Conditioned pain modulation in oarsmen compared to non-athletic controls - the ROW study

Pain is common phenomenon in athletes. In addition to sports-related injuries,
fanatic sportsmen even continue to exercise while they suffer from severe
injury. On the other hand, physical activity is part of most pain treatment
programs. There is consistent evidence that physical activity activates
generalized endogenous pain modulation mechanisms, such as exercise-induced or
stress-induced hypoalgesia. These phenomena involve reduced pain perception
during or shortly after intense stress or exercise.1
In contrast to the acute effects of exercise, there is controversial evidence
of the effect of chronic exercise on pain perception or pain modulation in
athletes. Two recent studies addressed the more long-term alterations in
endogenous pain modulation in endurance athletes. In the first study, it was
shown that triathletes had a greater conditioned pain modulation (CPM)
magnitude,2 which is an experimental measure of endogenous pain inhibition,
whereas the second study demonstrated reduced capability of endurance athletes
to engage CPM pathways.3 The latter was possibly explained by exhaustion of the
endogenous pain inhibitory pathways due to chronic overstressing. The
discrepancies between the two study outcomes may be related to the type of
athletes, methodological or analytical differences.
A second factor known to affect the CPM magnitude is ageing. There is
increasing evidence in literature that older adults have decreased CPM
engagement. This phenomenon is thought to contribute to the reduced ability of
older adults to cope with severe persistent pain states and to the greater
prevalence of pain in older age.4-6
Oarsmen are competitive rowers with high frequency training schedules,
sometimes up to 6 training exercises per week. Training and competitions induce
a considerable amount of stress and pain endurance over several hours per day.
Rowers frequently suffer from rib injuries and low back pain. Competitive
rowers and experimental pain tests have been the topic of one publication,
where pain thresholds and pain tolerances in 20 male rowers in training and 20
male controls were assessed.7 Pain tolerances were higher for competitive
rowers,7 indicating that pain modulation may be altered in these athletes.
Competitive rowing athletes provide a unique opportunity to study the effect of
chronic physical exercise on the endogenous pain modulation system. We have the
opportunity to test *young* and *old* oarsmen, whereby we can assess the effect
of intensified sports and ageing on CPM magnitude.

The aim of this study is to assess differences in the endogenous pain
modulation paradigm in older and younger subjects, with and without an
intensified sports regimen (ie. oarsmen).

Heat pain model. A thermal stimulus will be applied on the volar side of the
forearm using the thermal probe (a 3 X 3 cm thermode) of the Pathway or Q-sense
devices (Medoc Ltd, Ramat Yishai, Israel). These are computer-controlled
devices capable of generating highly reproducible thermal stimuli. The Visual
Analogue Scale (VAS) will be measured electronically using a slide
potentiometer that can be moved from the left (0 or no pain) to the right (10
cm or most intense pain imaginable).
Electrical pain model. We will use the Computer-Interfaced Current Stimulator
(CICS) that was developed at LUMC by R. Sloos (Technical Department) and E.
Olofsen (Dept. of Anesthesiology). Transcutaneous electrical stimulation will
applied to the skin via two surface electrodes (Red Dot; 3M, London, Ontario).
The electrodes are applied on the proximal forearm at a mid-position between
the volar and dorsal aspects (avoiding stimulation above a main nerve trunk).
In this study we will use fixed stimuli and will use the numerical rating scale
(NRS) to score the electrical pain intensity. We will use a stimulus that will
cause an NRS score of 6.
Cold pain. Cold pain will be applied to the foot of the patient. For baseline
measurements, the foot of the patient will be immersed in a cold water bath
ranging from 6-12 oC. The temperature that evokes an NRS of 3 will be used in
the remainder of the study.
Baseline measurements. Prior to the test sequence, the patients will be
familiarized with the experimental set up. The procedure will be explained to
them in detail. Next, a set of test stimuli is applied to obtain setting for
fixed pain scores of 6 for heat stimuli and for electrical stimulation. To that
end, multiple in strength increasing stimuli are applied. For heat pain the
ramp starts at 38 oC and the heat will increase to 43 oC and plateau for 10 sec
before returning to baseline. The patient will score the pain using the VAS.
When the pain rating is < 6, a next stimulus will be applied (from 38 to 44
oC). Again, when the pain score is < 6 cm next stimulus will be applied. The
first temperature at which the pain score is 6 or greater will be used in the
study.
For the electrical pain protocol a similar approach is applied. Each stimulus
is rated verbally, and the first stimulus that gives an NRS of 6 or greater
will be used in the study. Steps start at 0.5 mA and will increase by 0.5
mA/sec.
Conditioned Pain Modulation (CPM). The test stimulus is made up of a heat
stimulus to the volar side of the non-dominant forearm and is applied using the
Medoc 3 x 3 cm thermode using the Q-sense or Pathway devices. The temperature
of the thermode is maintained at 32 oC for 20 seconds followed by a temperature
that gives an NRS of 6 (this stimulus is defined at baseline). The stimulus is
maintained for 10 seconds. The patients will score pain perception using the
electronic VAS system. Then the thermode is removed and 3 electrical stimuli
(again aimed at NRS 6) are applied with a 1 s interval. The stimuli are applied
to the non-dominant arm. The patient will verbally rate each of the stimuli on
NRS.
First the test stimuli will be applied alone (ie. first pain). After a 10 min
break the test stimulus will be applied together with the second pain stimulus,
the conditioning stimulus. The conditioning stimulus is a cold water stimulus
to the foot (NRS 3), contralateral of the dominant arm. The foot will be
immersed (t = 0 s), and the test sequence (heat and electrical pain) will be
started 20 seconds after foot immersion. See also figure 1. The CPM test will
take about 15 min and will be repeated three times per test with a 5 minute
rest period between tests (total test duration 40 minutes).

For this study, volunteers will be recruited. The study involves sensory
testing models with short pain stimuli. The estimated risk for the participant
is therefore minimal. The potential damage that could be inflicted onto the
subjects is also mild, as these tests are known to be well tolerated. The risk
for the subjects in our opinion is therefore negligible.