Repetition-induced activity-related summation of pain in patients
Dorothée Ialongo Lambina, Pascal Thibaultb, Maureen Simmondsc, Christian Lariviered,
Michael J.L. Sullivanb,⇑
aDepartment of Psychology, University of Montreal, Montreal, QC, Canada
bDepartment of Psychology, McGill University, Montreal, QC, Canada
cSchool of Physical and Occupational Therapy, McGill University, Montreal, QC, Canada
dOccupational Health and Safety, Research Institute Robert-Sauvé, Montreal, QC, Canada
Sponsorships or completing interests that may be relevant to content are disclosed at the end of this article.
a r t i c l ei n f o
Received 11 November 2010
Received in revised form 2 February 2011
Accepted 10 February 2011
Low back pain
Summation of pain
Fear of pain
a b s t r a c t
This study compared individuals with fibromyalgia (FM) and individuals with chronic low back pain
(CLBP) on repetition-induced summation of activity-related pain (RISP). Fear of movement, pain catastro-
phizing and depression were examined as potential mediators of group differences. The sample consisted
of 50 women with FM and 50 women with CLBP who were matched on age, pain severity and pain dura-
tion. Participants were asked to lift a series of 18 weighted canisters. In one trial, participants were asked
to rate their pain after each lift. In a second trial, participants estimated the weight of each of the canis-
ters. An index of repetition-induced summation of pain was derived as the change in pain ratings across
repeated lifts. Analyses revealed that women with FM obtained higher scores on the index of RISP than
women with CLBP. The heightened sensitivity to RISP in individuals with FM was not due to generalized
hyperalgesia or a greater work output. Consistent with previous research, fear of movement was posi-
tively correlated with RISP. Pain disability was also associated with greater RISP, but not pain catastro-
phizing or depression. Discussion addresses the processes by which individuals with FM might have
increased RISP responses. The findings of this study point to possible neurophysiological mechanisms
that could help explain the high levels of pain-related disability seen in individuals with FM.
? 2011 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.
Fibromyalgia (FM) is characterized by chronic widespread pain
and hypersensitivity to a variety of noxious stimuli . Individu-
als with FM often experience a number of symptoms other than
pain, including fatigue, sleep disturbances and a variety of neuro-
psychiatric problems such as memory difficulties, slowed informa-
tion processing, and depressive symptoms . These symptoms
can be associated with a high level of disability that is often unre-
sponsive to traditional pharmacological or rehabilitative interven-
tion approaches for chronic pain [10,19].
Research has shown that individuals with FM experience
increasing pain in response to noxious stimulation of constant
intensity [32,28]. The term ‘temporal summation’ (TS) of pain (ie,
windup) has been used to describe progressive increases in pain
severity as a function of repeated noxious stimulation [28,33]. TS
has been demonstrated in response to thermal stimulation, electri-
cal stimulation or pressure with standardized duration of stimula-
tion and inter-stimulus intervals [1,9,43]. There are indications
that TS occurs centrally in second-order neurons in the spinal cord
as a consequence of sustained C-fiber afferent input [22,28]. En-
hanced TS has been observed in pain conditions in which the path-
ophysiology of the disorder is thought to involve a maladaptive
degree of sensitization to noxious stimuli . It has been sug-
gested that TS might contribute to pain-related disability in indi-
viduals with persistent pain conditions .
Recent reports have described a phenomenon that has been
termed ‘‘repetition-induced summation of activity-related pain’’
(RISP) [35,37]. In one study, patients with chronic pain were asked
to rate their pain as they lifted a series of 18 weighted canisters. A
subset of participants reported increasing levels of pain over suc-
cessive lifts even though the overall physical demands of the task
remained constant . To date, RISP has been demonstrated in
0304-3959/$36.00 ? 2011 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.
⇑Corresponding author. Address: Department of Psychology, McGill University,
1205 Docteur Penfield, Montreal, QC, Canada H3A 1B1. Tel.: +1 514 398 5677; fax:
+1 514 343 4896.
E-mail address: email@example.com (M.J.L. Sullivan).
?152 (2011) 1424–1430
individuals with CLBP and in individuals with whiplash injuries
[35,37]. It is unclear whether the mechanisms underlying RISP
are similar to those underlying TS.
It is possible that RISP might be more pronounced in individuals
with FM than individuals with CLBP. Increased RISP in individuals
with FM might explain, at least in part, why individuals with fibro-
myalgia show more severe pain-related disability than individuals
with CLBP. For example, research shows that individuals with FM
report greater intolerance for physical activity than individuals
with CLBP [25,26]. In addition, activity-based rehabilitation inter-
ventions that benefit individuals with CLBP have been shown to
be less effective for individuals with FM [19,30]. To date, RISP
has not been examined in individuals with FM.
The primary objective of the present study was to compare indi-
viduals with FM and individuals with CLBP on an index of RISP.
Participants completed a simulated occupational lifting task de-
signed to generate an index of RISP. It was predicted that individ-
uals with FM would show higher levels of RISP than individuals
with CLBP, and that the index of RISP would be associated with
measures of functional disability. The role of pain-related psycho-
logical variables as mediators of group differences in RISP was also
The study sample consisted of 50 women with FM and 50 wo-
men with CLBP. Participants were recruited through local pain
treatment centers and newspaper ads in Montreal, Quebec. Partic-
ipants in both groups were matched in terms of age (+ or ?3 years),
pain severity (+ or ?2 on a 0–10 scale) and pain duration (+ or ?3
years). The mean age of the sample was 44 years (SD = 8 years),
with a range of 25 to 59 years. The mean duration of pain was
12.1 years (SD = 6.0 years). The mean number of years of education
was 12.7 (SD = 2.0). The majority of the sample (86%) was married
or living in common-law. All participants underwent a medical
evaluation in order to ascertain diagnosis and ensure there were
no medical contraindications to performing the physical manoeu-
vres of the lifting task. FM participants were diagnosed using the
American College of Rheumatology (1990) criteria for fibromyalgia.
2.2.1. Pain severity
Participants rated the severity of their pain symptoms on a
numerical rating scale (NRS) with the endpoints (0) no pain at all
and (10) excruciating pain.
The Pain Disability Index (PDI)  was used as a self-report
measure of pain-related disability. Participants rated their level
of disability on 7 dimensions of every day life (home, social, recre-
ational, occupational, sexual, self-care, life support). The PDI has
been shown to be correlated with objective measures of disability
2.2.3. Fear of movement/re-injury
The Tampa Scale for Kinesiophobia (TSK)  was used to as-
sess fear of movement and re-injury associated with pain. Respon-
dents indicated their level of agreement with each of 17
statements reflecting worries or concerns about the consequences
of participating in physical activity. The TSK has been shown to be
internally reliable (coefficient alpha = .77) , and has been used
with FM population [5,13].
The Pain Catastrophizing Scale (PCS)  was used to assess
catastrophic thinking associated with pain. Respondents rated
the frequency with which they experienced each of 13 different
thoughts and feelings when in pain. The PCS has been shown to
have high internal consistency (coefficient alpha = .87), and to be
associated with pain experience, pain behavior and disability .
The Beck Depression Inventory II (BDI-II)  was used to mea-
sure the severity of depressive symptoms. Respondents were asked
to select phrases that best described how they had been feeling
during the past two weeks. The BDI-II has been shown to be a reli-
able and valid index of depressive symptoms in chronic pain pa-
2.2.6. Activity-related pain
Patients provided verbal ratings of pain intensity as they lifted a
series of 18 canisters (described in more detail below). Pain ratings
were made on an 11-point NRS with the endpoints 0 (no pain) and
10 (excruciating pain). Mean activity-related pain (MARP) was
computed as the mean of pain ratings provided for all 18 canister
2.3. Procedure and apparatus
This research received ethical approval from the Institutional
Review Board of the Centre de recherche interdisciplinaire en réadap-
tation du Montréal métropolitain (CRIR). Participants signed a con-
sent form as a condition of enrolment in the study.
Participants responded to advertisements seeking individuals
with FM or CLBP for a research study on physical and psychological
factors associated with pain and disability. Once the target sample
size of 50 participants with FM had been reached, participants with
FM were matched to individuals with CLBP on age, pain severity
and pain duration. Since patients with FM and CLBP differ on a
number of demographic and condition-related parameters, it was
necessary to select matched CLBP patients from a substantively lar-
ger sample. The data for participants with CLBP were drawn from a
larger sample (N = 265) of individuals, tested over the same time
period, who had also participated in the same experimental proto-
col and were tested by the same research assistants. Portions of the
data used as the CLBP comparison group have been reported in a
previous paper . A pain-free control group was not included
as pilot testing revealed that the canister-lifting task did not gener-
ate pain in individuals without a pain condition characterized by
Participants were asked to complete the NRS, the PDI, TSK, PCS
and the BDI-II prior to completing the canister-lifting task. Partic-
ipants were informed that the study was aimed at developing a
new assessment procedure for individuals suffering from persis-
tent pain. In the consent form, participants were made aware that
the lifting task might lead to temporary increases in discomfort in
the 24 to 48 hour period following completion of the task. How-
ever, no leading information was provided suggesting that partici-
pants might experience increases in pain during the lifting task.
Participants were made aware that they were free to withdraw
their participation at any point. There were no cases of participant
The lifting task was the same as the one described in Sullivan
et al . Participants were asked to stand in front of a height-
adjustable table, and to lift 18 canisters (4-liter size paint canisters)
that were partially filled with sand. The canisters weighed 2.9, 3.4
or 3.9 kilograms and were arranged in 6 columns with 3 canister
positions. The canisters were positioned such that each weight
was represented twice in each location of a double latin square.
D.I. Lambin et al./PAIN
?152 (2011) 1424–1430
The height of the table was adjusted so that the handle of the
canisters in the first row (ie, closest to the participant) was at
standing elbow height. Participants were asked to lift the canisters,
with their dominant arm, in a pre-determined sequence (ie, col-
umn 1, first, second, third row; column 2, first, second, third
row; etc). The experimenter demonstrated the lift of the first three
canisters to minimize inter-individual variations in the approach to
the lifting task.
Participants assumed three postural positions to perform the
lifting task. For canisters in the first row (ie, closest to the body),
the participant stood erect with his or her elbow bent at 90 de-
grees; for canisters in the second row, the participant stood erect
with his or her arm fully extended; for canisters in the third row,
the participant’s trunk was forward flexed with his or her arm fully
extended . Fig. 1 depicts the weight and position configuration
of the canister-lifting task. In a previous study, it was estimated
that mean net moments (ie, force ? distance corresponding to
the weight and body segments) was approximately equivalent
across columns, varying from 17.3 to 17.9 Nm at the shoulder
and from 34.0 to 35.0 Nm at the back (L4/L5 joint) . The corre-
sponding mean percentage of strength varies from 40.3% to 41.5%
at the shoulder and from 20.2 to 20.7% at the back.
The participants performed the lifting task twice; in one trial,
participants were asked to provide a verbal rating of their pain as
they lifted each of the 18 canisters. In a second trial, participants
were asked to provide a verbal estimate of the weight of each of
the canisters they lifted. Participants could provide their estimates
either in imperial or metric units. All weight estimates were trans-
formed into metric units for analysis. The order of the pain rating
and weight estimation trials was counterbalanced across partici-
pants. Participants were asked to proceed through the pain rating
and weight estimate tasks at a comfortable pace. There was an
interval of one minute between the two trials of the lifting task,
during which time the experimenter provided instructions for
the second trial.
Participants were video taped while performing the canister-
lifting task. From the video records, two trained coders calculated
the duration of each lift (defined as the number of seconds that a
canister was off the table) and calculated the duration of inter-lift
intervals (defined as the number of seconds elapsed from the re-
lease of the canister handle to the time the next canister was lifted
off the surface of the table). The correlations between coders’ cal-
culations of canister lift duration and inter-lift intervals were .92
and .94. The average of the coders’ calculations of lift duration
and inter-lift interval were used in analyses.
2.4. Data analytic approach
As described above, participants lifted 18 canisters arranged in
6 ‘‘columns’’ of 3 canisters. Each weight was represented within
each column, thus equalizing columns in terms of total weight
lifted and in terms of net loading (torque) at the shoulder and
the back. Mean pain ratings within ‘‘Column’’ were computed. An
index of RISP was derived by subtracting the mean for pain ratings
for canisters in the 1st Column from the mean pain ratings for can-
isters in the 6th Column. Higher values on the index of RISP reflect
greater increase in pain across successive lifts. Average lift duration
and average duration of inter-lift rest periods were also computed
for the first and last columns of canister lifts.
The difference between the participants’ weight estimates and
the actual weight of the canisters was used as an indirect measure
of perceived work demands . Differences between actual
weights and weight estimates were averaged within ‘column’. Po-
sitive values reflect over-estimates of weight and negative values
reflect under-estimates of weight. In previous research using the
canister-lifting task, weight estimates have been shown to vary
according to postural position and repetition of lifts [7,38]. As such,
there is support for the view that the weight estimates during the
lifting task reflect, at least in part, the perception of physical
Means and standard deviations were computed for demo-
graphic and dependent measures. Separate t-tests for independent
samples were used to examine group differences on demographic
and dependent measures. Mixed analysis of variance was used to
Fig. 1. Force demands at the shoulder and at the back (L5/L5 joint) for the canister lifting task.
D.I. Lambin et al./PAIN
?152 (2011) 1424–1430
examine group differences in pain ratings and weight estimates
over successive lifts. The pain ratings and weight estimation data
from the lifting task were initially analyzed as a three-way mixed
factorial with Diagnosis (FM, CLBP) and Task order (pain rating
first, weight estimation first) as between groups factors and Col-
umn (Columns 1 through 6) as the within groups factor. Initial
analyses revealed main effects for Task order such that pain rat-
ings, F (1, 96) = 4.1, p < .05, and weight estimates, F (1, 96) = 3.8,
p < .05, were significantly greater when the respective tasks were
performed second. Task order did not interact with any other fac-
tor. As such, task order was not included as a factor in the analyses
reported below. For the repeated measures analyses of variance, in
cases where sphericity was violated, the Greenhouse–Geisser cor-
rected F is reported.
3.1. Sample characteristics
Mean scores on measures of condition-related pain severity,
fear of movement/re-injury (TSK), disability (PDI), depression
(BDI-II), and catastrophizing (PCS) for FM and CLBP participants
are presented in Table 1. T-tests for independent samples revealed
that participants with FM obtained higher scores on the PDI (t
(98) = 3.1, p < .01), than participants with CLBP. Participants with
FM rated their pain during the canister-lifting task (averaged
across 18 canister lifts) as marginally more intense than partici-
pants with CLBP (t (98) = 1.9, p = .06). Participants with FM ob-
tained higher scores on the index of RISP than participants with
CLBP (t (98) = 3.2, p < .001). No significant differences were found
between participants with FM and CLBP on the TSK, the BDI-II
and the PCS. Means and standard deviations on these measures
are comparable to values that have been reported in previous re-
search with individuals with FM [8,15,42].
3.2. Correlations among measures
Correlations among the three pain indices (ie, NRS, MARP, the
index of RISP), and the psychological variables (PCS, TSK, BDI-II,
PDI) are presented in Table 2. All psychological variables were sig-
nificantly inter-correlated. The index of RISP was significantly cor-
related with MARP and the PDI. Consistent with previous research,
the index of RISP was significantly correlated with the TSK .
The BDI-II and the PCS were not significantly correlated with the
index of RISP.
3.3. Repetition-induced activity-related pain
A two-way (Diagnosis ? Column) repeated measures analyses
of variance (ANOVA) revealed a significant main effect for Column,
F (5, 490) = 36.6, p < .001, and a significant Diagnosis ? Column
interaction, F (5, 490) = 6.7, p < .001. The main effect for Diagnosis
was marginally significant, F (1, 98) = 3.5, p = .060. The results of
this analysis are presented in Fig. 2. Tests of simple effects revealed
that participants with FM and CLBP did not differ significantly for
pain ratings provided for the first column of canisters. Participants
with FM reported more intense pain for canisters lifted in the sec-
ond, fifth and sixth columns, ps < .05.
Diagnosis-related differences on the index of RISP were also
examined in relation to the number of participants who showed
an increase in pain ratings of three points or greater from the first
to the sixth column of canister lifts. An increase of three points or
greater on a 0–10 severity scale would be considered clinically sig-
nificant . Results of this analysis showed that 28% of partici-
pants with FM experienced an increase in pain of three points or
greater from the first to the sixth column of canisters, compared
to only 6% of participants with CLBP (v2= 8.7, p < .001).
3.4. Task-related influences on pain summation: work output
An index of work output was computed by dividing lift duration
by the duration of rest periods between lifts (for the pain rating
task). Higher scores on this measure reflect greater energy
Means and SDs on demographic and pain-related variables.
FM (n = 50) CLBP (n = 50)
Pain duration (years)
FM, fibromyalgia; CLBP, chronic low back pain; PCS, Pain Catastrophizing Scale;
TSK, Tampa Scale for Kinesiophobia; BDI-II, Beck Depression Inventory II; PDI, Pain
Disability Index; MARP, Mean Activity-Related Pain; RISP, Repetition Inducted
Summation of Activity-Related Pain.
Values in parentheses are SDs; significance values are 2-tailed.
Correlations among pain indices and psychological variables.
1. Pain (0–10)
Note: n = 100.
Pain, condition-related pain severity; PCS, Pain Catastrophizing Scale; TSK, Tampa
Scale for Kinesiophobia; BDI-II, Beck Depression Inventory II; PDI, Pain Disability
Index; MARP, mean activity-related pain; RISP, repetition-induced summation of
Values in parentheses are SDs.
*P < .05.
**P < .01.
Fig. 2. Pain ratings as a function of lift repetition and diagnosis. Pain intensity
values represent the mean pain ratings provided for 3 canister lifts in each column.
D.I. Lambin et al./PAIN
?152 (2011) 1424–1430
expenditure relative to rest periods. A two-way (Diagnosis ? Col-
umn) ANOVA was conducted in order to examine whether the
work output of performing the lifting task also showed a repeti-
tion-induced summation effect and varied according to Diagnosis.
This analysis was conducted to examine whether higher scores on
the index of RISP by participants with FM were due to greater work
output during the task. The analysis yielded a significant main ef-
fect of Column, F (5, 490) = 8.0, p < .001, and a significant two-way
interaction, F (5, 490) = 2.3, p < .05. The main effect for Diagnosis
was not significant. As shown in Fig. 3, there was no indication that
participants with FM expended more energy during the task than
participants with CLBP.
Change in work output was inversely correlated with index of
RISP, r = ?.24, p < .01, indicating that participants expended less
energy during the lifting task as their pain levels increased.
3.5. Perceived work demands (accuracy of weight estimates)
Indices of perceived work demands were derived as the mean
discrepancy of actual and estimated weights for each of the three
weights (ie, 2.9 kilograms, 3.4 kilograms, 3.9 kilograms). A two-
way (Diagnosis ? Weight) ANOVA revealed significant main effects
for Diagnosis, F (1, 98) = 4.1, p < .05, and Weight (FGreenhouse–Geis-
ser(2, 196) = 30.1, p < .001). As shown in Table 3, participants with
FM estimated the weights to be heavier than participants with
CLBP, and the weight of heavier canisters was overestimated com-
pared to the weight of lighter canisters.
The findings of the present study replicate previous findings
showing that a subset of individuals with persistent pain condi-
tions experience clinically significant increases in pain in response
to repeated low or moderate level physical activity . The find-
ings extend previous research in showing that individuals with FM
are more susceptible than individuals with CLBP to activity-related
increases in pain, as quantified with the RISP index. There were no
differences between the two groups on level of condition-related
(ie, spontaneous) pain or in pain ratings provided during the lift
of the first three canisters (ie, activity evoked pain). As such, the
higher index of RISP in participants with FM cannot be attributed
to generalized hyperalgesia. It is also unlikely that the heightened
index of RISP in individuals with FM was the result of increased
force or energy expenditure. A measure of work output was de-
rived as a ratio of lift duration to inter-lift rest periods. Analyses re-
vealed that work output decreased over successive lifts for both
groups, and no significant effect of diagnosis was observed.
The results of the present study replicate previous research
showing that the index of RISP was positively correlated with fear
of movement . It has been suggested that fear of movement
might impact on the index of RISP by contributing to increased
muscle tension, in turn, leading to ischemic changes that could, di-
rectly or indirectly, increase nociceptive input to the spinal cord
. However, it is also necessary to consider an alternate causal
pathway. Fears of movement might be more likely to develop in
individuals who experience increasing pain in response to repeated
movement. Although fear of movement was correlated with RISP,
there were no indications that fear of movement accounted for
diagnosis-related differences in RISP.
Numerous research investigations have shown that while exer-
cise leads to hypoalgesic responses in pain free individuals, exer-
cise appears to contribute to hyperalgesia in individuals with FM
. It has been suggested that proinflammatory responses to
muscle activation might be one of the processes that underlies
exercise-induced hyperalgesia in individuals with FM . These
same proinflammatory responses might also underlie the greater
susceptibility to RISP in individuals with FM. It has also been sug-
gested that central processing of nociceptive stimuli from the mus-
cles might be upregulated in patients with FM [14,21].
At present, the precise mechanisms underlying RISP remain un-
clear. As noted earlier, TS of pain has been discussed in relation to
central sensitisation consequent to sustained or repeated C-fiber
input to the dorsal horn . It is possible that similar mechanisms
contribute to RISP. However caution must be exercised in extrapo-
lation TS research to RISP. One of the distinguishing features of TS
and RISP paradigms concerns the temporal component of noxious
stimulation. In most studies examining TS in individuals with FM,
TS has been observed with stimulus presentations of heat pulses of
approximately 1 second duration and inter-stimulus intervals of
approximately 3 seconds. Extending the inter-stimulus duration
to more than 3 seconds appears to attenuate the TS effect. There
is one study, however, that showed that in response to repeated
mechanical pressure to peripheral muscles, TS effects were
observed in individuals with FM even when the inter-stimulus
interval was 5 seconds . In the present study, over 95% of the
inter-lift intervals were between 2 and 5 seconds. It is possible that
the inter-stimulus period necessary to elicit summation of pain
might differ for signals emanating from mechanical stimulus on
the skin or the muscle.
Another difference between paradigms used to generate TS and
RISP is that RISP arises from self-initiated and self-paced muscle
activation whereas the stimuli used to generate TS are experi-
menter controlled. A host of central and peripheral processes asso-
ciated with self-initiation of movement might play a role in RISP
but not in TS. The self-initiation component of RISP might afford
it greater ecological relevance, at least in relation to the level of
disability observed in individuals with FM. Participation in daily
activities requires self-initiated muscle activation and many activ-
ities of daily living require repeated muscle activation. It is inter-
esting to note that although participants with FM did not differ
Fig. 3. Work output as a function of lift repetition and diagnosis. The work output
values represent the mean of work output (duration/rest) values for 3 canister lifts
in each column.
Perceived work demands as a function of pain condition.
Canister weight FM (n = 50)CLBP (n = 50)
Note: Index of perceived work demands was computed as the difference between
actual canister weights and estimated canister weights. Values in parentheses are
FM, fibromyalgia; CLBP, chronic low back pain.
D.I. Lambin et al./PAIN
?152 (2011) 1424–1430
significantly from individuals with CLBP on the severity of their
condition-related pain, or on the intensity of the pain they experi-
enced in response to the first three canister lifts, participants with
FM showed greater RISP and more pronounced disability than par-
ticipants with CLBP. RISP might be one of the factors that contrib-
utes to heightened disability in individuals with FM.
The clinical implications of the present findings warrant reflec-
tion. The results of the present study might offer insight into the
factors underlying poor rehabilitation treatment outcomes for
individuals with FM. Intervention approaches that incorporate
exposure to repeated physical activity might augment the pain
experience of individuals with FM. The results of the present study
showed that increases in pain were associated with decreases in
work output, which could reflect efforts to compensate for in-
creases in pain experience. If individuals with FM approach reha-
bilitation exercises with the same compensatory approach, they
will likely benefit less from treatment. It is also possible that in-
creases in pain during rehabilitation treatment might impact neg-
ativelyon individuals’ expectancies
outcome, or their motivation to comply with treatment.
Research on the therapeutic benefits of activity/exercise sug-
gests that hypoalgesic effects of exercise may follow an initial per-
iod of hyperalgesia [4,23]. Following more extensive repetition of
activity, processes associated with descending inhibition might
lead to an attenuation or reduction in RISP. Research conducted
to date has not addressed whether RISP values continue to increase
until pain tolerance is reached, or whether some type of habitua-
tion occurs. It is possible that FM might be associated with a de-
layed onset, as opposed to absence, of descending inhibitory
processes in response to physical activity.
Caution must be taken in the interpretation of the results of the
present study. First, the pain induced in the laboratory setting may
not be comparable to the pain experience of individuals in their
day-to-day activities. In the laboratory, participants are reassured
about the safety of the task and the benign nature of the pain they
are likely to experience. Such reassurance does not accompany the
pain exacerbations that individuals with persistent pain might
experience. The rigor demands of experimentation necessarily
tradeoff a certain degree of ecological validity. It is also important
to note that the average increase in pain during the lifting task was
less than 2 points on an 11-point severity scale. As such, the clin-
ical relevance of the findings warrants cautious reflection. Finally,
although participants with FM and CLBP were matched on their
condition-related pain (ie, spontaneous pain), they were not
matched on number of pain sites. By definition, FM is associated
with widespread pain. It is possible that the demand of the lifting
task engaged more affected muscle groups for participants with FM
than with CLBP. In other words, it is possible that differences in
RISP were the result of group differences in the number of pain
sites as opposed to factors specific to the pathophysiology of FM.
It is also important to note that although the absolute value of
weight lifted was constant across columns, the net moments (ie,
force required to lift the canisters) varied slightly across columns.
In order to rule out the possibility that RISP effects were due to
slight variations in force demands across columns, the data were
re-analysed using an index of RISP computed as the difference in
pain ratings provided for the lift of the 2.9 kilograms canister in
column 1, from the pain ratings provided for the lift of the 2.9 kilo-
grams canister in column 6. Both canisters are in the third position,
thus equalizing force demands in terms of weight and position. Re-
analysis of the data yielded a pattern of findings identical to that
reported with the index of RISP computed as the difference in
mean pain ratings from the first to the 6th column of canisters.
For ethical reasons, it was necessary to keep the physical de-
mands of the task within acceptable limits. The disadvantage how-
ever was that many participants provided activity-related pain
ratings, particularly for the 2.9 kilograms canisters that were below
their spontaneous pain severity. To date, little research has been
conducted on how individuals ‘compartmentalize’ their pain expe-
rience when multiple sources of pain are being experienced. How-
ever, the process appears more involved than the cummulative sum
of the different sources of pain. Previous studies using measures of
evoked and spontaneous pain have reported similar findings .
In spite of limitations, the results of the present study show that
individuals with FM are more susceptible to RISP than individuals
with CLBP. The findings are consistent with empirical and anec-
dotal accounts of the difficulties that individuals with FM experi-
ence in the context of repeated physical activity. Future research
aimed at elucidating the parameters of RISP could provide the
empirical foundation for the development of more effective reha-
bilitation approaches for individuals with FM.
Conflict of interest
The authors have no financial interests related to the findings
reported in this paper.
This research was supported by a grant from the Canadian Insti-
tutes of Health Research. The authors thank Nicole Davidson and
Beatrice Garfinkiel for their assistance in recruitment, testing,
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