, deformation of the TLF.
2.4. Statistical Analysis
All data met the criteria for parametric testing. Descriptive statistics are reported as the
mean, standard deviation (SD), minimum, maximum and 95%
confidence interval (95% CI).
The intraclass correlation coefficient (ICC) between the
in vitro
TLFD_US and the
distance ground truth was calculated using a 2-way random effects model (absolute agree-
ment, ICC
(2,k)
, multiple raters, k = 2). The resulting values from the ICC calculation
were categorized as “poor” (<0.50), “moderate” (0.50 to 0.75), “good” (0.75 to 0.90), and
“excellent” (>0.90) according to Koo and Li [
34
]. A Bland–Altman plot was created to pro-
vide further visual information on the limits of agreement between the manual TLFD_US
and the real values. The agreement between TLFD_US and the speed ground truth was
presented in a Bland–Altman difference quantile–quantile plot (Q-Q) in addition to the
descriptive evaluation.
The Pearson’s product moment correlation coefficient was calculated between
in vivo
TLFD_US and speckle tracking analysis and interpreted according to Cohen [
35
] as ”small”
(0.1 to 0.3), “medium” (0.3 to 0.5) or “large” (0.5 to 1.0) correlations.
A multiple linear regression model with the speckle tracking analysis of TLF as the
dependent variable and dermis as well as the erector spinae muscle sliding as independent
variables was performed to predict TLFD adhesion properties to adjacent tissue layers.
Effect sizes (adjusted R
2
) were interpreted according to Cohen as “small” (0.01–0.08),
“medium” (0.09–0.24), and “large” (>0.25) [35].
The significance level was set at p= 0.05. Analyses were performed using Jamovi 2.3
(The jamovi project, https://www.jamovi.org, accessed on 4 February 2025).
3. Results
A total of 36 images for distance and 10 images for velocity were derived and analyzed
to verify the concurrent validity of TLFD_US and ground truth
in vitro
. For
in vivo
valida-
tion, the concurrent validity of TLFD_US and speckle tracking analysis was performed in
20 participants.
J. Clin. Med. 2025,14, 1736 7 of 13
3.1. In Vitro Concurrent Validity with Ground Truth
An excellent degree of agreement was found between TLFD_US and distance ground
truth (ICC(2,2) = 0.99, 95% CI [0.998, 0.999], F(35.0, 36.0) = 1846, p< 0.001).
The Bland–Altman diagram showed that almost all points were within the small limits
of agreement. The mean difference was less than 0.05 mm, indicating that there were no
systematic errors. Outliers exceeding the limits of agreement were only observed at the
absolute end of the measurement range below 3.7 and above 19.6 mm (Figure 5).
J. Clin. Med. 2025, 14, 1736 7 of 14
3. Results
A total of 36 images for distance and 10 images for velocity were derived and ana-
lyzed to verify the concurrent validity of TLFD_US and ground truth in vitro. For in vivo
validation, the concurrent validity of TLFD_US and speckle tracking analysis was per-
formed in 20 participants.
3.1. In Vitro Concurrent Validity with Ground Truth
An excellent degree of agreement was found between TLFD_US and distance ground
truth (ICC(2,2) = 0.99, 95% CI [0.998, 0.999], F(35.0, 36.0) = 1846, p < 0.001).
The Bland–Altman diagram showed that almost all points were within the small lim-
its of agreement. The mean difference was less than 0.05 mm, indicating that there were
no systematic errors. Outliers exceeding the limits of agreement were only observed at the
absolute end of the measurement range below 3.7 and above 19.6 mm (Figure 5).
Figure 5. The Bland–Altman plot of the differences between TLFD_US and distance ground truth.
TLFD_US, ultrasound measurement of the deformation of the thoracolumbar fascia.
The absolute mean difference between TLFD_US and the speed ground truth was
0.215 mm, 95% CI [−0.410, −0.035]. As shown in the Q–Q plot, sample quantiles follow the
theoretical quantiles and only deviate in a small range (Figure 6). Descriptive data for this
analysis are shown in Table 1.
Table 1. Descriptive data of TLFD_US measurements and speed ground truth.
95% CI
Mean Lower Upper SD Min Max
Ground truth (mm) 20.0 20.0 20.0 0.006 20.0 20.0
TLFD_US (mm) 20.2 20.0 20.4 0.246 19.9 20.5
TLFD_US, ultrasound measurement of the deformation of the thoracolumbar fascia; SD, standard
deviation; CI, confidence interval; Min, minimum; Max, maximum.
Figure 5. The Bland–Altman plot of the differences between TLFD_US and distance ground truth.
TLFD_US, ultrasound measurement of the deformation of the thoracolumbar fascia.
The absolute mean difference between TLFD_US and the speed ground truth was
0.215 mm, 95% CI [
−
0.410,
−
0.035]. As shown in the Q–Q plot, sample quantiles follow
the theoretical quantiles and only deviate in a small range (Figure 6). Descriptive data for
this analysis are shown in Table 1.
J. Clin. Med. 2025, 14, 1736 8 of 14
Figure 6. A quantile–quantile plot of the differences between TLFD_US and speed ground truth.
3.2. In Vivo Concurrent Validity with Speckle Tracking Analysis
A total of 20 participants were analyzed for the in vivo validation between TLFD_US
and the speckle tracking analysis. The sample characteristics are listed in Table 2.
Table 2. Sample characteristics.
95% CI
Group N Mean Lower Uppe
r
SD
Sex (woman/men) Healthy 10 6/4
aLBP 10 4–6 month
Age (years) Healthy 10 38.97 28.21 49.73 15.04
aLBP 10 43.55 32.18 54.93 15.89
Weight (kg) Healthy 10 66.04 54.92 77.16 15.55
aLBP 10 75.52 66.31 84.73 12.87
Height (m) Healthy 10 1.67 1.59 1.76 0.12
aLBP 10 1.72 1.67 1.78 0.07
BMI Healthy 10 23.48 20.29 26.67 4.45
aLBP 10 25.57 22.35 28.79 4.50
VAS (mm) aLBP 10 5.27 3.50 7.04 2.47
ODQ (0–100) aLBP 10 49.40 37.35 61.45 16.84
Pain duration (days) aLBP 10 9.10 6.44 11.76 3.72
aLBP, acute low back pain patients; Healthy, healthy individuals; BMI, body mass index; ODQ,
Oswestry Disability Questionnaire.
Pearson’s product moment correlation coefficient was large between the TLFD_US
and speckle tracking analysis (r(18) = 0.701; p < 0.001).
3.3. Thoracolumbar Fascia Deformation in Relation to Other Tissue Layers
The overall linear model fit was F(2,17) = 6.19, p = 0.01, adjusted R2 = 0.353. The model
explained 35% of the TLFD variability, emphasizing the significant influence of the in-
cluded predictors and a large effect. Dermis sliding as a predictor of TLFD showed a sig-
nificant effect (B = 0.780, 95% CI [0.15, 0.93]; standardized B = 0.541, 95% CI [0.22, 1.34]; p
= 0.009). In contrast, erector spinae muscle sliding as a predictor of TLFD showed no sig-
nificant effect (B = 0.386, 95% CI [−0.04, 0.81]; standardized B = 0.356, 95% CI [−0.03, 0.75];
p = 0.071).
Figure 6. A quantile–quantile plot of the differences between TLFD_US and speed ground truth.
J. Clin. Med. 2025,14, 1736 8 of 13
Table 1. Descriptive data of TLFD_US measurements and speed ground truth.
95% CI
Mean Lower Upper SD Min Max
Ground truth (mm) 20.0 20.0 20.0 0.006 20.0 20.0
TLFD_US (mm) 20.2 20.0 20.4 0.246 19.9 20.5
TLFD_US, ultrasound measurement of the deformation of the thoracolumbar fascia; SD, standard deviation;
CI, confidence interval; Min, minimum; Max, maximum.
3.2. In Vivo Concurrent Validity with Speckle Tracking Analysis
A total of 20 participants were analyzed for the
in vivo
validation between TLFD_US
and the speckle tracking analysis. The sample characteristics are listed in Table 2.
Table 2. Sample characteristics.
95% CI
Group N Mean Lower Upper SD
Sex (woman/men) Healthy 10 6/4
aLBP 10 4–6
month
Age (years) Healthy 10 38.97 28.21 49.73 15.04
aLBP 10 43.55 32.18 54.93 15.89
Weight (kg) Healthy 10 66.04 54.92 77.16 15.55
aLBP 10 75.52 66.31 84.73 12.87
Height (m) Healthy 10 1.67 1.59 1.76 0.12
aLBP 10 1.72 1.67 1.78 0.07
BMI Healthy 10 23.48 20.29 26.67 4.45
aLBP 10 25.57 22.35 28.79 4.50
VAS (mm) aLBP 10 5.27 3.50 7.04 2.47
ODQ (0–100) aLBP 10 49.40 37.35 61.45 16.84
Pain duration (days) aLBP 10 9.10 6.44 11.76 3.72
aLBP, acute low back pain patients; Healthy, healthy individuals; BMI, body mass index; ODQ, Oswestry
Disability Questionnaire.
Pearson’s product moment correlation coefficient was large between the TLFD_US
and speckle tracking analysis (r(18) = 0.701; p< 0.001).
3.3. Thoracolumbar Fascia Deformation in Relation to Other Tissue Layers
The overall linear model fit was F
(2,17)
= 6.19, p= 0.01, adjusted R
2
= 0.353. The model
explained 35% of the TLFD variability, emphasizing the significant influence of the included
predictors and a large effect. Dermis sliding as a predictor of TLFD showed a significant
effect (B = 0.780, 95% CI [0.15, 0.93]; standardized B = 0.541, 95% CI [0.22, 1.34]; p= 0.009). In
contrast, erector spinae muscle sliding as a predictor of TLFD showed no significant effect
(B = 0.386, 95% CI [−0.04, 0.81]; standardized B = 0.356, 95% CI [−0.03, 0.75]; p= 0.071).
4. Discussion
This is, to our knowledge, the first validation study to examine a US method for
quantifying the sliding and deformation properties of the TLF. Given the growing evidence
of a link between TLFD and LBP and the global economic burden of LBP, an
in vitro
validation was performed to determine whether the US measurements of TLFD correspond
to real-world values. In addition, the method was validated
in vivo
in concurrence with
speckle tracking analysis, which is commonly used in laboratory settings. A post-analysis
of speckles from US videos was then used to deduce in which tissue layer surrounding the
TLF (dermis and erector spinae muscle) did deformation occur and in what proportion.
J. Clin. Med. 2025,14, 1736 9 of 13
4.1. In Vitro Concurrent Validity with Ground Truth
The degree of agreement between TLFD_US and the distance truth was excellent and,
apart from a small area at the end of the measurement range, the US values matched the
real values exactly. The TLFD_US is based on a trunk extension task performed by the
participant. For ease of use in daily practice of this protocol, the speed at which the task
is performed is only given in a rough range. Therefore, the accuracy of the method at
different speeds was also investigated. The absolute mean difference was small and within
a low 95% CI. To summarize, the TLFD_US has proven its high precision in an
in vitro
environment, and it can be assumed that the measured values represent the actual values.
Many authors have performed calculations of the physical properties of the TLF based
on US analyses [
6
,
17
–
19
]. However, there is an urgent need to ensure that the calculations
are based on valid data. This work therefore provides a trustworthy basis. In the course of
developing the
in vitro
procedure, we have published the entire ground truth evaluation
with the tissue sliding device in an open repository (https://zenodo.org/records/11402043,
accessed on 4 February 2025). Work is currently underway with these data to validate
certain speckle tracking analyses [
27
], and other authors are also invited to evaluate their
various US methods of TLF against ground truth as well.
4.2. In Vivo Concurrent Validity with Speckle Tracking Analysis
TLFD_US and speckle tracking analysis showed large correlations. This analysis
was performed
in vivo
with symptomatic aLBP patients and healthy individuals to reflect
the population commonly seen in a physician’s office. Speckle tracking analysis using
Kinovea, version 0.9.5, software in a recent study showed a high correlation of r = 0.97
with a minimal detectable change of 0.21 mm with the ground truth data, demonstrating
similar concurrent
in vitro
validity [
36
]. The large
in vivo
relation of r = 0.70 between
the two different methods proves the reliable use of the TLFD_US as a more practical
and time-efficient measurement method for use in daily practice, which is well above
the limit of the recommended minimum percentage agreement of 60% for diagnostic
procedures in manual and musculoskeletal medicine [
29
,
37
]. In recent years, attempts
have been made to characterize TLF properties using simple methods such as compressive
stiffness (e.g., ultrasound elastography, myotonometry or intendometry) or TLF thickness
measurements [
38
,
39
]. However, compressive stiffness is unlikely to detect stiffness changes
in the thin TLF [
24
,
25
] and the measurement of TLF thickness is significantly influenced
by the observer [
40
]. Furthermore, these measurements only consider one dimension in
space, which gives TLFD_US an advantage as an additional method (a two-dimensional
measurement of tissue sliding in an XY Cartesian coordinate system) for diagnosing and
monitoring LBP [21].
4.3. Thoracolumbar Fascia Deformation in Relation to Other Tissue Layers
We performed multiple linear regression to determine which tissue layers the TLF
deforms
in vivo
and to what extent. The overall model fit showed a large effect size in this
regard, meaning that the included components of the dermis and erector spinae muscle
significantly influenced TLFD. The linear model showed that mainly the dermis influenced
TLFD. In total, 70% of the dermis sliding was transferred to the TLF. The 39% proportion
of the erector spinae muscle was not significant in the modal but showed a trend with a
p-value of 0.07.
Previous studies have primarily looked at the relationship between the TLF and the
erector spinae muscle [
6
,
23
], just as most research on fascia has tended to focus on deeper
layers of tissue [
41
]. In recent years, the role of the overlying layers of the TLF, particularly
the superficial fascia, also known as Scarpa’s fascia, has been studied in more detail,
J. Clin. Med. 2025,14, 1736 10 of 13
and there is increasing evidence of the importance of these structures for the mechanical
influence on the TLF [
41
,
42
]. Our study results are consistent with the observation of van
Amstel et al. [
43
] on the direct effect of skin displacement on the range of motion of the
spine, pelvis and hip. Wilke et al. [
44
] have demonstrated force transmission between
muscle and superficial fascia. This study provides an initial indication that the tissue
layers overlying the TLF may have a greater influence on TLFD and lumbar mobility than
previously considered.
Pirri et al. [
19
] and Willard et al. [
10
] have spoken of a kind of “frozen back” in
LBP pathologies. Tomita et al. [
17
] calculated the internal shear strain of the TLF based
on a Lagrangian approach to solid mechanics and found greater stress within the TLF
in LBP patients. This sounds plausible considering that a decrease in sliding against
the surrounding tissue must increase the shear strain within the TLF during movement
(Figure 7). This could have inter-related pathological effects of various tissue overloads that
lead to angiogenesis failure, hypoxia, inflammation, fibrosis, the excitation of nociceptors
and others [
9
,
45
]. Our results of the influence of dermis on TLFD and, although not
significant, a trend of the erector spinae muscle could confirm this hypothesis. However,
due to the small sample size, no sub-analysis could be performed to separate the results
of symptomatic and healthy participants. Further research could take this into account
and use the effect sizes from our validity study in addition to the earlier study on intra-
and inter-rater reliability to conduct more comprehensive follow-up work. It would be
of particular interest to investigate the contribution of each tissue layer to TLFD, which
should also include a heterogeneous group and appropriate subgroup analysis.
J. Clin. Med. 2025, 14, 1736 10 of 14
4.3. Thoracolumbar Fascia Deformation in Relation to Other Tissue Layers
We performed multiple linear regression to determine which tissue layers the TLF
deforms in vivo and to what extent. The overall model fit showed a large effect size in this
regard, meaning that the included components of the dermis and erector spinae muscle
significantly influenced TLFD. The linear model showed that mainly the dermis influ-
enced TLFD. In total, 70% of the dermis sliding was transferred to the TLF. The 39% pro-
portion of the erector spinae muscle was not significant in the modal but showed a trend
with a p-value of 0.07.
Previous studies have primarily looked at the relationship between the TLF and the
erector spinae muscle [6,23], just as most research on fascia has tended to focus on deeper
layers of tissue [41]. In recent years, the role of the overlying layers of the TLF, particularly
the superficial fascia, also known as Scarpa’s fascia, has been studied in more detail, and
there is increasing evidence of the importance of these structures for the mechanical influ-
ence on the TLF [41,42]. Our study results are consistent with the observation of van Am-
stel et al. [43] on the direct effect of skin displacement on the range of motion of the spine,
pelvis and hip. Wilke et al. [44] have demonstrated force transmission between muscle
and superficial fascia. This study provides an initial indication that the tissue layers over-
lying the TLF may have a greater influence on TLFD and lumbar mobility than previously
considered.
Pirri et al. [19] and Willard et al. [10] have spoken of a kind of “frozen back” in LBP
pathologies. Tomita et al. [17] calculated the internal shear strain of the TLF based on a
Lagrangian approach to solid mechanics and found greater stress within the TLF in LBP
patients. This sounds plausible considering that a decrease in sliding against the sur-
rounding tissue must increase the shear strain within the TLF during movement (Figure
7). This could have inter-related pathological effects of various tissue overloads that lead
to angiogenesis failure, hypoxia, inflammation, fibrosis, the excitation of nociceptors and
others [9,45]. Our results of the influence of dermis on TLFD and, although not significant,
a trend of the erector spinae muscle could confirm this hypothesis. However, due to the
small sample size, no sub-analysis could be performed to separate the results of sympto-
matic and healthy participants. Further research could take this into account and use the
effect sizes from our validity study in addition to the earlier study on intra- and inter-rater
reliability to conduct more comprehensive follow-up work. It would be of particular in-
terest to investigate the contribution of each tissue layer to TLFD, which should also in-
clude a heterogeneous group and appropriate subgroup analysis.
Figure 7. A model of thoracolumbar fascia deformation. (A) The extended position of the trunk. (B)
Deformation during trunk flexion in healthy individuals. The dermis and the subcutaneous adipose
tissue (represented as retinacula and superficial fascia) slide in relation to the TLF, which in turn is
relatively fixed by the aached tissues, i.e., the latissimus dorsi muscle, the transversus abdominis
muscle and the spinous processes. The erector spinae muscle can slide relatively freely under the
TLF. In this respect, deformation is possible with the involvement of the surrounding connective
tissue. (C) Deformation during trunk flexion in patients with low back pain. The red asterisks mark
possible densification and/or the loss of liquid, which lead to adhesion zones with reduced gliding
properties. The TLF cannot deform against the over- and underlying tissue layers. The internal shear
stress on the TLF increases. TLF, thoracolumbar fascia.
Figure 7. A model of thoracolumbar fascia deformation. (A) The extended position of the trunk.
(B) Deformation
during trunk flexion in healthy individuals. The dermis and the subcutaneous
adipose tissue (represented as retinacula and superficial fascia) slide in relation to the TLF, which
in turn is relatively fixed by the attached tissues, i.e., the latissimus dorsi muscle, the transversus
abdominis muscle and the spinous processes. The erector spinae muscle can slide relatively freely
under the TLF. In this respect, deformation is possible with the involvement of the surrounding
connective tissue. (C) Deformation during trunk flexion in patients with low back pain. The red
asterisks mark possible densification and/or the loss of liquid, which lead to adhesion zones with
reduced gliding properties. The TLF cannot deform against the over- and underlying tissue layers.
The internal shear stress on the TLF increases. TLF, thoracolumbar fascia.
4.4. Limitations
This study has a number of limitations. First, various internal shear stresses occur
during movement in vital biomaterials. This behavior can hardly be mimicked
in vitro
.
We monitored the material deformation of the gel pad layers of the tissue sliding device
and found no undesired shear stresses in the material. However, the approach is a two-
dimensional simplification of real biomechanics in living subjects. Therefore, we also
investigated its validity against the speckle tracking analysis
in vitro
with real patients and
healthy individuals. The speckle tracking analysis showed strong validity against ground
truth (r = 0.97) and small minimal detectable changes of 0.21 mm in a recent study [
26
]. This
justifies the comparison of both methods in terms of their concurrent validity in this study.
Third, we thoroughly calculated the sample size for the
in vivo
validation with a sufficient
effect size to detect both statistically and clinically significant correlations. However, this
J. Clin. Med. 2025,14, 1736 11 of 13
calculation did not include a subgroup analysis to analyze aLBP patients and healthy
individuals separately. This should be considered in future work.
5. Conclusions
TLFD_US showed excellent
in vitro
validity, and it can be assumed that the measure-
ments also apply to the real-world data. Furthermore, it proved its resistance to variable
tissue velocities with a stable measurement within small deviation ranges.
There was a large correlation between TLFD_US and speckle tracking analysis in an
in vivo
validation involving 10 aLBP patients and 10 healthy subjects. Additionally, speckle
tracking analysis showed that TLFD was primarily caused by dermis sliding, although
erector spinae muscle movement also appeared to have an impact. These results are a
promising starting point for further research and may support the idea that LBP patients
have a “frozen back”.
In addition to its previously established reliability, the TLFD_US has demonstrated its
validity. When screening for LBP patients, the technique can be suggested to capture an
extra morphologic TLF parameter.
Author Contributions: Conceptualization, A.B.; methodology, A.B.; formal analyses, A.B.; investi-
gation, A.B.; writing—original draft preparation, A.B.; writing—review and editing, A.B. and R.S.;
visualization, A.B.; supervision, R.S.; project administration, A.B. and R.S. All authors have read and
agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: The study was conducted according to the guidelines of the
Declaration of Helsinki and approved by the Ethics Committee of the Diploma Hochschule, Germany
(Nr. 1014/2021, 27 October 2021).
Informed Consent Statement: Informed consent was obtained from all subjects involved in
the study.
Data Availability Statement: Data can be made available by the author upon request.
Conflicts of Interest: The authors declare no conflicts of interest.
Abbreviations
The following abbreviations are used in this manuscript:
aLBP Acute low back pain
LBP Low back pain
TLF Thoracolumbar fascia
TLFD Deformation of the thoracolumbar fascia
TLFD_US Ultrasound method to measure the deformation of the thoracolumbar fascia
US Ultrasound
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