Self-reported pain and disability outcomes from an endogenous model of muscular back pain

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Self-reported pain and disability outcomes from an endogenous model of muscular back pain
Series Title:
BMC Musculoskeletal Disorders 2011,12:35 doi:10.1186/1471-2474-12-35
Bishop, Mark
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BMC Musculoskeletal
Biomed Central
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Background: Our purpose was to develop an induced musculoskeletal pain model of acute low back pain and examine the relationship among pain, disability and fear in this model. Methods: Delayed onset muscle soreness was induced in 52 healthy volunteers (23 women, 17 men; average age 22.4 years; average BMI 24.3) using fatiguing trunk extension exercise. Measures of pain intensity, unpleasantness, and location, and disability, were tracked for one week after exercise. Results: Pain intensity ranged from 0 to 68 with 57.5% of participants reporting peak pain at 24 hours and 32.5% reporting this at 48 hours. The majority of participants reported pain in the low back with 33% also reporting pain in the legs. The ratio of unpleasantness to intensity indicated that the sensation was considered more unpleasant than intense. Statistical differences were noted in levels of reported disability between participants with and without leg pain. Pain intensity at 24 hours was correlated with pain unpleasantness, pain area and disability. Also, fear of pain was associated with pain intensity and unpleasantness. Disability was predicted by sex, presence of leg pain, and pain intensity; however, the largest amount of variance was explained by pain intensity (27% of a total 40%). The second model, predicting pain intensity only included fear of pain and explained less than 10% of the variance in pain intensity. Conclusions: Our results demonstrate a significant association between pain and disability in this model in young adults. However, the model is most applicable to patients with lower levels of pain and disability. Future work should include older adults to improve the external validity of this model.
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Bishop et al. BMC Musculoskeletal Disorders 2011, 12:35

BMusculoskeletal Disorders
Musculoskeletal Disorders

Self-reported pain and disability outcomes from

an endogenous model of muscular back pain

Mark D Bishop13*, Maggie E Horn2, Steven Z George13, Michael E Robinson3'4

Background: Our purpose was to develop an induced musculoskeletal pain model of acute low back pain and
examine the relationship among pain, disability and fear in this model.
Methods: Delayed onset muscle soreness was induced in 52 healthy volunteers (23 women, 17 men; average age
22.4 years; average BMI 24.3) using fatiguing trunk extension exercise. Measures of pain intensity, unpleasantness,
and location, and disability, were tracked for one week after exercise.
Results: Pain intensity ranged from 0 to 68 with 57.5% of participants reporting peak pain at 24 hours and 32.5%
reporting this at 48 hours. The majority of participants reported pain in the low back with 33% also reporting pain
in the legs. The ratio of unpleasantness to intensity indicated that the sensation was considered more unpleasant
than intense. Statistical differences were noted in levels of reported disability between participants with and
without leg pain.
Pain intensity at 24 hours was correlated with pain unpleasantness, pain area and disability. Also, fear of pain was
associated with pain intensity and unpleasantness. Disability was predicted by sex, presence of leg pain, and pain
intensity; however, the largest amount of variance was explained by pain intensity (27% of a total 40%). The
second model, predicting pain intensity only included fear of pain and explained less than 10% of the variance in
pain intensity.
Conclusions: Our results demonstrate a significant association between pain and disability in this model in young
adults. However, the model is most applicable to patients with lower levels of pain and disability. Future work
should include older adults to improve the external validity of this model.

Musculoskeletal pain is the most common form of
chronic or recurrent pain [1,2] that has a high societal
cost [3], making it a public health priority [1,2,4].
Current literature suggests that there is a potentially
complex interaction of factors that contribute to low
back pain (LBP). Psychological factors, for example, pro-
long recovery and may predict disability for patients
with LBP [5]. One limitation to clinical studies is that
there can be little experimental control of the pain
experience. Using a model of experimentally induced
pain, the type of the pain stimulus, as well as the general
area where pain is experienced can be controlled [6].

* Correspondence bish@ufl edu
'Department of Physical Therapy, University of Florida, Gainesville, Florida,
Full list of author information is available at the end of the article

O BioMed Central

Exercise, an endogenous method of inducing muscle
pain, can produce pain during [7] and after activity [8].
Performance of eccentric (lengthening) muscle actions
in muscles unaccustomed to such forces causes damage
to muscle fibers. Pain, hyperalgesia, allodynia, edema,
and weakness can also result [9-13]. These symptoms
and signs typically completely resolved within two
weeks [14,15] and are referred to as delayed onset mus-
cle soreness (DOMS). DOMS has been reported in the
trunk muscles after floorball training [16] and perfor-
mance of the tests associated with a functional capacity
evaluation [17,18]. However, each of these models
involved significant time to induce the DOMS. For
example, participants in the study by Hjortskov et al,
played handball for two hours while participants in
work described by Soer et al completed a functional
capacity examination, a procedure that involves twelve
separate strength tasks.

2011 Bishop et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (, which permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.

Bishop et al. BMC Musculoskeletal Disorders 2011, 12:35

In DOMS models that involve peripheral (non-
axial) muscles common protocols target a primary
muscle or muscle group in which the DOMS is to be
induced. Active trunk exercises have been used to
generate DOMS in the trunk [19,20]. Therefore we
had two general goals to extend previous work in this
area. Our first goal was to describe the pain and dis-
ability outcomes resulting from DOMS to establish
the external validity of DOMS as a model of LBP. We
aimed to quantify pain intensity and self-report of
disability, examine the distribution of any pain that
might occur expecting that it would be in anatomic
regions consistent with LBP reported by patients
seeking intervention for their pain, and characterize
the quality of any pain reported. Additionally we
sought to identify whether peripheral sensitization
occurred following DOMS in the trunk, as indicated
by reductions in mechanical pain thresholds over the
targeted muscles.
Our next goal was to determine the relevance of this
model to patients with low back pain. We hypothesized
that there would be a relationship between pain inten-
sity and self-report of disability [21-23] and we also
expected variability in pain intensity to be related to
psychological factors, such as pain related fear, and psy-
chophysical measures [24,25]. Then we compared pain
and disability reports by participants in the current
study to a group of age-matched patients seeking inter-
ventions for LBP. Demographics and the characteristics
of the patients' pain experiences were tracked as part of
a separate randomized trial of interventions for LBP
[26]. We hypothesized that there would be overlap
between the two groups (healthy participants with
DOMS and patients with LBP) in reports of pain inten-
sity. The overriding rationale for these hypotheses was
that if measures collected after DOMS were consistent
with observations made of patients seeking intervention
for LBP this would support the external validity of the

52 healthy pain-free volunteers (23 women, 17 men;
average age 22.4 years; average BMI 24.3) read and
signed an informed consent form approved by the
University Institutional Review Board. Participants were
excluded if they met any of the following criteria: pre-
vious participation in a conditioning program specific to
trunk extensors, any current back pain, any chronic
medical conditions that may affect pain perception,
kidney dysfunction, major psychiatric disorder, history
of previous injury including surgery to the lumbar spine,
cardiac conditions, osteoporosis, or liver dysfunction, or
performance of any intervention for symptoms induced

by exercise before the termination of their participation
in the protocol.

All tests and measures were collected in a research
laboratory setting before exercise and at 24, 48,
96 hours and one week after the exercise protocol was
administered. In addition, participants complete pain
and disability questionnaires 2 weeks, 4 weeks and
12 weeks after exercise to examine any long term effects
of participation.
Pain intensity, unpleasantness, and location
Pain intensity was measured with a visual analog scale
(VAS) consisting of a 100 mm line anchored at one end
with "none" and at the other with "'worst imaginable."
A previous study has indicated that the VAS is a valid
ratio measure [27]. Participants rated "worst pain in the
back or legs today" by placing a mark along the
100 mm line. The VAS for pain unpleasantness con-
sisted of a 100 mm line with anchors of "not at all
unpleasant" and "most unpleasant imaginable." Pain
intensity measures the sensory-discriminative dimension
of pain, and pain unpleasantness measures the affective-
cognitive dimension of pain [28]. The affective ratio
(unpleasantness divided by intensity) provides informa-
tion about the quality of the pain [29]. Clinical pain
often is associated with unpleasantness being similar to,
or greater than, intensity [30]. Participants also com-
pleted pain drawings to indicate the spatial distribution
of any back or leg pain. Data were coded to indicate if
the participant reported any leg pain defined in this
study as pain below the gluteal fold.
Self-report of low back-related disability was assessed
with a modified version of the Oswestry Disability Ques-
tionnaire (ODQ) [31]. This ten-item questionnaire has a
range of 0 (no disability due to back pain) to 100 (com-
pletely disabled due to back pain) and is typically
reported in percentages. The ODQ has been extensively
used as a measure of disability in those patients with
LBP during clinical management and experimental stu-
dies. The modified ODQ has very good test-retest
repeatability [31] and a recent consensus statement
included the ODQ as an important measure of physical
functioning in patients with LBP [32].
Pain-related fear
Two questionnaires were used to measure pain-related
fear. The Fear of Pain Questionnaire (FPQ) is a 30-item,
5-point rating scale with a range from 30 to 150, devel-
oped to measure fear about specific situations that may
cause pain. We used the total score for the FPQ, as we
were most interested in measuring participants' general
fear of pain [33]. High levels of internal consistency
(>0.8) have been reported in clinical and non-clinical

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Bishop et al. BMC Musculoskeletal Disorders 2011, 12:35

groups of participants [33,34]. This questionnaire was
used because it can be completed by healthy, pain free
participants and we have found it to be associated with
experimental pain responses in our previous studies
A shortened version of the Tampa Scale of Kinesio-
phobia (TSK-11) was used to assess the fear of move-
ment or injury [35]. The original TSK included 17
questions and was used in multiple studies in partici-
pants with low back pain [36-38]. The TSK-11 provides
a measure of 2 separate domains: somatic focus and fear
of reinjury [39]. The TSK-11 excludes 6 questions from
the original Tampa Scale of Kinesiophobia (TSK). The
TSK-11 has demonstrated similar factor structure, relia-
bility (ICC = 0.82), and validity to the original version of
the TSK [35]. The items are scored on a 4-point scale
from 1 (strongly disagree) to 4 (strongly agree). Lower
scores on the TSK-11 indicate less pain-related fear of
Pain Sensitivity
Mechanical Pressure Threshold (MPT): Lowered
mechanical pain threshold has been previously reported
in DOMS models suggesting sensitization of muscle
nociceptors in response to mediators of the inflamma-
tory process [6,40].Tenderness in paraspinal muscle tis-
sue was assessed using a hand-held dynamometer
(Microfet 2, Hoggan Health Industries, Inc, West
Jordan, UT). The tip of the dynamometer is equipped
with a rubber foot-plate of 1-cm diameter. During test-
ing, the participant was positioned in prone and force
was applied until the participant reported that the sen-
sation changed from pressure to pain. At that point the
participant rated pain they experienced using a numeric
rating scale (NRS) anchored at 0 (no pain sensation at
all) and 100 (worst pain imaginable) and the applied
force to reach this threshold was recorded in kg-force.
Threshold measures were evoked in the paraspinal mus-
cles bilaterally 2.5 cm from the spinous processes of L1,
L5 and S2 for a total of six ratings. These were averaged
to provide a single measure of MPT. Between session
reliability for MPT has been shown to be high (ICC >
0.87) in the posterior trunk muscles [41].

Exercise Protocol
Prior to exercise all participants completed a submaxi-
mal effort warm-up session consisting of riding the sta-
tionary bicycle at a speed of 50-60 RPM and 1 Kp of
resistance and static passive stretching (held for 30 sec-
onds) of the lower extremities and posterior trunk. Par-
ticipants performed an isometric (static) test of total
torque of the trunk extensor muscles through their
available trunk flexion range of motion (ROM) using a
MedX lumbar extension exercise machine following the
standardized protocol [42]. The repeatability of

isometric torque production is well-established in parti-
cipants without pain [42] and groups of patients with
LBP [43]. Participants were seated in the MedX machine
with the stabilizing straps attached across the pelvis and
knees. The participant was moved through the ROM of
the machine in lumbar flexion and extension to deter-
mine his or her available ROM. The device was locked
into place in maximal flexion and the participant was
instructed to build up force gradually against a pad in
contact with the mid and lower back. The torque gener-
ated by the participant was displayed graphically on the
data collection computer. Once peak effort was observed
by the research assistant, the participant was instructed
to relax, the device released and the participant returned
to an upright position for at least 10 seconds. Isometric
testing was administered seven times in positions that
ranged from the participant's maximum available trunk
extension to maximal trunk flexion. The isometric tor-
que collected at each test angle was summed to give
measure of total torque produced across the entire
range of motion.
After baseline total torque was recorded, participants
performed bouts of dynamic exercise to the point of
volitional fatigue. To perform the exercise bout, the par-
ticipants were seated and restrained in a MedX lumbar
extension exercise machine. Participants performed as
many repetitions as possible using a weight load equal
to approximately 80% of the peak torque measured dur-
ing the isometric test. Each repetition was performed
through the full available ROM and the participants
were encouraged to perform the lifting portion (con-
centric) in two seconds and the lowering (eccentric) in
four seconds. Repetitions continued until the patient
reported being unable to move through a full range of
motion (volitional fatigue). Once this occurred, the iso-
metric torque test was performed again. If the total tor-
que measured during the repeat isometric test was 50%
or less of the baseline total torque, the protocol was
complete. If this didn't occur, the exercise bout was
repeated. Participants repeated this sequence of dynamic
exercise and isometric testing until total measured tor-
que decreased to 50% of the baseline measurement. Par-
ticipants were instructed not to initiate any medication
in the next 48 hours, or apply any intervention, such as
ice or heating pacs to the lumbar spine.

Descriptive statistics were generated for all baseline
Effect of DOMS
The effect of exercise on pain intensity and disability
was compared over time using separate repeated mea-
sures analysis of variance (ANOVA) models. We specifi-
cally expected pain reports to peak at 24 or 48 hours

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Bishop et al. BMC Musculoskeletal Disorders 2011, 12:35

and return to baseline over time. Additionally, we calcu-
lated the affective ratio to examine the change in quality
of the pain. These ratios represent the ratio of unplea-
santness to intensity and were calculated beginning at
24 hours. To examine changes in pain sensitivity that
occurred in response to DOMS we used a repeated-
measures ANOVA. We also compared participants with
and without leg pain on measures of pain, disability and
Changes in muscle performance were also examined.
A two-way repeated measures ANOVA was used to
examine whether decrements in isometric torque pro-
duction were specific to a particular angle or angles, or
whether changes occurred across all testing angles
equally. Additionally the total isometric torque was
tracked to determine the timeframe over which muscle
performance returned to baseline levels of performance.
Model Validity
Next we examined the extent to which pain reports
explained self-reported disability after the induction of
pain. We used the values of pain and disability at
24 hours following the induction of muscle pain for
these analyses. First, zero order correlations were calcu-
lated among the potential predictor and outcome vari-
ables. Sex differences in pain intensity and disability
were tested using independent t-tests. The first model
was fit for disability using the ODQ as the dependent
variable. We built a hierarchical regression model using
demographic variables as the first block. The second
block consisted of psychological measures for which the
zero order correlation with pain was significant at p <
0.1. Pain intensity was entered as the final block. Hier-
archical regression was chosen to allow us to examine
the contributions of each 'block' to the overall variation
in the dependent variable. We examined the change in
R2 and standardized betas to determine significant con-
tributions to the variance.
A second model was build to examine factors that
explained variability in pain intensity after exercise. The
first block entered consisted of demographic variables.
The second block consisted of psychological measures
for which the zero order correlation with pain was sig-
nificant at p < 0.1.
Our last test of validity was to compare the experi-
mentally induced LBP to clinical LBP. Reported pain
intensity from participants in our study was compared
to age matched patients with low back pain from a pre-
vious clinical trial [26] using Mann-Whitney U test.
Demographic details of patients in the clinical trial are
summarized in Table 1.
Repeated measures ANOVAs were performed using
StatView 5.0.1 (SAS Institute, Cary, NC, USA), and
regression analyses were performed using SPSS for Win-
dows 15.1. Type 1 error was maintained at 5%.

Demographic and descriptive data are summarized in
Table 1. 52 participants completed the evaluations with-
out drop-outs and no adverse events were reported. 38
participants indicated that they had not performed any
type of regular exercise (more than one a week) for the
past month. Five indicated participating in weight-train-
ing (no training of the trunk extensors), and seven indi-
cated that they performed aerobic exercise (running,
walking, or bicycling) twice a week.

Effects of DOMS
There was a significant main effect of time for pain
intensity in the back or legs (F7,58 = 45.7, p < 0.001).
The general time course of DOMS is shown in
Figure 1. 30 participants reporting peak pain at
24 hours and 17 reporting this at 48 hours. One parti-
cipant reported peak pain at 96 hours after exercise
and 4 participants reported no pain at any time per-
iod.18 participants reported pain in the back and legs.
No participants reported pain below the knee at any
time point. A sample pain diagram is shown in Figure
2 illustrating the distribution of pain reported by this
participant at 48 hours. The pain intensity for 45 of
participants had returned to zero within one week
after the induction of DOMS and only 1 participant
reported pain at 2 weeks (10 mm). Similarly, the distri-
bution of disability scores increased from baseline to
24 hours and then returned to zero over time (F7,58 =
13.2, p < 0.001). MPT at 24 and 48 hrs was signifi-
cantly lower than baseline (p = 0.004 and p = 0.012,
respectively) and returned to baseline by 96 hours.
These values are shown Table 2.

Table 1 Characteristics of participants performing the
exercise protocol and age matched-patients with LBP
Healthy Patients with L
participants [26] (n = 33
(n = 52)
Variable Median Range Median Ran
Age (years) 23 18,32 24 18,3
Sex (% female) 31 60% 22 66%

African American
Native American
Hawaiian/Pacific Islander
More than one race
Pain intensity
Fear of pain (FPQ-111)
Kinesiophobia (TSK)



42 81%
5 10%

1 3%
0 0%
1 3%

0, 63
0, 30
30, 110
11, 26

50 0, 70
28 0, 74

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Bishop et al. BMC Musculoskeletal Disorders 2011, 12:35

Table 2 Changes in Mechanical Pain Threshold
Force (kgf) Rating (NRS)
Baseline 28.8 + 1.4 23.1 + 2.3
24 hours 236 + 13 26,9 + 2,6

48 hours
96 hours

24.8 + 1.2
27.7 + 1.2

Baseline 24 48 96 1wk 2wk lmth 3mth

Figure 1 Pain intensity reported after induction of DOMS

We assessed the affective ratio over the course of one-
week. The affective ratio did not follow the same pattern
as pain intensity and disability; however there were dif-
ferences across testing sessions (F1,51 = 8.81, p = 0.004).
The ratio of unpleasantness to intensity statistically
increased from 24 to 48 hours (p = 0.010) and remained
elevated one week after the exercise protocol (p =
0.007) following a different time course from measures
of intensity only. The ratio of unpleasantness to inten-
sity remained greater than one indicating that the sensa-
tion was considered more unpleasant than intense.

When immediate changes in muscle performance after
exercise were examined, a significant interaction was
noted between time and angle (F6,312 = 15.01, p < 0.001).
The greatest decrements in isometric torque production
occurred at test angles 12, 24 and 72 degrees correspond-
ing to the mid-range of trunk motion and in full trunk
flexion, respectively. Post-hoc testing indicated that the
decrement in torque at 12 degrees and at 24 degrees was
largerer than in the loss of torque in full extension. The
percent deficit at each angle is shown in Figure 3. Total
isometric torque remained depressed at one week after
the exercise protocol was performed and returned to
baseline totals by two weeks after exercise (Figure 4).

Model validity
The pain intensity at 24 hours for the participants ran-
ged from 0 to 63 on the VAS. Statistical differences
were noted in levels of self-reported disability when
comparing participants with and without leg pain (parti-
cipants reporting leg pain had higher levels of self-
reported disability) but no differences were noted in
pain intensity.
The zero order correlations among demographic, psy-
chological, and pain variables, and self-reports of disabil-
ity at 24 hours are summarized in Table 3. Pain intensity
at 24 hours was correlated with pain unpleasantness,

Page 5 of 10

24.1 + 2.4
22.7 + 2.2

0 12 24 36 48 60 Full flex









Figure 3 Decrements in isometric torque production
immediately following the exercise protocol. The line represents
differences among testing angles (p < 0.002 after Bonferroni

Bishop et al. BMC Musculoskeletal Disorders 2011, 12:35

pain area and self-reports of disability. Also, fear of pain
was associated with pain intensity and pain
Our first model, predicting self-report of disability at
24 hours, included sex, as the only demographic vari-
able, the presence of leg pain, and pain intensity. Each
variable contributed unique amounts of variance to the
final model predicting disability as indicated by the sig-
nificant changes in the adjusted R2 (see Table 4). How-
ever, the largest amount of variance in disability, an
additional 27%, was explained by pain intensity after
controlling for sex and the presence of leg pain. The
final model accounted for 40% of the total variance in
self-reported disability and is shown in Table 4.
No other predictor variables of interest were related to
pain-intensity except fear of pain; therefore no regres-
sion analyses were performed. The association between
fear of pain and pain intensity is shown in Table 3.
Thirty-four patients in the comparison clinical trial [26]
were between the ages of 18 and 32 years. Demographic
details of this group of patients is presented in Table 1.
Data from these patients was compared to the reports of
pain from participants in this DOMS study. Overall, the
reports of pain after exercise were significantly lower
than the reports of pain of the patients (U = 256.5, p <
0.001). Visual inspection of the quartile ranges in both
data sets suggested that the highest pain intensity reports
of the experimental group (50th percentile and greater)
overlapped with the lowest pain reports (50th percentile
or lower) of the patients seeking intervention for their
back pain 16 to 68 mm, and 0 to 50 mm respectively.

We induced acute pain using an exercise protocol to
create DOMS in the low back and followed participants
for 12 weeks to collect pertinent outcomes. DOMS

models are potentially very useful because they mimic
musculoskeletal pain in loss of range of motion, pain
with movement and self care behaviors [12,13,44]. The
reports of pain generated in this current study followed
a time course consistent with other DOMS models with
pain peaking at 24 or 48 hours and resolving within
approximately one week after exercise [45,46].
The reported pain intensity ranged from 0 to 68 mm
in the low back and, sometimes, legs. These anatomic
areas are consistent with regions described by patients
seeking intervention for LBP. In patients with low back
and leg pain, the leg pain may be referred from ana-
tomic structures in the lumbar spine. For example,
structures that have been demonstrated to cause referral
of pain into the posterior thigh include the dura and
nerve roots [47] as well as the intraspinous ligament
[48] and the lumbar multifidii [49,50]. Magnetic reso-
nance imaging of the posterior trunk muscles after per-
forming the exercise protocol used in this manuscript
indicates that primary changes identified after exercise
occur in the lumbar multifidii (paper in review). The leg
pain in our current study may also have resulted from
the sustained isometric contractions performed by mus-
cles of the lower extremity during the exercise protocol.
While these muscles were not subjected to the forces
that occurring during eccentric (lengthening) actions,
there is evidence to suggest that restricted blood flow to
a working muscle may cause DOMS in that muscle [51].
However, this range of reported pain intensity is lower
than that reported by Udermann et al, 2002 who also
used trunk extension exercise to cause DOMS. These
authors present data indicating that five participants
who completed 50 repetitions of trunk extension using a
weight that was 100% of the isometric maximum
reported pain intensity of approximately 8 using a
10 cm VAS that peaked at 24 hours after exercise. This
exercise intensity was somewhat greater than we used in
our model although no information regarding anchors
on the VAS or confidence intervals reported, and no
error bars are shown in the graph. When developing the
model used in the current study we were very cautious
about the magnitude of the loading being placed on the
lumbar spines of the participants. This was a concern of
the funding agency and of our own institutional review
board. Consequently, our protocol is less aggressive than
is seen in models developed in the extremities and
included both concentric and eccentric muscle actions.
Additionally participants in our current study were
asked to rate pain that they were experiencing at rest,
not during movement or activity. These factors com-
bined may explain why the pain intensity reported is
less than other models of DOMS.
Consistent with other DOMS models, participants
experienced an increase in the affective-cognitive

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Bishop et al. BMC Musculoskeletal Disorders 2011, 12:35

Table 3 Bivariate associations among measures
Pain Unpleasantness

Pain intensity
Pain Unpleasantness
Pain Area


Disability: Oswestry Disability Questionnaire.
BMI: Body Mass Index.
TSK: Tampa Scale of Kinesiophobia.
FPQ: Fear of Pain.
*- p < 0.05, **-p < 0.01.

component of the pain experience
[44,52]. The affective ratio showed a
pattern of change over time. At eac]
ratio was greater than one indicating t
station was perceived as more unpleas
This also supports model clinical releva
ical pain often is associated with unpl
similar to, or greater than, intensity. TI
at 48 hours and remained so at the oni
By this time, pain intensity had decrease
most participants. Fields et al, 1999 h:
for a sensation to be recognized as pa
be unpleasantness associated with the
sity. We did not specifically query parti
quality of any ongoing sensation (other
ness) that they may have been feeling
legs but we speculate that participants
ued to experience 'soreness' but not pai
Additionally, participants reported
induction of DOMS using an outcome
clinical studies of patients with LBP. I
intensity was the primary predictor of
disability. This is relationship is also
clinical populations. For example, in
pain this correlation was 0.69 [21], com
in our study. The presence of leg

Table 4 Model Predicting Self-report of
Step Beta Sig. Adjuste

1 (Constant) 2.08
Sex 3.03
2 (Constant) 0.61
Sex 3.52
Leg pain 3.53
3 (Constant) -2.31
Sex 3.36
Leg pain 2.85
Pain intensity 0.20


Page 7 of 10

Pain Area Disability Age BMI TSK FPQ



0.06 0.26*
0.17 0.35*
0.05 <0.01
0.10 0.20
-0.02 -0.11
-0.15 003

unpleasantness) associated with disability in our model of LBP and this
slightly different finding is likewise consistent with the clinical literature
h time point the regarding LBP. Selim et al, 1998 compared groups of
hat the pain sen- patients with LBP who were then further classified by
ant than intense, the location of leg pain (none, to the knee, below the
nce because clin- knee). These authors reported that disability increased
easantness being as the involvement of leg pain increased [53].
ie ratio increased However, the range of scores using the self-report of
e week follow-up, disability reported in our study was 0 to 30. This range
ed to baseline for suggests that, while participants perceived some disabil-
as suggested that ity from DOMS, it was a minimal amount of disability.
inful there must This is not unexpected given that participants are told
sensation inten- during the informed consent process to expect the pain
cipants about the that they experience to be of short duration and instruc-
than unpleasant- tional set regarding what to expect after a procedure has
in their back and been demonstrated to influence the outcome after that
may have contin- procedure [54]. However, this is also a strength of the
in per se. model as there were a range of pain intensity values
I disability after generated and it may be unethical to have induced mod-
measure used in els that cause high disability conditions. Such a model -
n our study, pain long duration of high pain and disability would no
this self-report of longer be a model per se but rather the clinical condi-
consistent with tion being modeled!
nurses with back Lowered mechanical pain threshold has been reported
[pared to r = 0.50 in DOMS models involving differing somatic locations
pain was also such as the biceps [29], shoulder [55], and hand [10].
A proposed mechanism of DOMS is that muscle noci-
Disability ceptors become sensitized in response to mediators of
d R2 Sig. F Change the inflammatory process, thereby lowering stimulus
0.06 0.035 thresholds (peripheral sensitization) [6,40]. The finding
of local reduction in mechanical pain threshold after
DOMS was consistent with other studies of DOMS
[22,56]. In addition, isometric torque producing ability
of the trunk muscles remained impaired for approxi-
mately 2 weeks after exercise. These findings in combi-
nation confirm that the exercise protocol produced
muscle changes in the target muscles.
Pain intensity in our model was only related to base-
line fear of pain and not to kinesiophobia as measured

Bishop et al. BMC Musculoskeletal Disorders 2011, 12:35

by the TSK. The most likely explanation for this finding
is the type of participants in our study. The version of
the TSK used in our study was TKS-11. This question-
naire includes questions about the current pain experi-
enced by the individual completing the form. All our
volunteers were pain-free and none had experienced
back pain or had an accident. More recently the TSK-
general has been published. This may have been a more
appropriate measure of kinesiophobia in our partici-
pants. In contrast, the FPQ measures general fear of
pain and may represent a trait measure of fear.
Our findings are also different from those of Soer
et al, 2009. These authors report that sex was a signifi-
cant predictor of pain intensity in their model of DOMS
after an FCE. For our study, sex predicted the self-report
of disability, not pain.
LBP is characterized by heterogeneous mechanisms of
onset and pathoanatomic pain generators. Additionally,
multiple studies have indicated that identification of
pathoanatomy is difficult because of the many condi-
tions that are also present in asymptomatic participants
[57-63]. Development of an exercise-induced model of
LBP is essential therefore to better control experimental
investigations related to management of LBP. We
believe that DOMS models will be useful for future stu-
dies of LBP. With these models we can test participants
when they are pain free and examine factors that contri-
bute to their pain experience after induction of LBP. In
addition, we have the potential to establish comparison
groups in whom the mechanism of onset is consistent
resulting in the ability to use true experimental design
in studies of LBP potentially allowing us to test
responses to intervention in a homogenous LBP model.
Also, there were participants who reported that they
experienced no pain in the back or legs after performing
the exercise protocol. Consequently, this presents the
opportunity to study potential factors that might be pro-
tective of experiencing DOMS. This finding suggests
additional relevance of our model because not all indivi-
duals who experience the same stress or stimulus go on
to develop and complain of LBP. This is not possible in
clinical studies and remains a novel part of this current
Models of endogenous muscle pain are particularly
useful because they mimic the most common form of
chronic pain, musculoskeletal pain [6,64]. Other endo-
genous experimental models of LBP have been recently
presented that use injection of saline into spinal liga-
ments [65] and mulitifidus muscles [66]. Models using
DOMS, such as presented in this current study and
others [17-20], may have more clinical validity than
these injection models given that DOMS is non-invasive
and the course of symptoms lasts for days rather than
hours or minutes potentially mimicking LBP for which

an individual might seek intervention more closely.
There was overlap between reports of pain of the high-
est 50% of participants with DOMS in our study (16 to
68) and the lowest 50% of the patients seeking interven-
tion (0 to 50) [26] suggesting that the model may have
the best comparison to patients with lower levels of
pain and disability.
One important consideration, however, is that pain
related to DOMS is likely to be of shorter duration than
clinical pain. Additionally, pain was generated in this
study in young healthy participants. Given the limited
age range of participants in our study, future work will
examine the development of pain in older adults to
improve the external validity of DOMS as a model of
LBP for which a patient might seek intervention. Other
factors, such as catastrophizing, will need to be included
in follow-up studies to better understand psychological
influence and other work suggests an interaction
between genetic and psychological factors contributes to
pain and disability in the upper extremity [67]. Another
limitation to consider is that we assume that the exer-
cise protocol is the cause of the reported of pain inten-
sity and disability at 24 and 48 hours. Without a group
of participants performing sham exercise or in a control
group we cannot be completely certain that the results
of our study are attributable to participation in exercise.

None the less, our results suggest overlap with clinical
LBP for self-reports of pain as well as demonstrating
association between pain and disability as would be
expected in clinical pain. This model therefore may be
most applicable to younger adults with lower levels of
pain and disability. This work also supports the concept
of multiple factors interacting to explain the develop-
ment of pain and disability in LBP; however larger sam-
ples will be needed to develop a complete model.

We acknowledge the assistance of Lauren Bernloehr, Nicholas DiSarro,
Adrienne Driggers and Carlos Riveros during data collection for this study
This research was supported by a University of Florida Opportunity Fund
grant, and funding from the National Institute of Arthritis and
Musculoskeletal and Skin Diseases (K01 AR054331-01A2) Publication of this
article was funded in part by the University of Florida Open-Access
Publishing Fund

Author details
'Department of Physical Therapy, University of Florida, Gainesville, Florida,
USA Rehabilitation Doctoral Program, College of Public Health and Health
Professions, University of Florida, Gainesville, Florida, USA Center for Pain
and Behavioral Health, University of Florida, Gainesville, Florida, USA
4Department of Clinical and Health University of Florida,
Gainesville, Florida, USA

Authors' contributions
MDB conceived, and participated in the design of, the study; procured
funding; participated in data collection; performed and interpreted statistical

Page 8 of 10

Bishop et al. BMC Musculoskeletal Disorders 2011, 12:35

analyses; and helped to draft the manuscript MEH participated in data
collection and helped to draft the manuscript SZG conceived, and
participated in the design of, the study; interpreted statistical analysis; and
helped to draft the manuscript MER conceived, and participated in the
design of, the study
All authors read and approved the final manuscript

Competing interests
The authors declare that they have no competing interests

Received: 13 August 2010 Accepted: 2 February 2011
Published: 2 February 2011

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Cite this article as: Bishop et al Self-reported pain and disability
outcomes from an endogenous model of muscular back pain. BMC
Musculoskeletal Disorders 2011 1235

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RESEARCHARTICLEOpenAccess Self-reportedpainanddisabilityoutcomesfrom anendogenousmodelofmuscularbackpain MarkDBishop 1,3* ,MaggieEHorn 2 ,StevenZGeorge 1,3 ,MichaelERobinson 3,4 Abstract Background: Ourpurposewastodevelopaninducedmusculoskeletalpainmodelofacutelowbackpainand examinetherelationshipamongpain,disabilityandfearinthismodel. Methods: Delayedonsetmusclesorenesswasinducedin52healthyvolunteers(23women,17men;averageage 22.4years;averageBMI24.3)usingfatiguingtrunkextensionexercise.Measuresofpainintensity,unpleasantness, andlocation,anddisability,weretrackedforoneweekafterexercise. Results: Painintensityrangedfrom0to68with57.5%ofparticipantsreportingpeakpainat24hoursand32.5% reportingthisat48hours.Themajorityofparticipantsreportedpaininthelowbackwith33%alsoreportingpain inthelegs.Theratioofunpleasantnesstointensityindicatedthatthesensationwasconsideredmoreunpleasant thanintense.Statisticaldifferenceswerenotedinlevelsofreporteddisabilitybetweenparticipantswithand withoutlegpain. Painintensityat24hourswascorrelatedwithpainunpleasantness,painareaanddisability.Also,fearofpainwas associatedwithpainintensityandunpleasantness.Disabilitywaspredictedbysex,presenceoflegpain,andpain intensity;however,thelargestamountofvariancewasexplainedbypainintensity(27%ofatotal40%).The secondmodel,predictingpainintensityonlyincludedfearofpainandexplainedlessthan10%ofthevariancein painintensity. Conclusions: Ourresultsdemonstrateasignificantassociationbetweenpainanddisabilityinthismodelinyoung adults.However,themodelismostapplicabletopatientswithlowerlevelsofpainanddisability.Futurework shouldincludeolderadultstoimprovetheexternalvalidityofthismodel. Background Musculoskeletalpainisthemostcommonformof chronicorrecurrentpain[1,2]thathasahighsocietal cost[3],makingitapublichealthpriority[1,2,4]. Currentliteraturesuggeststhatthereisapotentially complexinteractionoffactorsthatcontributetolow backpain(LBP).Psychologicalfactors,forexample,prolongrecoveryandmaypredictdisabilityforpatients withLBP[5].Onelimitationtoclinicalstudiesisthat therecanbelittleexperimentalcontrolofthepain experience.Usingamodelofexperimentallyinduced pain,thetypeofthepainstimulus,aswellasthegeneral areawherepainisexperiencedcanbecontrolled[6]. Exercise,anendogenousmethodofinducingmuscle pain,canproducepainduring[7]andafteractivity[8]. Performanceofeccentric(lengthening)muscleactions inmusclesunaccustomedtosuchforcescausesdamage tomusclefibers.Pain,hyperalgesia,allodynia,edema, andweaknesscanalsoresu lt[9-13].Thesesymptoms andsignstypicallycompletelyresolvedwithintwo weeks[14,15]andarereferredtoasdelayedonsetmusclesoreness(DOMS).DOMShasbeenreportedinthe trunkmusclesafterfloorballtraining[16]andperformanceofthetestsassociatedwithafunctionalcapacity evaluation[17,18].However,eachofthesemodels involvedsignificanttimetoinducetheDOMS.For example,participantsinth estudybyHjortskovetal, playedhandballfortwohourswhileparticipantsin workdescribedbySoeretalcompletedafunctional capacityexamination,aproce durethatinvolvestwelve separatestrengthtasks. * 1 DepartmentofPhysicalTherapy,UniversityofFlorida,Gainesville,Florida, USA Fulllistofauthorinformationisavailableattheendofthearticle Bishop etal BMCMusculoskeletalDisorders 2011, 12 :35 2011Bishopetal;licenseeBioMedCentralLtd.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommons AttributionLicense(,whichpermitsunrestricteduse,distribution,andreproductionin anymedium,providedtheoriginalworkisproperlycited.


InDOMSmodelsthatinvolveperipheral(nonaxial)musclescommonprotocolstargetaprimary muscleormusclegroupinwhichtheDOMSistobe induced.Activetrunkexerciseshavebeenusedto generateDOMSinthetrunk[19,20].Thereforewe hadtwogeneralgoalstoextendpreviousworkinthis area.OurfirstgoalwastodescribethepainanddisabilityoutcomesresultingfromDOMStoestablish theexternalvalidityofDOMSasamodelofLBP.We aimedtoquantifypainintensityandself-reportof disability,examinethedistributionofanypainthat mightoccurexpectingthatitwouldbeinanatomic regionsconsistentwithLBPreportedbypatients seekinginterventionfortheirpain,andcharacterize thequalityofanypainreported.Additionallywe soughttoidentifywhetherperipheralsensitization occurredfollowingDOMSinthetrunk,asindicated byreductionsinmechanicalpainthresholdsoverthe targetedmuscles. Ournextgoalwastodeterminetherelevanceofthis modeltopatientswithlowbackpain.Wehypothesized thattherewouldbearelatio nshipbetweenpainintensityandself-reportofdisability[21-23]andwealso expectedvariabilityinpainintensitytoberelatedto psychologicalfactors,suchaspainrelatedfear,andpsychophysicalmeasures[24,25].Thenwecomparedpain anddisabilityreportsbyparticipantsinthecurrent studytoagroupofage-matchedpatientsseekinginterventionsforLBP.Demographicsandthecharacteristics ofthepatients ’ painexperiencesweretrackedaspartof aseparaterandomizedtrialofinterventionsforLBP [26].Wehypothesizedthattherewouldbeoverlap betweenthetwogroups(healthyparticipantswith DOMSandpatientswithLBP)inreportsofpainintensity.Theoverridingrationaleforthesehypotheseswas thatifmeasurescollectedaf terDOMSwereconsistent withobservationsmadeofpatientsseekingintervention forLBPthiswouldsupporttheexternalvalidityofthe model.MethodsParticipants52healthypain-freevolunteers(23women,17men; averageage22.4years;averageBMI24.3)readand signedaninformedconsentformapprovedbythe UniversityInstitutionalReviewBoard.Participantswere excludediftheymetanyofthefollowingcriteria:previousparticipationinaconditioningprogramspecificto trunkextensors,anycurrentbackpain,anychronic medicalconditionsthatmayaffectpainperception, kidneydysfunction,majorpsy chiatricdisorder,history ofpreviousinjuryincludingsurgerytothelumbarspine, cardiacconditions,osteoporosis,orliverdysfunction,or performanceofanyinterventionforsymptomsinduced byexercisebeforetheterminationoftheirparticipation intheprotocol.MeasuresAlltestsandmeasureswerecollectedinaresearch laboratorysettingbeforeexerciseandat24,48, 96hoursandoneweekaftertheexerciseprotocolwas administered.Inaddition,participantscompletepain anddisabilityquestionnaires2weeks,4weeksand 12weeksafterexercisetoexamineanylongtermeffects ofparticipation.Painintensity,unpleasantness,andlocationPainintensitywasmeasuredwithavisualanalogscale (VAS)consistingofa100mmlineanchoredatoneend with “ none ” andattheotherwith “’ worstimaginable. ” ApreviousstudyhasindicatedthattheVASisavalid ratiomeasure[27].Participantsrated “ worstpaininthe backorlegstoday ” byplacingamarkalongthe 100mmline.TheVASforpainunpleasantnessconsistedofa100mmlinewithanchorsof “ notatall unpleasant ” and “ mostunpleasantimaginable. ” Pain intensitymeasuresthesensory-discriminativedimension ofpain,andpainunpleasantnessmeasurestheaffectivecognitivedimensionofpain[28].Theaffectiveratio (unpleasantnessdividedbyin tensity)providesinformationaboutthequalityofthepain[29].Clinicalpain oftenisassociatedwithunpleasantnessbeingsimilarto, orgreaterthan,intensity[30].Participantsalsocompletedpaindrawingstoindicat ethespatialdistribution ofanybackorlegpain.Datawerecodedtoindicateif theparticipantreportedanylegpaindefinedinthis studyaspainbelowtheglutealfold.DisabilitySelf-reportoflowback-rel ateddisabilitywasassessed withamodifiedversionoftheOswestryDisabilityQuestionnaire(ODQ)[31].Thisten-itemquestionnairehasa rangeof0(nodisabilityduetobackpain)to100(completelydisabledduetobackpain)andistypically reportedinpercentages.TheODQhasbeenextensively usedasameasureofdisabili tyinthosepatientswith LBPduringclinicalmanagementandexperimentalstudies.ThemodifiedODQhasverygoodtest-retest repeatability[31]andarecentconsensusstatement includedtheODQasanimportantmeasureofphysical functioninginpatientswithLBP[32].Pain-relatedfearTwoquestionnaireswereusedtomeasurepain-related fear.TheFearofPainQuestionnaire(FPQ)isa30-item, 5-pointratingscalewitharangefrom30to150,developedtomeasurefearaboutspecificsituationsthatmay causepain.WeusedthetotalscorefortheFPQ,aswe weremostinterestedinmeasuringparticipants ’ general fearofpain[33].Highlevelsofinternalconsistency (>0.8)havebeenreportedinclinicalandnon-clinicalBishop etal BMCMusculoskeletalDisorders 2011, 12 :35 Page2of10


groupsofparticipants[33,34].Thisquestionnairewas usedbecauseitcanbecompletedbyhealthy,painfree participantsandwehavefoundittobeassociatedwith experimentalpainresponsesinourpreviousstudies [24,25]. AshortenedversionoftheTampaScaleofKinesiophobia(TSK-11)wasusedtoassessthefearofmovementorinjury[35].TheoriginalTSKincluded17 questionsandwasusedinmultiplestudiesinparticipantswithlowbackpain[36-38].TheTSK-11provides ameasureof2separatedomains:somaticfocusandfear ofreinjury[39].TheTSK-11excludes6questionsfrom theoriginalTampaScaleofKinesiophobia(TSK).The TSK-11hasdemonstratedsimilarfactorstructure,reliability(ICC=0.82),andvaliditytotheoriginalversionof theTSK[35].Theitemsarescoredona4-pointscale from1(stronglydisagree)to4(stronglyagree).Lower scoresontheTSK-11indicatelesspain-relatedfearof movement.PainSensitivityMechanicalPressureThreshold(MPT):Lowered mechanicalpainthresholdhasbeenpreviouslyreported inDOMSmodelssuggesting sensitizationofmuscle nociceptorsinresponsetomediatorsoftheinflammatoryprocess[6,40].Tendernessinparaspinalmuscletissuewasassessedusingah and-helddynamometer (Microfet2,HogganHealthIndustries,Inc,West Jordan,UT).Thetipofthedynamometerisequipped witharubberfoot-plateof1-cmdiameter.Duringtesting,theparticipantwaspositionedinproneandforce wasapplieduntiltheparticipantreportedthatthesensationchangedfrompressuretopain.Atthatpointthe participantratedpaintheyexperiencedusinganumeric ratingscale(NRS)anchoredat0(nopainsensationat all)and100(worstpainimaginable)andtheapplied forcetoreachthisthresholdwasrecordedinkg-force. Thresholdmeasureswereevokedintheparaspinalmusclesbilaterally2.5cmfromthespinousprocessesofL1, L5andS2foratotalofsixratings.Thesewereaveraged toprovideasinglemeasureofMPT.Betweensession reliabilityforMPThasbeenshowntobehigh(ICC> 0.87)intheposteriortrunkmuscles[41].ExerciseProtocolPriortoexerciseallparticipantscompletedasubmaximaleffortwarm-upsessionconsistingofridingthestationarybicycleataspeedof50-60RPMand1Kpof resistanceandstaticpassivestretching(heldfor30seconds)ofthelowerextremitiesandposteriortrunk.Participantsperformedanisom etric(static)testoftotal torqueofthetrunkextens ormusclesthroughtheir availabletrunkflexionrangeofmotion(ROM)usinga MedXlumbarextensionexercisemachinefollowingthe standardizedprotocol[42].Therepeatabilityof isometrictorqueproductioniswell-establishedinparticipantswithoutpain[42]andgroupsofpatientswith LBP[43].ParticipantswereseatedintheMedXmachine withthestabilizingstrapsattachedacrossthepelvisand knees.TheparticipantwasmovedthroughtheROMof themachineinlumbarflexionandextensiontodeterminehisorheravailableROM.Thedevicewaslocked intoplaceinmaximalflexionandtheparticipantwas instructedtobuildupforcegraduallyagainstapadin contactwiththemidandlowerback.Thetorquegeneratedbytheparticipantwasdisplayedgraphicallyonthe datacollectioncomputer.Oncepeakeffortwasobserved bytheresearchassistant,theparticipantwasinstructed torelax,thedevicereleasedandtheparticipantreturned toanuprightpositionforatleast10seconds.Isometric testingwasadministeredseventimesinpositionsthat rangedfromtheparticipant Â’ smaximumavailabletrunk extensiontomaximaltrunkflexion.Theisometrictorquecollectedateachtestanglewassummedtogive measureoftotaltorqueproducedacrosstheentire rangeofmotion. Afterbaselinetotaltorquewasrecorded,participants performedboutsofdynamicexercisetothepointof volitionalfatigue.Toperformtheexercisebout,theparticipantswereseatedandrestrainedinaMedXlumbar extensionexercisemachine.Participantsperformedas manyrepetitionsaspossibleusingaweightloadequal toapproximately80%ofthepeaktorquemeasuredduringtheisometrictest.Eachrepetitionwasperformed throughthefullavailableROMandtheparticipants wereencouragedtoperformtheliftingportion(concentric)intwosecondsandthelowering(eccentric)in fourseconds.Repetitionscontinueduntilthepatient reportedbeingunabletomovethroughafullrangeof motion(volitionalfatigue).Oncethisoccurred,theisometrictorquetestwasperformedagain.Ifthetotaltorquemeasuredduringtherepeatisometrictestwas50% orlessofthebaselinetotaltorque,theprotocolwas complete.Ifthisdidn Â’ toccur,theexerciseboutwas repeated.Participantsrepeatedthissequenceofdynamic exerciseandisometrictestinguntiltotalmeasuredtorquedecreasedto50%ofthebaselinemeasurement.Participantswereinstructednot toinitiateanymedication inthenext48hours,orapplyanyintervention,suchas iceorheatingpacstothelumbarspine.AnalysisDescriptivestatisticsweregeneratedforallbaseline variables.EffectofDOMSTheeffectofexerciseonpainintensityanddisability wascomparedovertimeusingseparaterepeatedmeasuresanalysisofvariance(ANOVA)models.Wespecificallyexpectedpainreportstopeakat24or48hoursBishop etal BMCMusculoskeletalDisorders 2011, 12 :35 Page3of10


andreturntobaselineovertime.Additionally,wecalculatedtheaffectiveratiotoexaminethechangeinquality ofthepain.Theseratiosrepresenttheratioofunpleasantnesstointensityandwerecalculatedbeginningat 24hours.Toexaminechangesinpainsensitivitythat occurredinresponsetoDOMSweusedarepeatedmeasuresANOVA.Wealsocomparedparticipantswith andwithoutlegpainonmeasuresofpain,disabilityand fear. Changesinmuscleperformancewerealsoexamined. Atwo-wayrepeatedmeasuresANOVAwasusedto examinewhetherdecrementsinisometrictorqueproductionwerespecifictoapa rticularangleorangles,or whetherchangesoccurredacrossalltestingangles equally.Additionallythetotalisometrictorquewas trackedtodeterminethetimeframeoverwhichmuscle performancereturnedtobaselinelevelsofperformance.ModelValidityNextweexaminedtheextenttowhichpainreports explainedself-reporteddisabilityaftertheinductionof pain.Weusedthevaluesofpainanddisabilityat 24hoursfollowingtheinductionofmusclepainfor theseanalyses.First,zeroordercorrelationswerecalculatedamongthepotentialpredictorandoutcomevariables.Sexdifferencesinpa inintensityanddisability weretestedusingindependentt-tests.Thefirstmodel wasfitfordisabilityusingtheODQasthedependent variable.Webuiltahierarchicalregressionmodelusing demographicvariablesasthefirstblock.Thesecond blockconsistedofpsychologicalmeasuresforwhichthe zeroordercorrelationwithpainwassignificantatp< 0.1.Painintensitywasenteredasthefinalblock.Hierarchicalregressionwaschosentoallowustoexamine thecontributionsofeach ‘ block ’ totheoverallvariation inthedependentvariable.Weexaminedthechangein R2andstandardizedbetastodeterminesignificantcontributionstothevariance. Asecondmodelwasbuildtoexaminefactorsthat explainedvariabilityinpainintensityafterexercise.The firstblockenteredconsistedofdemographicvariables. Thesecondblockconsistedofpsychologicalmeasures forwhichthezeroordercorrelationwithpainwassignificantatp<0.1. OurlasttestofvaliditywastocomparetheexperimentallyinducedLBPtoclinicalLBP.Reportedpain intensityfromparticipantsinourstudywascompared toagematchedpatientswithlowbackpainfromapreviousclinicaltrial[26]usingMann-WhitneyUtest. Demographicdetailsofpatientsintheclinicaltrialare summarizedinTable1. RepeatedmeasuresANOVAswereperformedusing StatView5.0.1(SASInstitute,Cary,NC,USA),and regressionanalyseswereperformedusingSPSSforWindows15.1.Type1errorwasmaintainedat5%.ResultsDemographicanddescriptivedataaresummarizedin Table1.52participantscompletedtheevaluationswithoutdrop-outsandnoadverseeventswerereported.38 participantsindicatedthat theyhadnotperformedany typeofregularexercise(morethanoneaweek)forthe pastmonth.Fiveindicatedparticipatinginweight-training(notrainingofthetrunkextensors),andsevenindicatedthattheyperformedaerobicexercise(running, walking,orbicycling)twiceaweek.EffectsofDOMSTherewasasignificantmaineffectoftimeforpain intensityinthebackorlegs(F7,58=45.7,p<0.001). ThegeneraltimecourseofDOMSisshownin Figure1.30participantsreportingpeakpainat 24hoursand17reportingthisat48hours.Oneparticipantreportedpeakpainat96hoursafterexercise and4participantsreportednopainatanytimeperiod.18participantsreportedpaininthebackandlegs. Noparticipantsreported painbelowthekneeatany timepoint.AsamplepaindiagramisshowninFigure 2illustratingthedistributionofpainreportedbythis participantat48hours.Thepainintensityfor45of participantshadreturnedtozerowithinoneweek aftertheinductionofDOMSandonly1participant reportedpainat2weeks(10mm).Similarly,thedistributionofdisabilityscoresincreasedfrombaselineto 24hoursandthenreturnedtozeroovertime(F7,58= 13.2,p<0.001).MPTat24and48hrswassignificantlylowerthanbaseline(p=0.004andp=0.012, respectively)andreturnedtobaselineby96hours. ThesevaluesareshownTable2. Table1Characteristicsofparticipantsperformingthe exerciseprotocolandagematched-patientswithLBPHealthy participants (n=52) PatientswithLBP [26](n=33) VariableMedianRangeMedianRange Age(years)2318,322418,32 Sex(%female)3160%2266% Race/Ethnicity White4281%2576% AfricanAmerican510%515% Asian48%13% NativeAmerican00%00% Hawaiian/PacificIslander12%13% Morethanonerace8Painintensity160,63500,70 Disability40,30280,74 Fearofpain(FPQ-III)74.730,110Kinesiophobia(TSK)18.511,26Bishop etal BMCMusculoskeletalDisorders 2011, 12 :35 Page4of10


Weassessedtheaffectiveratiooverthecourseofoneweek.Theaffectiveratiodidnotfollowthesamepattern aspainintensityanddisability;howeverthereweredifferencesacrosstestingsessions(F1,51=8.81,p=0.004). Theratioofunpleasantnesstointensitystatistically increasedfrom24to48hours(p=0.010)andremained elevatedoneweekaftertheexerciseprotocol(p= 0.007)followingadifferenttimecoursefrommeasures ofintensityonly.Theratioofunpleasantnesstointensityremainedgreaterthanoneindicatingthatthesensationwasconsideredmoreunpleasantthanintense. Whenimmediatechangesinmuscleperformanceafter exercisewereexamined,asignificantinteractionwas notedbetweentimeandangle(F6,312=15.01,p<0.001). Thegreatestdecrementsinis ometrictorqueproduction occurredattestangles12,24and72degreescorrespondingtothemid-rangeoftrunkmotionandinfulltrunk flexion,respectively.Posthoctestingindicatedthatthe decrementintorqueat12degreesandat24degreeswas largererthaninthelossoftorqueinfullextension.The percentdeficitateachangleisshowninFigure3.Total isometrictorqueremaineddepressedatoneweekafter theexerciseprotocolwasperformedandreturnedto baselinetotalsbytwoweeksafterexercise(Figure4).ModelvalidityThepainintensityat24hoursfortheparticipantsrangedfrom0to63ontheVAS.Statisticaldifferences werenotedinlevelsofself-reporteddisabilitywhen comparingparticipantswithandwithoutlegpain(participantsreportinglegpainhadhigherlevelsofselfreporteddisability)butnodifferenceswerenotedin painintensity. Thezeroordercorrelationsamongdemographic,psychological,andpainvariables,andself-reportsofdisabilityat24hoursaresummarizedinTable3.Painintensity at24hourswascorrelatedwithpainunpleasantness, Pain intensity (100mm VAS) Figure1 PainintensityreportedafterinductionofDOMS Figure2 Samplepaindiagramofsubjectreportinga “ peak paininthebackorlegs ” of30mmat48hours .Participants wereinstructedto “ usethediagramtomarktheareasinwhichyou arefeelingpainasaresultoftheexercisesdoneinthisstudy ” Table2ChangesinMechanicalPainThresholdForce(kgf)Rating(NRS) Baseline28.81.423.12.3 24hours23.61.326.92.6 48hours24. 96hours27. Figure3 Decrementsinisometrictorqueproduction immediatelyfollowingtheexerciseprotocol .Thelinerepresents differencesamongtestingangles(p<0.002afterBonferroni correction). Bishop etal BMCMusculoskeletalDisorders 2011, 12 :35 Page5of10


painareaandself-reportsofdisability.Also,fearofpain wasassociatedwithpainintensityandpain unpleasantness. Ourfirstmodel,predictingself-reportofdisabilityat 24hours,includedsex,astheonlydemographicvariable,thepresenceoflegpain,andpainintensity.Each variablecontributeduniqueamountsofvariancetothe finalmodelpredictingdisabilityasindicatedbythesignificantchangesintheadjustedR2(seeTable4).However,thelargestamountofvarianceindisability,an additional27%,wasexplain edbypainintensityafter controllingforsexandthepresenceoflegpain.The finalmodelaccountedfor40%ofthetotalvariancein self-reporteddisabilityandisshowninTable4. Nootherpredictorvariablesofinterestwererelatedto pain-intensityexceptfearof pain;thereforenoregressionanalyseswereperformed.Theassociationbetween fearofpainandpainintensityisshowninTable3. Thirty-fourpatientsinthecomparisonclinicaltrial[26] werebetweentheagesof18and32years.Demographic detailsofthisgroupofpatientsispresentedinTable1. Datafromthesepatientswascomparedtothereportsof painfromparticipantsinth isDOMSstudy.Overall,the reportsofpainafterexerciseweresignificantlylower thanthereportsofpainofthepatients(U=256.5,p< 0.001).Visualinspectionofthequartilerangesinboth datasetssuggestedthatthehighestpainintensityreports oftheexperimentalgroup(50thpercentileandgreater) overlappedwiththelowestpainreports(50thpercentile orlower)ofthepatientssee kinginterventionfortheir backpain-16to68mm,and0to50mmrespectively.DiscussionWeinducedacutepainusinganexerciseprotocolto createDOMSinthelowbackandfollowedparticipants for12weekstocollectpertinentoutcomes.DOMS modelsarepotentiallyveryusefulbecausetheymimic musculoskeletalpaininlossofrangeofmotion,pain withmovementandselfcarebehaviors[12,13,44].The reportsofpaingeneratedinthiscurrentstudyfollowed atimecourseconsistentwithotherDOMSmodelswith painpeakingat24or48hoursandresolvingwithin approximatelyoneweekafterexercise[45,46]. Thereportedpainintensityrangedfrom0to68mm inthelowbackand,sometimes,legs.Theseanatomic areasareconsistentwithregionsdescribedbypatients seekinginterventionforLBP.Inpatientswithlowback andlegpain,thelegpainmaybereferredfromanatomicstructuresinthelumbarspine.Forexample, structuresthathavebeendemonstratedtocausereferral ofpainintotheposteriorthighincludetheduraand nerveroots[47]aswellastheintraspinousligament [48]andthelumbarmultifidii[49,50].Magneticresonanceimagingoftheposteriortrunkmusclesafterperformingtheexerciseprotocolusedinthismanuscript indicatesthatprimarychangesidentifiedafterexercise occurinthelumbarmultifidii(paperinreview).Theleg paininourcurrentstudymayalsohaveresultedfrom thesustainedisometriccontractionsperformedbymusclesofthelowerextremityduringtheexerciseprotocol. Whilethesemuscleswerenotsubjectedtotheforces thatoccurringduringeccentric(lengthening)actions, thereisevidencetosuggestthatrestrictedbloodflowto aworkingmusclemaycauseDOMSinthatmuscle[51]. However,thisrangeofreportedpainintensityislower thanthatreportedbyUdermannetal,2002whoalso usedtrunkextensionexercisetocauseDOMS.These authorspresentdataindicatingthatfiveparticipants whocompleted50repetitionsoftrunkextensionusinga weightthatwas100%oftheisometricmaximum reportedpainintensityofapproximately8usinga 10cmVASthatpeakedat24hoursafterexercise.This exerciseintensitywassomewhatgreaterthanweusedin ourmodelalthoughnoinformationregardinganchors ontheVASorconfidenceintervalsreported,andno errorbarsareshowninthegraph.Whendevelopingthe modelusedinthecurrentstudywewereverycautious aboutthemagnitudeoftheloadingbeingplacedonthe lumbarspinesoftheparticipants.Thiswasaconcernof thefundingagencyandofourowninstitutionalreview board.Consequently,ourprotocolislessaggressivethan isseeninmodelsdevelopedintheextremitiesand includedbothconcentricandeccentricmuscleactions. Additionallyparticipantsinourcurrentstudywere askedtoratepainthattheywereexperiencingatrest, notduringmovementoractivity.Thesefactorscombinedmayexplainwhythepainintensityreportedis lessthanothermodelsofDOMS. ConsistentwithotherDOM Smodels,participants experiencedanincreaseintheaffective-cognitive Figure4 Timecoursetorecoveryofisometrictorque productioninthetrunkextensormuscles Bishop etal BMCMusculoskeletalDisorders 2011, 12 :35 Page6of10


componentofthepainexperience(unpleasantness) [44,52].Theaffectiveratioshowedaslightlydifferent patternofchangeovertime.Ateachtimepointthe ratiowasgreaterthanoneindicatingthatthepainsensationwasperceivedasmoreunpleasantthanintense. Thisalsosupportsmodelclinicalrelevancebecauseclinicalpainoftenisassociated withunpleasantnessbeing similarto,orgreaterthan,intensity.Theratioincreased at48hoursandremainedsoattheoneweekfollow-up. Bythistime,painintensityhaddecreasedtobaselinefor mostparticipants.Fieldsetal,1999hassuggestedthat forasensationtoberecognizedaspainfultheremust beunpleasantnessassociat edwiththesensationintensity.Wedidnotspecificallyqueryparticipantsaboutthe qualityofanyongoingsensation(otherthanunpleasantness)thattheymayhavebeenfeelingintheirbackand legsbutwespeculatethatparticipantsmayhavecontinuedtoexperience ‘ soreness ’ butnotpainperse. Additionally,participantsreporteddisabilityafter inductionofDOMSusinganoutcomemeasureusedin clinicalstudiesofpatientswithLBP.Inourstudy,pain intensitywastheprimarypredictorofthisself-reportof disability.Thisisrelation shipisalsoconsistentwith clinicalpopulations.Forexample,innurseswithback painthiscorrelationwas0.69[21],comparedtor=0.50 inourstudy.Thepresenceoflegpainwasalso associatedwithdisabilityi nourmodelofLBPandthis findingislikewiseconsistentwiththeclinicalliterature regardingLBP.Selimetal,1998comparedgroupsof patientswithLBPwhowereth enfurtherclassifiedby thelocationoflegpain(none,totheknee,belowthe knee).Theseauthorsreportedthatdisabilityincreased astheinvolvementoflegpainincreased[53]. However,therangeofscoresusingtheself-reportof disabilityreportedinourstudywas0to30.Thisrange suggeststhat,whileparticipantsperceivedsomedisabilityfromDOMS,itwasaminimalamountofdisability. Thisisnotunexpectedgiven thatparticipantsaretold duringtheinformedconsentprocesstoexpectthepain thattheyexperiencetobeofshortdurationandinstructionalsetregardingwhattoexpectafteraprocedurehas beendemonstratedtoinfluencetheoutcomeafterthat procedure[54].However,thisisalsoastrengthofthe modelastherewerearangeofpainintensityvalues generatedanditmaybeunethicaltohaveinducedmodelsthatcausehighdisabilityconditions.Suchamodellongdurationofhighpainanddisability-wouldno longerbeamodelpersebutrathertheclinicalconditionbeingmodeled! Loweredmechanicalpainthresholdhasbeenreported inDOMSmodelsinvolvingdif feringsomaticlocations suchasthebiceps[29],shoulder[55],andhand[10]. AproposedmechanismofDOMSisthatmusclenociceptorsbecomesensitizedinresponsetomediatorsof theinflammatoryprocess,therebyloweringstimulus thresholds(peripheralsensitization)[6,40].Thefinding oflocalreductioninmechanicalpainthresholdafter DOMSwasconsistentwithotherstudiesofDOMS [22,56].Inaddition,isometri ctorqueproducingability ofthetrunkmusclesremainedimpairedforapproximately2weeksafterexercise.Thesefindingsincombinationconfirmthattheexerciseprotocolproduced musclechangesinthetargetmuscles. Painintensityinourmodelwasonlyrelatedtobaselinefearofpainandnottokinesiophobiaasmeasured Table3BivariateassociationsamongmeasuresPainUnpleasantnessPainAreaDisabilityAgeBMITSKFPQ Painintensity0.53**0.33**0.56**-* PainUnpleasantness0.030.31*-** PainArea0.23-<0.01 Disability - Age 0.11-0.02-0.11 BMI -0.150.03 TSK 0.49**Disability:OswestryDisabilityQuestionnaire. BMI:BodyMassIndex. TSK:TampaScaleofKinesiophobia. FPQ:FearofPain. *-p<0.05,**-p<0.01. Table4ModelPredictingSelf-reportofDisabilityStepBetaSig.AdjustedR2Sig.FChange 1(Constant)2.080.0600.060.035 Sex3.030.035 2(Constant)0.610.6100.140.015 Sex3.520.012 Legpain3.530.015 3(Constant)-2.310.0470.41<0.001 Sex3.360.004 Legpain2.850.018 Painintensity0.20<0.001 Bishop etal BMCMusculoskeletalDisorders 2011, 12 :35 Page7of10


bytheTSK.Themostlikelyexplanationforthisfinding isthetypeofparticipantsinourstudy.Theversionof theTSKusedinourstudywasTKS-11.Thisquestionnaireincludesquestionsa boutthecurrentpainexperiencedbytheindividualcompletingtheform.Allour volunteerswerepain-fre eandnonehadexperienced backpainorhadanaccident.MorerecentlytheTSKgeneralhasbeenpublished.Thismayhavebeenamore appropriatemeasureofkinesiophobiainourparticipants.Incontrast,theFPQmeasuresgeneralfearof painandmayrepresentatraitmeasureoffear. OurfindingsarealsodifferentfromthoseofSoer etal,2009.TheseauthorsreportthatsexwasasignificantpredictorofpainintensityintheirmodelofDOMS afteranFCE.Forourstudy,sexpredictedtheself-report ofdisability,notpain. LBPischaracterizedbyheterogeneousmechanismsof onsetandpathoanatomicpaingenerators.Additionally, multiplestudieshaveindicatedthatidentificationof pathoanatomyisdifficultbecauseofthemanyconditionsthatarealsopresentinasymptomaticparticipants [57-63].Developmentofanexercise-inducedmodelof LBPisessentialthereforetobettercontrolexperimental investigationsrelatedtomanagementofLBP.We believethatDOMSmodelswillbeusefulforfuturestudiesofLBP.Withthesemodelswecantestparticipants whentheyarepainfreeandexaminefactorsthatcontributetotheirpainexperienceafterinductionofLBP.In addition,wehavethepotentialtoestablishcomparison groupsinwhomthemechanismofonsetisconsistent resultingintheabilitytousetrueexperimentaldesign instudiesofLBPpotentiallyallowingustotest responsestointerventioninahomogenousLBPmodel. Also,therewereparticipantswhoreportedthatthey experiencednopaininthebackorlegsafterperforming theexerciseprotocol.Consequently,thispresentsthe opportunitytostudypotentialfactorsthatmightbeprotectiveofexperiencingDOMS.Thisfindingsuggests additionalrelevanceofourmodelbecausenotallindividualswhoexperiencethesamestressorstimulusgoon todevelopandcomplainofLBP.Thisisnotpossiblein clinicalstudiesandremainsanovelpartofthiscurrent study. Modelsofendogenousmusclepainareparticularly usefulbecausetheymimicthemostcommonformof chronicpain,musculoskeletalpain[6,64].OtherendogenousexperimentalmodelsofLBPhavebeenrecently presentedthatuseinjectionofsalineintospinalligaments[65]andmulitifidus muscles[66].Modelsusing DOMS,suchaspresentedinthiscurrentstudyand others[17-20],mayhavemoreclinicalvaliditythan theseinjectionmodelsgiventhatDOMSisnon-invasive andthecourseofsymptomslastsfordaysratherthan hoursorminutespotentiallymimickingLBPforwhich anindividualmightseekinterventionmoreclosely. Therewasoverlapbetweenreportsofpainofthehighest50%ofparticipantswithDOMSinourstudy(16to 68)andthelowest50%ofthepatientsseekingintervention(0to50)[26]suggestingthatthemodelmayhave thebestcomparisontopatientswithlowerlevelsof painanddisability. Oneimportantconsideration,however,isthatpain relatedtoDOMSislikelytobeofshorterdurationthan clinicalpain.Additionally ,painwasgeneratedinthis studyinyounghealthyparti cipants.Giventhelimited agerangeofparticipantsinourstudy,futureworkwill examinethedevelopmentofpaininolderadultsto improvetheexternalvalidityofDOMSasamodelof LBPforwhichapatientmightseekintervention.Other factors,suchascatastrophizing,willneedtobeincluded infollow-upstudiestobette runderstandpsychological influenceandotherworksuggestsaninteraction betweengeneticandpsychologicalfactorscontributesto painanddisabilityintheupperextremity[67].Another limitationtoconsideristhatweassumethattheexerciseprotocolisthecauseofthereportedofpainintensityanddisabilityat24and 48hours.Withoutagroup ofparticipantsperformingshamexerciseorinacontrol groupwecannotbecompletelycertainthattheresults ofourstudyareattributabletoparticipationinexercise.ConclusionsNonetheless,ourresultssuggestoverlapwithclinical LBPforself-reportsofpainaswellasdemonstrating associationbetweenpainanddisabilityaswouldbe expectedinclinicalpain.Thismodelthereforemaybe mostapplicabletoyoungeradultswithlowerlevelsof painanddisability.Thisworkalsosupportstheconcept ofmultiplefactorsinteractingtoexplainthedevelopmentofpainanddisabilityinLBP;howeverlargersampleswillbeneededtodevelopacompletemodel.Acknowledgements WeacknowledgetheassistanceofLaurenBernloehr,NicholasDiSarro, AdrienneDriggersandCarlosRiverosduringdatacollectionforthisstudy. ThisresearchwassupportedbyaUniversityofFloridaOpportunityFund grant,andfundingfromtheNationalInstituteofArthritisand MusculoskeletalandSkinDiseases(K01AR054331-01A2).Publicationofthis articlewasfundedinpartbytheUniversityofFloridaOpen-Access PublishingFund. Authordetails1DepartmentofPhysicalTherapy,UniversityofFlorida,Gainesville,Florida, USA.2RehabilitationDoctoralProgram,CollegeofPublicHealthandHealth Professions,UniversityofFlorida,Gainesville,Florida,USA.3CenterforPain andBehavioralHealth,UniversityofFlorida,Gainesville,Florida,USA.4DepartmentofClinicalandHealthPsychology,UniversityofFlorida, Gainesville,Florida,USA. Authors Â’ contributions MDBconceived,andparticipatedinthedesignof,thestudy;procured funding;participatedindatacollection;performedandinterpretedstatisticalBishop etal BMCMusculoskeletalDisorders 2011, 12 :35 Page8of10


analyses;andhelpedtodraftthemanuscript.MEHparticipatedindata collectionandhelpedtodraftthemanuscript.SZGconceived,and participatedinthedesignof,thestudy;interpretedstatisticalanalysis;and helpedtodraftthemanuscript.MERconceived,andparticipatedinthe designof,thestudy. Allauthorsreadandapprovedthefinalmanuscript. Competinginterests Theauthorsdeclarethattheyhavenocompetinginterests. Received:13August2010Accepted:2February2011 Published:2February2011 References1.MantyselkaP,KumpusaloE,AhonenR,KumpusaloA,KauhanenJ, ViinamakiH,HalonenP,TakalaJ: Painasareasontovisitthedoctor:a studyinFinnishprimaryhealthcare. Pain 2001, 89(2-3) :175-180. 2.SternbachRA: Painand ‘ hassles ’ intheUnitedStates:findingsofthe Nuprinpainreport. Pain 1986, 27 :69-80. 3.FrymoyerJ,DurettC: Theeconomicsofspinaldisorders. In TheAdult Spine.Volume1. Editedby:FrymoyerJ.Philadelphia:Lippincott-Raven; 1997:143-150. 4.IJzelenbergW,BurdorfA: Patternsofcareforlowbackpaininaworking population. 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