Journal of

Functional Morphology
and Kinesiology

Review
Do Muscle Strength Imbalances and Low Flexibility
Levels Lead to Low Back Pain? A Brief Review
Cassio Victora Ruas 1,2, * and Adriane Vieira 2
1 Centre for Exercise and Sports Science Research (CESSR), School of Medical and Health Sciences,
Edith Cowan University, Joondalup, WA 6027, Australia
2 School of Physical Education, Federal University of Rio Grande do Sul (UFRGS),
Porto Alegre 90690-200, Brazil; adriane.vieira@gmail.com
* Correspondence: c.victoraruas@ecu.edu.au; Tel.: +61-8-6304-2736

Received: 15 July 2017; Accepted: 3 August 2017; Published: 4 August 2017

Abstract: Chronic low back pain (CLBP) has been related to hips, trunk and spine strength imbalances
and/or low flexibility levels. However, it is not clear if the assessment and normalization of these
variables are effective for prevention of low back pain (LBP) episodes and rehabilitation of patients
with CLBP. This brief review explored studies that have associated hip, trunk and spine strength
imbalances and/or low flexibility levels to LBP episodes or CLBP condition. Fourteen studies were
selected by accessing PubMed and Google Scholar databases. Collectively, the selected studies
demonstrate that trunk eccentric/concentric and flexion/extension strength imbalances may be
associated with CLBP or LBP episodes. However, the literature fails to demonstrate any clear
relationship between hip strength imbalances or low levels of spine flexibility with CLBP or LBP
episodes. In addition, there is no direct evidence to support the idea that the normalization of
these variables due to resistance and flexibility training leads to pain reduction and functionality
improvements in subjects with CLBP. Although further investigation is needed, the lack of a clear
direct association between hip strength imbalances or spine low flexibility levels to CLBP or LBP
episodes may demonstrate that these variables may have very low effect within the complexity of
these conditions.

Keywords: chronic low back pain; low back pain episodes; strength ratios; side-to-side asymmetry;
flexibility levels

1. Introduction
Chronic low back pain (CLBP) is considered one of the main health care problems of the modern
day society [1]. Up to 80% of the population report having low back pain (LBP) at some point of their
lives, and after a first LBP episode, 44–78% have relapses of pain, leading 26–37% of people to be
absent from work after an initial episode [2]. If the problem persists within approximately 12 weeks it
may become chronic [3]. The prevalence of nonspecific CLBP is approximately 23%, and 11–12% of
people may become disabled by this condition [2]. However, since CLBP is considered a multi-factorial
problem, which consists of patho-anatomical, psychological, social, neuro-physiological and physical
aspects, the treatment of this condition may be difficult, representing a major cost health problem for
society [3–6].
Although most cases of LBP are considered to be non-specific, previous studies have related these
conditions to compressive and repetitive muscle strength performance [7], as well as to bilateral and/or
unilateral strength imbalances, especially of trunk and hips, which ensure mechanical stability of the
spine [1,4,7–12]. For instance, antagonist/agonist, eccentric/concentric, and right/left muscle strength
imbalances have been associated to LBP occurrence [11–14]. In addition, low levels of flexibility and

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J. Funct. Morphol. Kinesiol. 2017, 2, 29 2 of 9

muscle activation have been reported in patients with CLBP, having been pointed out as possible
causes of this condition [1,15,16].
The knowledge of the effect of unilateral and bilateral strength imbalances and low flexibility
levels on the cause of LBP episodes is critical for the prescription of exercises to reduce the symptoms
of this condition. However, it is not clear if the assessment and normalization of these variables are
effective for the prevention of LBP episodes and rehabilitation of patients with CLBP. Therefore, the aim
of this study was to explore studies that have investigated associations between strength imbalances
and/or low flexibility levels of trunk, spine and hips and LBP episodes or CLBP conditions.

2. Materials and Methods

This review was based on 14 studies [1,4,7–18], published between 1983 and 2016. The articles
were found by accessing the databases PubMed and Google Scholar using the following search terms:
“Spine” or “Hips” or “Trunk” and “Muscle” and/or “Strength” and/or “Flexibility” and “Levels” or
“Imbalance” and “Chronic low back pain” or “Low back pain” (e.g., “Hips muscle strength imbalance
and low back pain”). Articles that did not match these terms were excluded, and only studies in the
English language were considered. Fifteen additional articles [2,3,5,6,19–29] were used to support the
variables investigated in the introduction and discussion of this review. In the present review, the term
“spine” refers to the spinal column, while the term “trunk” refers to the anterior and posterior muscles
of the trunk.

3. Results
Details of the main aims, samples, outcome measures and main results of the 14 selected studies
found through our search terms are described on Table 1. Based on the results of the selected studies,
three main topics were considered for discussion of LBP episodes and chronic prevalence: strength
and/or flexibility of trunk, spine and hips; unilateral trunk flexion/extension strength imbalances;
and hip and trunk side-to-side strength asymmetries.
J. Funct. Morphol. Kinesiol. 2017, 2, 29 3 of 9

Table 1. Summary of studies.

Topic Authors Main Aims Sample Outcome Measures Main Results

Women with CLBP had lower flexor and
To compare women with and without extensor trunk muscle peak torques,
CLBP on obesity, spine flexibility, levels greater BMI and lower spine ROM.
of pain, and trunk strength. In addition, Strength training resulted in greater flexor
45 women (25 with CLBP - BMI.
Bayramoglu et al. 2001 * to test the association between these and extensor trunk muscle strength, and
and 20 without CLBP). - Isokinetic trunk flexion-extension PT.
variables, and to investigate the effect of reduced trunk flexor/extensor ratio
a follow-up strength training on the (60◦ /sec only). No correlation between
CLBP group. levels of pain and trunk muscle strength or
spinal ROM.
Although there was a significant
82 amateur tennis players correlation between LBP and
To examine the relationship between
with (19 females and neuromuscular imbalance of erector spinae,
LBP, clinical symptoms, neuromuscular - Trunk extension isometric MVC
Renkawitz et al. 2006 27 males) and without no clear associations between LBP, trunk
imbalance and trunk strength extension - Spine mobility and flexibility.
(12 females and 24 males) extension strength, and clinical testings
before and after exercise program.
LBP. were found before and after the
exercise program.
To compare tennis players with and 60 male elite tennis players - Trunk extension, rotation, flexion and No significant difference between tennis
Grosdent et al. 2015 ** without LBP on trunk strength and with (38) and without lateral-flexion isometric MVC players with and without LBP on trunk
flexibility. (22) LBP. - Pelvic and lumbar flexion mobility. strength and spine flexibility.
Strength and/or Although ligamentous laxity or overuse
- Hip flexors flexibility.
Flexibility levels of To investigate if leg length discrepancy, were found to be potential predictors of
257 college athletes - Leg length discrepancy.
Trunk, Spine and Hips Nadler et al. 1998 lower extremity laxity and hip flexor LBP, leg length discrepancy and hip
(170 males and 87 females). - Ligamentous stability.
tightness were predictors of LBP. inflexibility were not associated to future
- Overuse syndrome.
LBP episodes.
Participants with history of LBP in the
following 5 years (8 male and 10 female)
To evaluate trunk strength weakness as did not differ from the non-LBP incidence
30 male and 37 female young - Isokinetic trunk flexion-extension and
Lee et al. 1999 * a risk factor of LBP in a 5-year group on trunk flexion-extension and
adults with no history of LBP. rotation PT.
prospective study. rotation PT. However, women with LBP
had lower extension/flexion ratio
compared to women without LBP.
No significant difference in hip strength
300 (100 with LBP + ITB; levels between LBP individuals with and
To investigate hip abductors strength - Hip abduction isometric MVC
Arab et al. 2010 100 with LBP only; without ITB. However, subjects without
and ITB tightness in subjects with LBP. - Length of ITB test.
100 without LBP). LBP had greater hip abductor strength
compared to subjects with LBP.
- Qualitative pain grade test.
Participants with CLBP improved 3-D
- Low back stabilization tests
To evaluate the effects of a short kinematics, body balance, pelvis
6 subjects with one year of - Low back and pelvis kinematics.
Carpes et al. 2008 training program on trunk strength and inclination, and low back flexion ROM.
nonspecific LBP. - Body balance
stability of subjects with CLBP. Qualitative pain grade test indicated pain
- Strength and stability of low back and
decreased and pelvic strength increased.
pelvis complex.
J. Funct. Morphol. Kinesiol. 2017, 2, 29 4 of 9

Table 1. Cont.

Topic Authors Main Aims Sample Outcome Measures Main Results

To compare trunk extensors and flexors Trunk flexion-extension PT was greater in
strength between different assessment 50 healthy subjects (25 males the siting posture with feet against the
positions of the isokinetic dynamometer, and 25 females) and 48 - Isokinetic concentric and eccentric floor. Flexion/extension and
Shirado et al. 1995
and flexor/extensor strength ratios patients with CLBP (26 males trunk flexion-extension PT. eccentric/concentric strength ratios were
between subjects with and and 22 females). greater in subjects with CLBP at almost all
without CLBP. isokinetic speeds compared to non-CLBP.
CLBP patients showed less fatigue and did
To compare lumbar muscle strength, - Trunk extension isometric MVC. not produce a “true” maximal torque.
neuromuscular fatigue, and muscle 34 male adults (20 healthy - EMG over L1, L2 and L5 levels of Muscle imbalances were present in both
Oddsson et al. 2003
imbalances between subjects with and and 14 CLBP patients). lower back groups, although greater in CLBP when
Unilateral Trunk without CLBP. - Subjective pain tests. averaged. Patients with CLBP had altered
Flexion/Extension lumbar back EMG.
strength Imbalances - Trunk flexion, extension and side
bridge isometric endurance strength. Players with LBP had lower muscle
To compare trunk endurance time,
35 (28 males and 7 females) - History of LBP incidence EMG of activation of trunk extensor muscles, less
Correia et al. 2016 muscle activation and fatigue in tennis
national level tennis players. rectus abdominis, external oblique, abdominal endurance, and less trunk
players with and without low back pain.
iliocostalis lumborum and extensor co-contraction patterns.
longuissimus thoracis.
To determine if trunk levels of strength Patients with CLBP had lower muscle
and fatigue are related to low-back-pain 140 (90 patients with strength and greater muscle fatigue of
Suzuki et al. 1983 syndrome, and the association between persistent LBP and 40 healthy - Isokinetic trunk flexion-extension PT. trunk flexors than extensors. In addition,
trunk muscle strength and lumbar free pain adults) participants. trunk strength and fatigue were negatively
lordosis. associated to lumbar lordosis.
Only female athletes with LBP or LE
To investigate the association between
presented greater side-to-side hip extensors
LBP and previous LE on hip abduction 210 (140 males and - Hip abduction and extension isometric
asymmetry compared to athletes without
Nadler et al. 2000 and extension side-to-side strength 70 females) Division I mean MVC.
LBP or LE. There was no side-to-side
asymmetries of Division I collegiate athletes. - History of LBP incidence.
asymmetry difference between male tennis
collegiate athletes.
players with and without LBP or LE.
Side to side hip extensor strength
asymmetry was predictive of LBP
To investigate if athletes with hip
Hip and Trunk treatment over the ensuing year for female
side-to-side strength asymmetry would 163 (100 males and - Hip abduction and extension isometric
side-to-side strength athletes. No association was found
Nadler et al. 2001 be more likely to need treatment of LBP 63 females) Division I MVC
asymmetries between hip abductor side-to-side
over the subsequent year on Division I collegiate athletes. - History of LBP incidence.
asymmetry in women or hip abductor and
collegiate athletes.
hip extensor side-to-side asymmetries and
prediction of LBP in male athletes.
1st year: 166 (101 males and
To investigate the effects of 63 females); 2nd year: 236 Although core strengthening training
core-strengthening training on LBP (162 males and 74 females); - Hip extension isometric mean MVC. reduced hip extensor side-to-side
Nadler et al. 2002
incidence on Division I 3rd year: 225 (170 males and - Monitoring of LBP incidence. asymmetry, there was no effect of on LBP
collegiate athletes. 55 females) Division I incidence reduction.
collegiate athletes.
* Articles were also discussed in topic: Unilateral Trunk Flexion/Extension Strength Imbalances; ** Article was also discussed in topic: Hip and Trunk Side-to-Side Strength Asymmetries.
Abbreviations: CLBP = Chronic Low Back Pain; LBP = Low Back Pain; LE = Lower Extremity Injury; EMG = Electromyography; PT = Peak Torque; MVC = Maximum Voluntary
Contraction; ITB = Iliotibial Band; ROM = Range of Motion; BMI = Body Mass Index.
J. Funct. Morphol. Kinesiol. 2017, 2, 29 5 of 9

4. Discussion

4.1. Strength and/or Flexibility of Trunk, Spine and Hips

Low levels of strength and/or flexibility of trunk, spine and hips have been pointed out as
either causes, consequences or influencing factors for the prevalence of the CLBP condition [1,4,7–10].
Bayramoglu et al. [1] compared women with and without CLBP on obesity, spine flexibility and
trunk strength. Obesity was calculated by body mass index (BMI), flexibility and strength
by using an isokinetic dynamometer. Levels of pain and functionality were assessed by the
Oswestry Disability Questionnaire. Results demonstrated that women with CLBP had lower
flexor (60◦ /sec = 42.8 ± 12.8%; 120◦ /sec = 56.8 ± 35.0%; 120◦ /sec = 46.9 ± 33.3%) and extensor
(60◦ /sec = 57.7 ± 30.3%; 120◦ /sec = 73.7 ± 19.8%; 180◦ /sec = 46.4 ± 51.7%) trunk muscle peak
torques, as well as greater BMI (9.9 ± 0.5%) and a lower spine range of motion (ROM) (11.6 ± 42.6%).
In addition, a 15-day follow-up strength training program was performed by the CLBP group after
testing, which included double straight leg-lowers, sit-ups, and prone trunk extensions. Training
resulted in significantly greater flexor and extensor trunk muscle strength. However, although the
authors concluded that strength and flexibility training associated with weight reduction could
potentially improve the CLBP condition by improving trunk strength and spinal ROM, they were
unable to find significant correlations between functionality and pain with low levels of trunk strength
or spine ROM. In addition, the initial (38.64) and end (38.01) mean scores obtained by the Oswestry
Disability Questionnaire did not present a statistical difference.
This is in agreement with Renkawitz et al. [8], who did not find a direct association between
LBP and trunk extensor maximal isometric strength in tennis athletes after an intervention program
including exercises for strength, coordination, and mobility, as well as trunk and lumbar stretching
exercises. In addition, Grosdent et al. [7] failed to find differences between tennis players with or
without current LBP in terms of trunk isometric torque and spine ROM, which may also demonstrate
a low relationship between levels of strength and flexibility as cause or prevalence of CLBP condition.
In agreement with these studies, Nadler et al. [4] did not find any significant relationship between low
levels of hip flexibility and rate of collegiate athletes requiring treatment for LBP; Arab et al. [17] did
not find any significant difference in hip strength levels between LBP individuals with and without
iliotibial tightness, which is associated and common during CLBP, and Lee et al. [10] did not find
significant differences between young adults with and without LBP in trunk flexor and extensor
isokinetic peak torque.
In contrast, Carpes et al. [9], in a pilot study, found that 20 sessions of training including trunk
strength and stability exercises led to low back pain reduction, improvements of limbo-pelvic strength,
spine and pelvis flexibility, reduction of lumbar lordosis, as well as improvements in postural stability
of a sample of 6 female participants. However, the authors concluded that the small sample size and
lack of control group for comparison of the effects of training on the investigated variables could
lead to misinterpretations, and required future studies addressing these methodological issues for
confirmation of their results.

4.2. Unilateral Trunk Flexion/Extension Strength Imbalances

Muscle strength ratios are commonly tested to describe unilateral antagonist to agonist strength
properties, functionality and imbalances [19,20,24–26,29]. An increased antagonist/agonist imbalance
may demonstrate failure of the antagonist muscles to produce enough strength to decelerate agonist
maximal torque actions during a required movement, increasing the likelihood of muscle and ligament
injuries during sports performance and functional activities [21,22,25,26,29]. Therefore, unilateral
imbalances have also been investigated as possible causes leading to a CLBP condition [1,12,18].
Shirado et al. [12] compared trunk extensor and flexor strength between different assessment
positions of the isokinetic dynamometer, as well as trunk flexor/extensor strength ratios in men
and women with and without CLBP. They found that concentric and eccentric flexion/extension
J. Funct. Morphol. Kinesiol. 2017, 2, 29 6 of 9

strength ratios were greater in subjects with CLBP at almost all isokinetic speeds tested compared
to non-CLBP. This demonstrates that a CLBP condition may result in greater than normal trunk
flexion strength than extension when trunk extension movements are performed, resulting in a trunk
strength imbalance. Interestingly, Bayramoglu et al. [1] found that a short strength training program in
which two trunk flexion (double straight leg-lowers, sit-ups) and one trunk extension (prone trunk
extensions) exercises were included lead to a significant decrease on concentric flexion/extension
ratio at a slow isokinetic speed (60◦ /sec = 32.2 ± 29.3%), but did not significantly change the ratio
at intermediate (120◦ /sec = 14.9 ± 37.1%) and high isokinetic speeds (180◦ /sec = 4.5 ± 36.1%) in
participants with CLBP. This may show that subjects with CLBP may need to greatly activate trunk
flexor (antagonists) strength to decelerate trunk extensors (agonists) during trunk extension actions.
However, Lee et al. [10] demonstrated that subjects with LBP had lower trunk extension/flexion ratio
at 60◦ /sec compared to subjects without LBP (men = 22.0 ± 3.6%; women = 23.0 ± 15.8%), which shows
that trunk extension strength is reduced when trunk flexion movements are performed. Contrary to
these findings, Suzuki et al. [18] did not find any difference in trunk flexion/extension strength ratio
between patients with persistent LBP compared to healthy participants, but the fatigability of trunk
flexors was significantly greater (18.3 ± 1.4%) for the group with LBP. Therefore, these results show
that subjects with LBP may use different strategies and distinct strength patterns to perform trunk
extension and flexion movements due to increased pain or caution.
Shirado et al. [12] also found that subjects with CLBP had approximately 2 to 4 times greater
unilateral eccentric/concentric ratios of trunk flexors and extensors through all slow-fast tested
isokinetic velocities compared to non-CLBP. The authors assumed that this ratio would be an important
muscle strength balance measurement in CLBP patients as trunk extensors and flexors are activated
either concentrically or eccentrically in several daily life tasks. Since eccentric strength is used to
decelerate and control movements [24–28], this may demonstrate that subjects with CLBP constantly
perform extreme and unnecessary levels of eccentric strength when performing trunk extension and
flexion daily tasks such as picking up an object from the floor or returning it to its original place [12].
However, in this study, some subjects reported back pain and discomfort when performing strength
tests, which may have increased their muscle inhibition and decreased performance.
Furthermore, Oddsson et al. [15] showed that subjects with the presence of CLBP could not
perform maximal effort during trunk extensors isometric strength tests, and had reduced control of
lumbar muscle activation. Correia et al. [16] also found that tennis players with symptoms of LBP
had significantly reduced iliocostalis lumborum and longuissimus thoracis muscle activation patterns
during trunk extension isometric test compared to players without LBP. Although Shirado et al. [12]
were not specific as to whether pain was aggravated during eccentric strength tests, this may call into
question whether or not the subjects with CLBP accurately performed maximal trunk flexion and
extension eccentric strength, which may have adversely affected their results. However, since patients
with CLBP report pain when performing several strength tasks, the fact that they felt pain and
discomfort may also approximate results in functional situations of their daily life, especially when
performing tasks that involve eccentric strength.

4.3. Hip and Trunk Side-to-side Strength Asymmetries

Screening for side-to-side strength asymmetry has become a common practice for identifying muscle
strength differences between right and left or dominant and non-dominant sides of the body [27,28].
High levels of side-to-side asymmetry have been associated with an increased risk of injuries [23], postural
problems and rate of LBP episodes [11,14].
Nadler et al. [11] investigated the relationship between LBP episodes and hip side-to-side
asymmetries in 210 male and female Division I collegiate athletes. Both right-to-left hip extensors
and abductors were assessed on a dynamometer attached to a specially designed anchoring station.
The history of lower extremity or LBP was recorded through interviews and previous injury records,
when available. Results demonstrated that hip extensors of female athletes with LBP had 9.6%
J. Funct. Morphol. Kinesiol. 2017, 2, 29 7 of 9

significantly greater side-to-side asymmetry compared to female athletes without LBP. Therefore,
the authors concluded that screening for side-to-side asymmetry may be an important tool on the
prevention of LBP. However, there are potential limitations to fully supporting the assumption in
this study, such as: (1) There were no significant differences between hip abductor or extensor
side-to-side asymmetries in male athletes; (2) There were no significant differences between hip
abductor side-to-side asymmetries in female athletes; an (3) LBP was screened through interview
and previous injury records, but no other common research methods for recording levels of pain and
functionality were used, such as the Oswestry Disability Questionnaire.
Nevertheless, although these results are in agreement with their subsequent cohort study with 163
Division I collegiate athletes [14], where a greater concentric strength side-to-side asymmetry of hip
extensors was predictive (p = 0.05) of LBP treatment over the ensuing year for women only, in a third
study, Nadler et al. [13] did not find any effect of a core resistance training protocol (focused on
normalizing hip side-to-side asymmetries by strengthening abdominal, paraspinal, and hip extensors)
on LBP episodes reduction in college athletes. Similarly, although Grosdent et al. [7] demonstrated that
tennis players had greater nondominant than dominant lateral-flexors and rotator strength compared
to sedentary participants, there was no significant difference in trunk strength imbalances when tennis
players with and without current LBP were compared.

5. Conclusions
This brief review explored studies that investigated potential associations between CLBP or LBP
episodes with strength imbalances and/or flexibility levels of the trunk, spine and hips. Collectively,
the selected studies demonstrate that trunk eccentric/concentric and flexion/extension strength
imbalances may be associated to episodes or chronic prevalence of LBP. However, the literature fails
to demonstrate any clear relationship between hip strength imbalances or spine flexibility levels
with this condition. In addition, there is no direct evidence to support that the normalization of
these variables through resistance and flexibility training leads to pain reduction and functionality
improvements in subjects with CLBP. Most studies to date demonstrate methodological limitations on
the assessment of LBP episodes, difficulty of testing the strength and flexibility of participants with
CLBP who are constantly in pain and have functional limitations, or small sample sizes for inferences
of results for the general population. In addition, the different sample sizes from the selected studies
of this review also limit great generalizations of the investigated topics. Therefore, future acute and
chronic longitudinal studies are needed to further understand how effective screening and training are
for decreasing strength imbalances and increasing flexibility levels on the cause and effect of CLBP.
However, since LBP episodes leading to CLBP can be related to multi-factorial problems, which rely
heavily on psychosocial aspects, it is possible that these variables may have very low effect within the
complexity of this condition.

Conflicts of Interest: The authors declare no conflict of interest.

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