Joint Bone Spine 73 (2006) 43–50
http://france.elsevier.com/direct/BONSOI/
Review
Spinal muscle evaluation using the Sorensen test:
a critical appraisal of the literature
Christophe Demoulin *, Marc Vanderthommen, Christophe Duysens, Jean-Michel Crielaard
Physical Medicine and Rehabilitation Unit, Liège University, Liège, Belgium
Received 3 May 2004; accepted 4 August 2004
Available online 05 October 2004
Abstract
The first test for evaluating the isometric endurance of trunk extensor muscles was described by Hansen in 1964. In 1984, following a study
by Biering-Sorensen, this test became known as the “Sorensen test” and gained considerable popularity as a tool reported to predict low back
pain within the next year in males. The test consists in measuring the amount of time a person can hold the unsupported upper body in a
horizontal prone position with the lower body fixed to the examining table. This test has been used in many studies, either in its original
version or as variants. Although its discriminative validity, reproducibility, and safety seem good, debate continues to surround its ability to
predict low back pain; in addition, the gender-related difference in position-holding time remains unexplained and the influence of body
weight unclear. A contribution of the hip extensor muscles to position holding has been established, but its magnitude remains unknown. The
influence of personal factors such as motivation complicates the interpretation of the results. Despite these drawbacks, the Sorensen test has
become the tool of reference for evaluating muscle performance in patients with low back pain, most notably before and after rehabilitation
programs.
© 2005 Elsevier SAS. All rights reserved.
Keywords: Muscle endurance; Low back pain; Trunk extensor muscles; Physical evaluation; Spinal muscles
1. Introduction
Despite growing research efforts, nonspecific low back pain
remains a major public health burden throughout the industrialized world. Epidemiological data indicate an annual
prevalence of about 39–54% [1,2] and a lifetime prevalence
of 60–65% [1–3]. Costs to society stem mainly from chronic
forms, which account for only 5–10% of cases [4,5]. Chronic
nonspecific low back pain results in both physical and psychological deconditioning that traps the patient in a vicious
circle characterized by decreased physical performance, exacerbated nociceptive sensations, impaired social functioning,
work disability, and depression. The physical component of
deconditioning involves both stiffness of the lumbar spinepelvic-femoral unit, decreased muscle strength and endurance, loss of cardiorespiratory adjustment to physical exertion, and neuromuscular inhibition (kinesiophobia) [6].
* Corresponding author. Département de Médecine Physique et
Kinésithérapie-Réadaptation, ISEPK, B21, Allée des Sports 4,
B-4000 Liege, Sart Tilman, Belgium.
E-mail address: mvanderthommen@ulg.ac.be (C. Demoulin).
1297-319X/$ - see front matter © 2005 Elsevier SAS. All rights reserved.
doi:10.1016/j.jbspin.2004.08.002
Several studies suggest that patients with chronic nonspecific low back pain may benefit from an active multidisciplinary approach involving an individually tailored reconditioning program [6–8]. Tools capable of quantitating
individual deficiencies and of documenting the effects of
reconditioning would be useful. Evaluating the endurance of
trunk extensor muscles seems to have greater discriminative
validity than evaluation of maximal voluntary contractile force
[9–14].
In 1964, Hansen developed the first test for evaluating the
isometric endurance of trunk extensor muscles and validated
it in 168 healthy individuals and 90 patients who had had
surgery for low back pain within the last 3–4 weeks [15]. In
this test, the patient is in the prone position with the lower
body fixed to the examining table and the upper body extending beyond the edge of the table. The test consists in holding
the upper body horizontal as long as possible. In 1972, Troup
et al. evaluated muscle fatigability by performing surface electromyography in patients during the test [16]. After a
1984 study by Biering-Sorensen [9] published in Spine, the
test became known as the “Sorensen test”. Biering-Sorensen
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C. Demoulin et al. / Joint Bone Spine 73 (2006) 43–50
used the test together with several other evaluations in over
900 individuals and concluded that a shorter positionholding time during the Sorensen test predicted low back pain
within the next year in males.
2. Methods
We conducted a Medline search for articles reporting the
use of the Sorensen test to evaluate the back muscles. The
keywords used for the search were “Sorensen test”, “back
pain”, “muscle endurance”, “static endurance”, “muscle
fatigue”, “function test”, “back extension”, and “trunk extensors”. In addition, the reference list of each article retrieved
by the Medline search was examined for additional related
articles.
3. Description of the Sorensen test
The patient lies on the examining table in the prone position with the upper edge of the iliac crests aligned with the
edge of the table. The lower body is fixed to the table by three
straps, located around the pelvis, knees, and ankles, respectively. With the arms folded across the chest, the patient is
asked to isometrically maintain the upper body in a horizontal position (Fig. 1). The time during which the patient keeps
the upper body straight and horizontal is recorded. In patients
who experience no difficulty in holding the position, the test
is stopped after 240 s.
The Sorensen test is the most widely used test in published studies evaluating the isometric endurance of trunk
extensor muscles. It has been used either as described initially or with a number of modifications, as listed below.
• Arm position: the test has been used with the arms bent,
the elbows held out, and the hands on the ears [17], forehead [18], or nape of the neck [19,20]; in another variant,
the arms are held along the sides [13,21,22]. Because arm
position influences the location of the center of gravity,
these modifications affect the mass moment of the upper
body and, therefore, test performance [23].
• Location of the edge of the table: in several studies, the
anterior–superior iliac spines were placed at the edge of
the table, instead of the upper edge of the iliac crests
[24,25].
Fig. 1. The original Sorensen test.
• Number of straps: two [19,26–28] to five straps [14] have
been used to hold the lower body to the table; in the Roman
chair variant (Fig. 2), the feet are fixed to the device and
no straps are needed [29–34].
• Starting position: in several studies [31,34], the test was
started with the upper body sloping downward toward the
floor so that a concentric contraction of the trunk extensor
muscles was needed initially to reach the horizontal position.
• Hip flexion: in theory, the hips remain fully extended
throughout the Sorensen test. However, hip flexion was 6°
in a study by Holmstrom et al. [10] and 40° in a study by
Dedering et al. [29,30] (Fig. 3).
• Method for documenting the horizontal position of the
upper body: whereas many authors (including BieringSorensen) simply trusted a visual evaluation [10,19,21,27,
31,34–36], others used measurement devices (inclinometer [24,37,38], goniometer [18], or photoelectric cell
[10,39]) or asked the patient to maintain contact between
the back and a stadiometer or weight hanging from the
ceiling [18,40,41].
• Criteria for stopping the test: in studies that used measurement devices or contact with an object to define the horizontal position, specific test-stopping criteria were used,
such as trunk downsloping by more than 5–10° [24,37,42]
or loss of contact with the object for more than 10 s [43].
Fig. 2. Roman chair used to evaluate isometric endurance of the trunk extensor muscles.
Fig. 3. Evaluation of the trunk extensor muscles as suggested by Dedering et
al [29].
C. Demoulin et al. / Joint Bone Spine 73 (2006) 43–50
45
Table 1
Results of the Sorensen test reported in the literature
Endurance (s)
Males
Prior LBP
Healthy
Biering-Sorensen [9] n > 900
Holmstrom et al. [10] n = 203
Mannion and Dolan [35] n = 229
Hultman et al. [39] n = 148
Mannion et al. [44] n = 200
Alaranta et al. [21] n = 475
Latimer et al. [24] n = 63
Simmonds et al. [22] n = 92
198 a
171.5 a
116 a
150 a
176 b
166.7 a
LBP
Healthy
163
137.5 b
197
Females
Prior LBP
Current
LBP
177
210
142 b
136 a
No LBP
100
132.6 a
77.8 a
85 b
141.7 a
Males and females
Prior LBP
85
107.7 b
123 a
Current LBP
85*–99**
94.6 b
39.5 b
The position-holding times are given in seconds; n is the number of individuals in the study; and LBP means low back pain. * and ** indicate populations of
LBP patients with and without an impact of their disease on their daily activities. Different letters in superscript indicate significant differences (P < 0.05).
• Test duration: some individuals can hold the position for
longer than 240 s [9,15,46], and Jorgenssen and Nicolaisen suggested that the test should be continued beyond
this time [46].
The populations also varied across studies, from patients
with prior low back pain [10,24] or current chronic low back
pain [20,31,39,42] to patients with no history of low back
pain [10,24,34,35] or 15-year-old students [47,48]. In addition, several studies failed to separate data from males and
females [22,24].
As expected, these numerous methodological variations
translate into considerable discrepancies in study findings
(Table 1). Biering-Sorensen [9] and Holmstrom et al. [10]
found high mean position-holding times, not only in healthy
individuals (198 and 171.5 seconds, respectively), but also in
patients with low back pain (163 and 137.5 s, respectively).
Most other studies found lower values in healthy individuals
[21,22,24,35,44].
4. Predictive validity of the Sorensen test
Biering-Sorensen [9] reported that a position-holding time
less than 176 s predicted low back pain during the next year
in males, whereas a time greater than 198 s predicted absence
of low back pain. Importantly, the test had no predictive validity in females. In a study by Luoto et al. [13], separating the
participants into three groups based on position-holding times
showed that a time less than 58 s was associated with a threefold increase in the risk of low back pain, as compared to a
time greater than 104 s.
Sjolie and Ljunggren et al. [49] and Adams et al. [50] found
that the Sorensen test predicted low back pain in both males
and females, whereas others found no predictive value
[51–54]. Mannion et al. [44] reported that the risk of low back
pain was independent from the position-holding time but was
correlated with muscle fatigability as measured by surface
electromyography during the test. In miners, Stewart et al.
[55] found no significant differences in position-holding time
between individuals with and without a history of low back
pain.
5. Discriminative validity of the Sorensen test
In many studies, the position-holding time was significantly decreased in patients with chronic low back pain
[9,11,15,22,24,39,56,57]. This finding suggests that chronic
low back pain may be associated with decreased isometric
endurance of the trunk extensor muscles.
6. Biometric characteristics
In several studies, neither body weight [42,58] nor mass
moment of the trunk [10] influenced the position-holding time.
Other studies, however, found a negative correlation between
body weight and position-holding time [9,19,21,
27,40]. The potential influence of age and stature remains
debated [9,10,21,27]. In contrast, there is general agreement
that differences exist between males and females. With a few
exceptions [21,42,59], studies found significantly longer
position-holding times in females [9,35,40,44,60]. Similarly,
spectral analysis of electromyography signals recorded during the test indicated greater muscle fatigability in males
[14,29,40,44,58]. Several hypotheses have been put forward
to explain this gender-related difference. In females, the
weight of the upper body is less and the center of gravity of
the trunk lowers, as compared to males [9,11]. However, the
position-holding time remained longer in females wearing
weights attached to the upper body [61] or performing isometric trunk muscle endurance tests in the standing position
[11]. The greater degree of lumbar lordosis in females may
afford a mechanical advantage by lengthening the lever arm
of the spinal erector muscles [62,63]. An influence of sex hormones has been suggested also [11,35]. Nevertheless, the most
compelling hypothesis involves differences in muscle composition. Mannion et al. [64] suggested that the spinal muscles
may show better adaptation to aerobic exercise in females as
a result of a larger proportion of slow Type I fibers in the
cross-sectional muscle area.
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C. Demoulin et al. / Joint Bone Spine 73 (2006) 43–50
7. Reproducibility
The reproducibility of the Sorensen test has been evaluated, but the studies either included small numbers of individuals [10,11,30,35,60] or used the correlation coefficient r,
which is not optimal for assessing test reproducibility
[10,21,34,36,38,57]. Investigations that relied on the intraclass coefficient of correlation (ICC) usually found that reproducibility was satisfactory (ICC > 0.75) [65] both in healthy
individuals and in patients with low back pain. Simmonds et
al. [22] reported that ICC values were 0.73, 0.68, and 0.99 for
within-session, session-to-session, and intraobserver reproducibility, respectively, in healthy individuals; corresponding values in patients with low back pain were 0.91, 0.88,
and 0.99. In a study of interobserver reproducibility, Latimer
et al. found ICC values of 0.77, 0.83, and 0.88 in individuals
with prior low back pain, no symptoms, and current low back
pain, respectively [24].
Reproducibility was poorer when a Roman chair was used
for the test. In a study of 12 healthy individuals conducted by
Mayer et al. [34], the correlation coefficient was 0.2. Keller
et al. [31] reported that the coefficient of variation was
20–21% in 31 patients with low back pain and 31 healthy
individuals.
8. Validity
Although Hansen [15] described the test as a tool for evaluating back strength, studies subsequently established that it
assesses isometric muscle endurance. Furthermore, the muscle
contractions elicited by the test were found to be no greater
than 40–52% of the maximal voluntary contractile force
[10,25,35,45,66,67]. Similarly, the electromyographic activity of the spinal erector muscles rarely exceeded 40% of its
maximal value [67,68].
The Sorensen test has been misinterpreted as a specific
tool for evaluating the back muscles [13]. Published studies
demonstrate that the test assesses the endurance of all the
muscles involved in extension of the trunk, which include
not only the paraspinal muscles, most notably the multifidus
muscle [25,69], but also the hip extensor muscles. The contribution of the gluteus maximus and hamstring muscles
remains controversial [42,70,71]. Sparto et al. [72] and Arokosky et al. [69] stated that these muscles played a minor
role, whereas others reported a correlation between the
position-holding time and the time-course of hip extensor fatigability as assessed by surface electromyography, suggesting
a significant role for the hip extensor muscles [40,42].
Another challenge to the validity of the Sorensen test comes
from evidence that the position-holding time is unrelated to
the cross-sectional area of the paraspinal muscles [39,73]. An
influence of individual factors such as motivation, pain tolerance, and competitiveness has been suggested [66,74]. In individuals who stop the test because of pain or breathing problems [9,24,27,28,35], the result may not reflect muscle
performance. In this situation, a submaximal Sorensen test
coupled with electromyography or a Borg scale evaluation
might be sufficient for documenting muscle fatigability while
limiting the role for individual factors such as motivation
[29,35].
9. Sensitivity to change
The position-holding time has been reported to increase
significantly after active rehabilitation therapy [38,75]. A
training program involving dynamic exercises performed on
a Roman chair regularly over several weeks was followed by
a significant increase in the position-holding time [71],
although measurements obtained using a dedicated dynamometer showed no increase in back extensor muscle strength
[32,71].
10. Spinal loads induced by the Sorensen test
Callaghan et al. [76] estimated that the compression load
imposed on the spine during the brief Sorensen test was
4000 N, which is slightly above the value recommended by
the National Institute of Occupational Security and Health in
1981 [77].
Pain, including spinal pain, may cause the patient to discontinue the test [9,24,27,28,35]. However, no persistent
adverse effects such as pain exacerbation have been reported.
Simmonds et al. [22] found high within-session reproducibility and recorded no instances of lasting pain induced by the
test. Measurements of the lumbar curvature with the trunk in
a horizontal position showed no increase in the normal lumbar lordosis [76]. Nevertheless, the suggestion by Tsuboi et
al. [14] that the test be performed with the trunk extended 5°
above the horizontal seems inappropriate.
11. Variants of the Sorensen test
11.1. Dynamometric measurements
Tests for evaluating isometric spinal muscle endurance
using a dynamometer have been developed [11,44,78]. Jorgenssen et al. [11] reported that dynamometric measurements obtained at a fixed percentage of the voluntary maximal force (usually 50–60%) were superior over the Sorensen
test with a 4-min maximum in several ways: the influence of
anthropometric factors was smaller, reproducibility was better (with a correlation coefficient of 0.89 as compared to 0.82),
discriminative validity was greater, and the time needed for
the test was shorter, resulting in a smaller influence of motivation. However, whereas the Sorensen test requires only submaximal muscle contraction, dynamometric tests require
determination of the maximal voluntary force, which may be
inappropriate in some patients. Furthermore, pain may result
C. Demoulin et al. / Joint Bone Spine 73 (2006) 43–50
47
in underestimation of the maximal voluntary force and therefore in spurious endurance test results.
11.2. The Ito test (Fig. 4)
Ito et al. [79] developed a test for evaluating isometric
endurance of the trunk extensor muscles. The patient lies in
the prone position with a pad under the abdomen and the arms
along the sides. When a signal is given, the individual lifts
the upper body while flexing the neck as much as possible
and contracting the gluteus maximus muscles to stabilize the
pelvis. The test consists in holding this position as long as
possible (without exceeding 5 min) while breathing normally. This simple test was described by Ito et al. as inducing
less lumbar lordosis than the Sorensen test. Shirado et al. [80]
reported that maximum neck flexion together with pelvic stabilization resulted in maximum activity of the erector spinae
muscles. In a study of 190 individuals, Ito et al. [79] found
that their test discriminated between healthy controls and
patients with low back pain. Evaluation of session-to-session
reproducibility showed that the ICC was 0.97 in healthy controls and 0.93 in low-back-pain patients [79].
The Ito test does not seem to induce pain and may result in
less spinal loading than the Sorensen test. Furthermore, as
the lower body is not fixed, the contribution of the hip extensor muscles may be smaller [68]. Factors that limit international recognition of the Ito test include the absence of studies, the absence of a standardized test procedure (type of pad,
extent of upper-body lifting, etc.), and the theoretical risk of
exaggerating the degree of lumbar lordosis.
11.3. McIntosh tests
Moreau et al. [81] noted that two other tests have been
described in the literature, for evaluating the isometric endurance of the upper and lower spinal extensor muscles, respectively. However, the only published study of these tests [82]
fails to provide information on validity, safety, and potential
use in low-back-pain patients.
11.4. Dynamic evaluations
Repetitive arch-up tests provide a dynamic evaluation of
the trunk extensor muscles without requiring the use of a dynamometer [21,59,83–86]. The position is derived from the
Sorensen test. The test consists in flexing the trunk at 45°
then returning to the horizontal position as many times as
possible, at a rate of one arch-up every 2–3 s.
Moreland et al. [28] used a test in which the lower limbs
were fixed to a triangular pad and the patient was asked to
Fig. 4. The Ito test [79].
Fig. 5. Moreland et al.’s [28] dynamic test for evaluating the trunk extensor
muscles.
flex the trunk so as to touch the table with the nose then to
return to the horizontal position at a rate of 25 arch-ups per
minute (Fig. 5). In a study by Uderman et al. [87], the test
was done on a Roman chair and the patients asked to arch up
repeatedly over a 90° angle.
Whereas the static version of the Sorensen test has been
widely used in published studies, the dynamic variant has
received less attention. Available data suggest that results may
be similar in males and females [61] and that the discriminative validity may be decreased as compared to the static test
[21,59]. As with the static test, the occurrence of pain or
cramps may limit the validity of the test [28,83]. In addition,
the contribution of the hip extensor muscles increases gradually from one repetition to the next, further limiting the validity of the test [88,89]. Thus, in-depth studies are needed to
evaluate the usefulness and limitations of these dynamic tests.
12. Conclusions
The Sorensen test allows for a rapid, simple, and reproducible evaluation of the isometric endurance of the trunk
extensor muscles. It discriminates between healthy individuals and patients with low back pain and may predict the occurrence of low back pain in the near future. Although the
Sorensen test has been extensively studied, the better performance among females remains partly unexplained and the
contribution of the hip extensor muscles is unknown. The
absence of a single standardized test procedure is an impediment to comparative studies. The role for motivation, the fact
that pain can lead patients to stop the test, and the impossibility of quantifying the relative muscle strength developed
by the individual constitute the major shortcomings of the
Sorensen test. Nevertheless, data in the literature argue in favor
of the Sorensen test for evaluating the isometric endurance of
the trunk extensor muscles. The Ito test and the dynamic variants of the Sorensen test need to be investigated further.
48
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