Clinical Biomechanics 49 (2017) 128–133

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Clinical Biomechanics
journal homepage: www.elsevier.com/locate/clinbiomech

Association of low back pain with muscle stiﬀness and muscle mass of thelumbar back muscles, and sagittal spinal alignment in young and middleaged medical workers

MARK

Mitsuhiro Masakia,b,⁎, Tomoki Aoyamac, Takashi Murakamid, Ko Yanasec, Xiang Jic,
Hiroshige Tateuchic, Noriaki IchihashicaDepartment of Physical Therapy, Niigata University of Health and Welfare, 1398 Shimami-cho, Kita-ku, Niigata 950-3198, Japan
Institute for Human Movement and Medical Sciences, Niigata University of Health and Welfare, 1398 Shimami-cho, Kita-ku, Niigata 950-3198, Japan
Department of Physical Therapy, Human Health Sciences, Graduate School of Medicine, Kyoto University, 53 Kawahara-cho, Shogoin, Sakyo-ku, Kyoto 606-8507,
Japand
Department of Rehabilitation, Kyoto Hakuaikai Hospital, 61 Kawaguchi-Bessho, Yawata 614-8114, Japanbc

A R T I C L E I N F O

A B S T R A C T

Keywords:
Low back pain
Paraspinal muscles
Muscle stiﬀness
Muscle thickness
PostureUltrasonography

Background: Muscle stiﬀness of the lumbar back muscles in low back pain (LBP) patients has not been clearly
elucidated because quantitative assessment of the stiﬀness of individual muscles was conventionally diﬃcult.
This study aimed to examine the association of LBP with muscle stiﬀness assessed using ultrasonic shear wave
elastography (SWE) and muscle mass of the lumbar back muscle, and spinal alignment in young and middle-aged
medical workers.
Methods: The study comprised 23 asymptomatic medical workers [control (CTR) group] and 9 medical workers
with LBP (LBP group). Muscle stiﬀness and mass of the lumbar back muscles (lumbar erector spinae, multiﬁdus,
and quadratus lumborum) in the prone position were measured using ultrasonic SWE. Sagittal spinal alignment
in the standing and prone positions was measured using a Spinal Mouse. The association with LBP was investigated by multiple logistic regression analysis with a forward selection method. The analysis was conductedusing the shear elastic modulus and muscle thickness of the lumbar back muscles, and spinal alignment, age,body height, body weight, and sex as independent variables.
Findings: Multiple logistic regression analysis showed that muscle stiﬀness of the lumbar multiﬁdus muscle andbody height were signiﬁcant and independent determinants of LBP, but that muscle mass and spinal alignment
were not. Muscle stiﬀness of the lumbar multiﬁdus muscle in the LBP group was signiﬁcantly higher than that in
the CTR group.
Interpretation: The results of this study suggest that LBP is associated with muscle stiﬀness of the lumbar multiﬁdus muscle in young and middle-aged medical workers.

1. Introduction
The occurrence rate of low back pain (LBP) within the lifetime ofadults is about 80% (Waddell, 1987). LBP is induced by stress on the
structures around the lumbar spine, such as intervertebral disks, intervertebral joints, ligaments, nerves, and lumbar back muscles. Thus,
clariﬁcation of the cause of LBP occurrence in rehabilitation is signiﬁcant. A previous study demonstrated that hard physical work, frequent trunk rotation or ﬂexion motion, and standing up motion areassociated with LBP occurrence (Xu et al., 1997). Medical workers, suchas nurses, care workers, and therapists, who work at hospitals

frequently perform these motions daily. LBP occurs at a high occurrence
rate in medical workers. In Japan, the occurrence rate of LBP within thepast month is about 50% in nurses who work at hospitals (Ando et al.,2000).
An electromyographic study demonstrated that the activities of thelumbar erector spinae muscle, gluteus maximus muscle, and hamstrings
increase during trunk rotation motion in LBP patients (Pirouzi et al.,2006). Moreover, the activities of the erector spinae and rectus abdominal muscles increase during walking in LBP patients (van der Hulst
et al., 2010). However, the relation of these electromyographic datawith muscle stiﬀness is unknown. Overuse of the lumbar erector spinae⁎
Corresponding author at: Department of Physical Therapy, Niigata University of Health and Welfare, Institute for Human Movement and Medical Sciences, 1398 Shimami-cho, Kitaku, Niigata 950-3198, Japan.
E-mail address: masaki@nuhw.ac.jp (M. Masaki).

http://dx.doi.org/10.1016/j.clinbiomech.2017.09.008
Received 6 December 2016; Accepted 12 September 20170268-0033/ © 2017 Elsevier Ltd. All rights reserved.

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M. Masaki et al.

2.2. Low back pain assessment

muscle caused by increased activity may lead to circulatory deﬁciency
within the muscle, resulting in increased muscle stiﬀness and LBP occurrence. On the other hand, LBP may contribute to increased muscle
stiﬀness (i.e., muscle spasm) of the lumbar back muscles, such as thelumbar erector spinae and lumbar multiﬁdus muscles in LBP patients.
Muscle stiﬀness of the lumbar back muscles in LBP patients is notclearly elucidated because the stiﬀness of the individual muscle distinguishing subcutaneous fat and ﬁbrous tissue has been diﬃcult toassess quantitatively. However, the assessment has recently become
possible by shear elastic modulus measured using ultrasonic shear wave
elastography (SWE). Ultrasonic SWE is a non-invasive and safe ultrasound imaging device. The shear elastic modulus, as an index of muscle
stiﬀness, is evaluated by measuring the shear wave propagation speed
in the tissues that is generated by an ultrasonic SWE. Previous studies
demonstrated that shear elastic modulus measured by SWE is associatedwith muscle elongation (Koo et al., 2013; Maïsetti et al., 2012) ormuscle strength (Ateş et al., 2015).
Previous studies evaluated muscle stiﬀness of the upper extremity
muscles (Leong et al., 2013; Rosskopf et al., 2016), such as the trapezius
and supraspinatus muscles; the lower extremity muscles, such as therectus femoris, gastrocnemius, and soleus muscles (Akagi et al., 2015);
the iliotibial band (Tateuchi et al., 2015, 2016); and the abdominal
muscles (Hirayama et al., 2015; MacDonald et al., 2016), such as therectus abdominis, external oblique, and internal oblique muscles.
However, no study has evaluated muscle stiﬀness individually andquantitatively in LBP patients. Furthermore, previous studies demonstrated the association of LBP with decreased muscle mass of thelumbar back muscles, and with changes in sagittal spinal alignment.
Decreased muscle mass of the lumbar multiﬁdus muscle (Barker et al.,
2004; Cooper et al., 1992; Hides et al., 1996, 2008; Hodges et al., 2006;
Keller et al., 2004) or changes in spinal alignment, such as decreased
lumbar lordosis in the standing position (Tsuji et al., 2001), cause stresson intervertebral disks or intervertebral joints, which may contribute to
LBP occurrence. A decreased muscle mass in the lumbar erector spinaeand quadratus lumborum muscles is also associated with LBP occurrence (Kamaz et al., 2007; Lee et al., 2011). Both muscle stiﬀness of thelumbar back muscles, and muscle mass of the lumbar back muscles andsagittal spinal alignment, are important factors that could be associatedwith LBP occurrence.
Therefore, this study aimed to examine the association of LBP withmuscle stiﬀness assessed using ultrasonic SWE and muscle mass of thelumbar back muscles, and sagittal spinal alignment in young andmiddle-aged medical workers.

The duration and degree of LBP, as well as the disabilities of dailyliving due to LBP, were assessed in the LBP group using a questionnaire.
The degree of LBP was examined using the NRS in both static (i.e.,lying, sitting, or standing) and dynamic situations (i.e., moving or
walking). The disabilities of daily living due to LBP were assessed using
the Oswestry disability index (ODI) (Fairbank and Pynsent, 2000). The
ODI consists of items, such as pain intensity of LBP during personal care(i.e., washing or dressing etc.), lifting, walking, sitting, standing,sleeping, sex life, social life, and travelling. The item of sex life, which
can be removed if not applicable, was not used in this study. The sum ofeach item was expressed as a percentage, and a large percentage indicated the severe disabilities of daily living due to LBP.
2.3. Ultrasound measurement
Images of the lumbar back muscles were taken using an ultrasoundimaging device with SWE (Aixplorer, Supersonic Imagine, Aix-enProvence, France). To assess the muscle mass of the lumbar backmuscles, longitudinal ultrasound images of the lumbar erector spinae,
multiﬁdus, and quadratus lumborum muscles were taken bilaterallyusing the B-mode of the ultrasound imaging device with a linear arrayprobe (SuperLinear 10-2), which was laid parallel to the muscle ﬁbers
in the prone position (Fig. 1). Ultrasound images were measured once
bilaterally for muscle thickness. The measurement sites were deﬁned as
7 cm lateral from the L3 spinous process for the lumbar erector spinaeand quadratus lumborum muscles, and 2 cm lateral to the L4 spinous
process for the lumbar multiﬁdus muscle (Masaki et al., 2015). All
measurements of the lumbar back muscles were performed with 58-dB
gain, 69-Hz dynamic range, and time gain compensation with theneutral position. Dynamic focus depth was also set to the depth of thelumbar back muscles. A previous study showed that the degree of intrarater reliability of the ultrasound technique is high for measuring
muscle thickness of the lumbar back muscles (Masaki et al., 2015).
The shear elastic modulus of the lumbar erector spinae and multiﬁdus muscles was evaluated twice bilaterally by measuring the shear
wave propagation speed in the tissues generated by an ultrasonic SWEin the prone position to assess the muscle stiﬀness of the lumbar backmuscles (Fig. 2). The shear elastic modulus of the quadratus lumborum
muscle, which was located deep within the body surface, was notmeasured in the present study. A linear array probe was set parallel tothe muscle ﬁbers to measure the shear elastic modulus accurately (Ebyet al., 2013). The circular regions of interest (ROIs) were set voluntarily
in the color-coded box presentation on a B-mode ultrasound imaging
with a scale from blue (soft) to red (hard) depending on the magnitudeof the shear wave speed. Three ROIs with a diameter of 10 mm were set
in the color-coded box, with 1 located at the center of the box and theother 2 beside the initial ROI. The mean shear elastic modulus values ineach ROI and the mean of the 3 ROIs were computed. The shear elastic
modulus (G) was computed from the shear wave propagation speed (v)and the muscle mass density (ρ) using the following equation:

2. Methods
2.1. Participants
Thirty-two young and middle-aged medical workers in Kyoto
Hakuaikai Hospital, Japan were included in the study. The subjects
were classiﬁed into control (CTR) (n = 23; mean age 34.7 ± 10.2) and
LBP groups (n = 9; mean age 44.7 ± 13.0) according to the presenceof LBP. The subjects in the CTR group had no LBP at the time of evaluation and no history of LBP lasting 3 or more months. The LBP groupconsisted of subjects with bilateral or central LBP (except for unilateral
LBP) with a severity rating of ≥3 on the numerical rating scale (NRS)
in both static (i.e., lying, sitting, or standing) and dynamic situations(i.e., moving or walking), lasting 3 months or more at the time ofevaluation. Medical workers included nurses, care workers, and therapists. Participants were excluded if they had severe orthopedic disorder
other than LBP; neurological, respiratory, or circulatory disorders in thepresent or past; or previous spinal surgery.
The protocol was approved by the Ethics Committee of the Kyoto
University Graduate School and Faculty of Medicine. All participants
provided written informed consent.

G = ρv 2
where ρ is presumed to be 1000 kg/m3 (Aubry et al., 2013). Enhanced
elastic shear modulus indicates an increase in muscle stiﬀness.
The mean values of muscle thickness in 1 measurement and theshear elastic modulus in 2 measurements for the right and left muscles
were used for statistical analyses. The determination of the ROIs andthe computation of muscle thickness and shear elastic modulus were
performed by 1 examiner who was blinded to information of thegroups.
To examine the intrarater reliability of the ultrasound technique formeasuring the shear elastic modulus of the lumbar back muscles, 2
images of each right muscle measured in the prone position were takenin 1 day in 52 young and middle-aged medical workers (age,129

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M. Masaki et al.

Fig. 1. Muscle thickness measurement of the lumbar back muscles in young and middle-aged medical workers.

kyphosis angle was calculated from the sum of the 11 segmental angles
from Th1/2 to Th11/12. The lumbar lordosis angle was calculated fromthe sum of the 6 segmental angles from Th12/L1 to L5/S1. The sacralanterior inclination angle was calculated from the diﬀerence between
the sacral angle and the vertical plane. The mean value of spinalalignment in 3 measurements was used for statistical analyses.

23.5 ± 1.5 years; height, 161.8 ± 7.9 cm; weight, 57.4 ± 12.6 kg).
2.4. Measurement of spinal alignment
The Spinal Mouse (Index Ltd., Tokyo, Japan) was used to measuresagittal spinal alignment in the standing position (thoracic kyphosis,
lumbar lordosis, and sacral anterior inclination angle) based on a previous study (Masaki et al., 2015). Spinal alignment in the prone position was also measured to identify whether muscle stiﬀness and muscle
mass of the lumbar back muscles were inﬂuenced by spinal alignment
in the position of ultrasound measurement. Spinal alignment wasmeasured 3 times.
The Spinal Mouse was guided along the midline of the spine,
starting at the C7 spinous process and ending at S3. The thoracic

2.5. Statistical analyses
Statistical analyses were performed using SPSS version 22.0 (IBMJapan; Tokyo, Japan). Intraclass correlation coeﬃcients [ICCs (1.1),
ICC (1.2)] were calculated to examine intrarater reliabilities of theshear elastic modulus measurements. Furthermore, the associations
with LBP were investigated by multiple logistic regression analysis with
Fig. 2. Muscle stiﬀness measurement of the lumbar backmuscles in young and middle-aged medical workers.

130

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M. Masaki et al.

important because the cause is attributed to diﬀerent factors. To thebest of our knowledge, this study is the ﬁrst to examine the associationof LBP with muscle stiﬀness of the lumbar back muscles assessed using
ultrasonic SWE in young and middle-aged medical workers. LBP wasfound to be associated with muscle stiﬀness of the lumbar multiﬁdus
muscle rather than muscle stiﬀness of the lumbar erector spinae muscle,
muscle mass of the lumbar back muscles, or sagittal spinal alignment inyoung and middle-aged medical workers. Furthermore, LBP was not
associated with muscle stiﬀness of the lumbar erector spinae muscle,
which was not consistent with our hypotheses.
Multiple logistic regression analysis showed that the muscle stiﬀnessof the lumbar multiﬁdus muscle in the LBP group was signiﬁcantly
higher than that in the CTR group in the prone position. The activity of
the lumbar multiﬁdus muscle in the prone position has not been clearly
elucidated, but the activity of the lumbar multiﬁdus muscle in LBP
patients has been demonstrated to decrease compared with the healthy
subjects during trunk motion (Ng et al., 2002). A possible reason for the
association of LBP with muscle stiﬀness of the lumbar back muscles in
the prone position is the frequent trunk ﬂexion or pelvic anterior tilt of
the medical workers in the standing position during medical treatment,
care, and rehabilitation, as well as frequent extension of their trunk
during transferring patients. The lumbar erector spinae muscle, which
is a member of the superior muscles of the trunk, is advantageous togenerate the extension moment because this muscle has a long extension moment arm for spine extension (Lin et al., 2001). On the other
hand, the lumbar multiﬁdus muscle, which is a deep muscle of thetrunk, is advantageous to stabilize the lumbar spine (Bergmark, 1989;
MacDonald et al., 2006). However, it is disadvantageous to generate theextension moment because it has a shorter extension moment arm of
the spine than the lumbar erector spinae muscle (Bogduk et al., 1992).
The frequent activity of lumbar multiﬁdus muscle during motions maylead to muscle overuse in medical workers because the activity tends tobe higher than that of the lumbar erector spinae during trunk extension
motions (Ng and Richardson, 1994). Circulatory diﬃculty within the
lumbar multiﬁdus muscle caused by overuse during the motions may
contribute to an increase in muscle stiﬀness and LBP occurrence.
Muscle stiﬀness measured using ultrasonic shear wave elastography
is inﬂuenced by muscle elongation or muscle activity. Another possible
reason for the relation of LBP with muscle stiﬀness in the LBP group
could be muscle spasm due to pain. LBP, which is caused from stress onintervertebral disks or intervertebral joints, may induce muscle spasmof the lumbar multiﬁdus muscle. Muscle stiﬀness of the lumbar multiﬁdus muscle is assumed to increase by muscle contraction. In this case,the overuse caused by muscle spasm of the lumbar multiﬁdus muscle
may lead to circulatory diﬃculty within the muscle, which contributes

Table 1
Characteristics and LBP status in the CTR and LBP groups.

Characteristics
Age (years)
Height (cm)Weight (kg)Sex (male/female)
LBP status
Duration (months)
NRS (static)
NRS (dynamic)
ODI (%)CTR group (n = 23)

LBP group (n = 9)

Mean (SD)

Range

Mean (SD)Range

34.7 (10.2)
164.1 (7.5)
56.9 (8.9)
8/15

22.0–52.0
153.0–177.0
42.0–73.0

44.7 (13.0)
157.4 (6.6)
52.1 (9.4)
1/8

29.0–67.0
151.0–170.0
41.0–70.0

―
―
―
―

―
―
―
―

98.0 (73.1)
5.0 (1.4)
5.0 (1.7)
19.6 (7.8)

6.0–240.0
4.0–8.0
2.0–8.0
6.0–30.0

CTR: control, LBP: low back pain, NRS: numerical rating scale, ODI: Oswestry disability
index, SD: standard deviation.

a forward selection method. This analysis was conducted using theshear elastic modulus and muscle thickness of the lumbar back muscles,
and sagittal spinal alignment, age, body height, body weight, and sex as
independent variables. P values of < 0.05 were considered signiﬁcant.
3. Results
Table 1 presents characteristics and LBP status in the CTR and LBP
groups. Muscle stiﬀness and muscle mass of the lumbar back muscle,and spinal alignment are shown in Table 2.
In the reliability analysis of the shear elastic modulus measurement,
the ICC values of 1.1 for the erector spinae muscle and lumbar multiﬁdus muscle were 0.784 and 0.913, and the ICC values of 1.2 were0.879 and 0.954, respectively.
Multiple logistic regression analysis showed that the shear elastic
modulus of the lumbar multiﬁdus muscle (odds ratio, 4.13) and bodyheight (odds ratio, 0.82) were signiﬁcant and independent determinants of LBP. The shear elastic modulus of the lumbar multiﬁdus muscle
in the LBP group was signiﬁcantly higher than that in the CTR group.
The height in the LBP group was signiﬁcantly lower than that in the
CTR group (Table 3). Multiple logistic regression analysis also showedthat the other factors were not signiﬁcant independent determinants of
LBP.
4. Discussion
Practicing rehabilitation based on the cause of LBP occurrence isTable 2
Muscle stiﬀness and muscle mass of the lumbar back muscles, and spinal alignment in the CTR and LBP groups.
CTR group (n = 23)

Shear elastic modulus (kPa)Lumbar erector spinae
Lumbar multiﬁdus
Muscle thickness (cm)Lumbar erector spinae
Lumbar multiﬁdus
Quadratus lumborum
Spinal alignment (standing) (°)
Thoracic kyphosis
Lumbar lordosis
Sacral anterior inclination
Spinal alignment (prone) (°)
Thoracic kyphosis
Lumbar lordosis
Sacral anterior inclination

LBP group (n = 9)

Mean (SD)

Range

Mean (SD)Range

3.5 (1.1)
4.8 (0.8)

1.8–6.4
3.6–6.1

3.7 (1.1)
5.6 (1.1)

2.2–5.9
3.3–6.8

2.82 (0.67)
2.86 (0.39)
0.93 (0.27)

1.55–4.58
2.27–3.94
0.49–1.84

2.45 (0.19)
2.77 (0.33)
0.88 (0.17)

2.11–2.67
2.37–3.29
0.56–1.21

41.1 (8.4)
25.7 (9.6)
12.9 (5.7)

26.0–58.0
2.0–41.0− 2.0 to 22.0

41.8 (12.0)
25.4 (8.4)
10.3 (5.6)

24.0–63.0
14.0–38.0
1.0–18.0

21.5 (10.6)
21.1 (7.6)
102.0 (6.5)

0–33.0
7.0–37.0
92.0–114.0

19.9 (6.6)
20.3 (7.2)
100.9 (5.3)

10.0–29.0
9.0–30.0
94.0–108.0

CTR: control, LBP: low back pain, SD: standard deviation.

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Table 3Results of multiple logistic regression analysis with a forward selection method.
Dependent variables

Low back pain
(Yes = 1, No = 0)
χ2 value P = 0.002

Independent variables

Non-standard partial regression coeﬃcient

P value

Odds ratio

95% conﬁdence interval
Lower

Upper

Shear elastic modulus of lumbar multiﬁdus (kPa)

1.42

0.03

4.13

1.17

14.63

Height (cm)

− 0.20

0.02

0.82

0.69

0.97

ﬂexion of the lumbar spine, which compensatorily becomes excessive
during medical treatment, care, rehabilitation, and transferring patientsin medical workers, who have shorter body height (i.e., shorter upper
extremities). Thus, shorter body height may contribute to increased
muscle stiﬀness of the lumbar multiﬁdus muscles, or the stress on intervertebral disks or intervertebral joints.
The present study has several limitations. First, the measurements ofmuscle stiﬀness and muscle mass targeted only a part of the lumbar
back muscles. Second, whether an increase in muscle stiﬀness of thelumbar multiﬁdus muscle is caused by overuse or muscle spasm is unclear because the activities of the lumbar back muscles were not measured using electromyography during ultrasound measurement. Third,
not only nonspeciﬁc LBP patients but also speciﬁc LBP (i.e., the diseaseof the lumbar spine) patients might have been included because the LBPof the subjects was not diagnosed in the present study. Therefore, examination of the association of LBP with muscle stiﬀness and muscle
mass of the lumbar back muscles, and spinal alignment after classifying
LBP patients in detail based on the disease of the lumbar spine is necessary.
The present study suggests that LBP is associated with muscle
stiﬀness of the lumbar multiﬁdus muscle in young and middle-aged
medical workers. Further studies should examine training for improving muscle stiﬀness of the lumbar multiﬁdus muscle eﬀectively.
Furthermore, assessment of the association of LBP with muscle stiﬀnessof the lumbar back muscles in LBP patients who have diﬀerent occupations or ages from those of the subjects in the present study is needed.

to secondary LBP occurrence in the future. There is the possibility thatthe muscle activity and muscle stiﬀness of the lumbar multiﬁdus
muscle, not only in the prone position, but also during the motions inthe standing, increases with muscle spasm in LBP patients. Thus, thelevel of muscle stiﬀness of the lumbar multiﬁdus muscle measured in
the prone position using ultrasonic SWE may reﬂect the condition ofthat muscle during the motions in LBP patients.
Previous study (Chan et al., 2012) demonstrated muscle stiﬀness of
the lumbar back muscles using a strain imaging method of the ultrasound imaging device, which is diﬀerent from ultrasonic SWE used in
the present study. They targeted only the lumbar multiﬁdus muscles inthe LBP patients and suggested that no signiﬁcant diﬀerence exists inthe muscle stiﬀness of the lumbar multiﬁdus muscle in the prone position between healthy subjects and LBP patients. However, measuring
muscle stiﬀness quantitatively using the strain imaging method is difﬁcult because the evaluation depends on the rater's control of thecompression added to the probe, and the absolute value cannot bemeasured. The ultrasonic SWE used in the current study can measuremuscle stiﬀness more quantitatively than a strain imaging method because the former is less dependent on the evaluation technique.
No signiﬁcant diﬀerence in the muscle stiﬀness of the lumbar
erecter spinae muscle was found between the CTR and LBP groups inthe present study. Although the activity of the lumbar erector spinae
muscle is reported to increase during trunk rotation motion (Pirouzi
et al., 2006) or walking (van der Hulst et al., 2010), the activity of thelumbar erecter spinae muscle in the LBP group might not have increased during motions in the present study.
Signiﬁcant diﬀerence was not observed in the muscle mass of thelumbar back muscles between the CTR and LBP groups in the present
study. Previous studies using computed tomography and magnetic resonance imaging demonstrated that the muscle mass of the lumbar backmuscles, such as the lumbar erector spinae, multiﬁdus, and quadratus
lumborum muscles, either decreases (Kamaz et al., 2007; Lee et al.,2011; Wallwork et al., 2009) or does not decrease (Danneels et al.,2000) in the LBP patients. The results of the present study were consistent with those of the previous study, which demonstrated that LBP is
not associated with the muscle mass of the lumbar back muscles. No
signiﬁcant diﬀerence was found in each angle of spinal alignment in theprone position between the CTR and LBP groups. Thus, muscle stiﬀness
and muscle mass were assumed to be not inﬂuenced by spinal alignment in the prone position, which is the position of ultrasound measurement.
No signiﬁcant diﬀerence existed in each angle of spinal alignment in
the standing position between the CTR and LBP groups. Previous studies demonstrated that lumbar lordosis in the standing position decreases (Tsuji et al., 2001) or does not decrease (Chaléat-Valayer et al.,2011; Laird et al., 2014) in the LBP patients. The results of the present
study were consistent with those of the previous studies that demonstrate no association with spinal alignment in the standing position and
LBP.
Furthermore, body height in the LBP group was signiﬁcantly lower
than that in the CTR group. Previous study (Heuch et al., 2015) longitudinally examined the association of LBP with body height. Thisprevious study demonstrated that taller subjects have a higher risk of
LBP recurrence because the stress on tissues around the lumbar spine
increases during motions. This inconsistency may be attributed to the

5. Conclusions
The results of the present study suggest that LBP is associated withmuscle stiﬀness of the lumbar multiﬁdus muscle rather than muscle
stiﬀness of the lumbar erector spinae muscle, muscle mass of thelumbar back muscles, or sagittal spinal alignment in young and middleaged medical workers.
Conﬂicts of interest statement
No funding sources and potential conﬂicts of interest were disclosed
for the present study.
Acknowledgments
The authors thank all of the individuals who participated in thepresent study.
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