Deep tissue hyperalgesia after computer work

2.1 Subjects

A pain group consisting of 14 women and 10 men with chronic shoulder and neck pain and a reference group of 16 women and 12 men, all healthy and pain-free, were recruited through advertisements in local papers and the Internet.

Inclusion criteria for the pain group were: self reported pain in the shoulders or neck for at least 2–3 days per week during the previous 4 weeks and tender points in the upper trapezius muscles, age between 18 and 45 years, and working more than 80% full time and working with a computer more than 20% of the working time. The subjects had to be familiar with the Norwegian language to perform the text-editing task. Exclusion criteria were: fibromyalgia; cervicobrachialgia; rotator tendinosis or other shoulder disorders; inflammatory, metabolic or cardiac diseases; regular medication of importance for circulation; pregnancy; alcohol or medicament abuse; or dyslexia. The reference group was recruited with the same inclusion- and exclusion criteria as for the pain group, except that subjects with current musculoskeletal pain were excluded.

Two women in each group were excluded due to reporting pain as a consequence of the laser Doppler probes insertion in the trapezius muscle (see Refs. [31,32]). The mean age for those in the pain group was 39 ± (1SD) 6 years vs. 33 ± 6 years for those in the reference group, p < 0.001, and the body mass index respectively 23.2 ± 2.8 kg/m² vs. 23.6 ± 3.2 kg/m², p = 0.62. All except two were right handed. For further characteristics of the subjects, see Ref. [32].

All participants received written information and signed an informed consent. The Norwegian Regional Committee for Medical Research Ethics and the Norwegian Social Science Data Services approved the study.

2.2 Experimental protocol

All subjects reported to the laboratory twice; a pretest session and an experimental session. On average 5.5 (range 1–12) days separated the two sessions for subjects in the pain group vs. 4.2 (1–15) days for those in the reference group (p = 0.12).

At the pretest all subjects were examined by a specialist of physical medicine and rehabilitation. The examiner was aware of the subject’s pain status, thus not blinded. The examination included range of motion of the cervical spine and shoulders, tests for nerve compression in the neck and upper extremities, and tests for subacromial problems. A neurological examination of muscle force, reflexes (biceps, triceps and brachioradialis tendon reflexes), and sensory function was performed on the upper extremities to identify any exclusion criteria. In addition, tender points in the neck and shoulder muscles, defined as localized pain occurring on a thumb pressure below 4 kg, and the typical areas of tenderness in fibromyalgia, were examined. None of the subjects fulfilled the ACR criteria for fibromyalgia [33], but all subjects in the pain group had tender points in the upper trapezius muscles (i.e. perceived pain on testing). The pretest further included familiarization with the measurements and procedures, and test trials of pressure pain thresholds before and after a 15 min computer work task training. During the computer training session the subject was encouraged to work as fast and accurately as possible without any pauses. The performance of this training session served to set the subject’s standard for time pressure (see below), although the subjects were not aware of this.

At the experimental day the subjects reported to the laboratory in the morning. Pre-work PPTs were determined approximately 90 min before starting the computer work task, and before mounting the measurement equipments (the present study is part of a larger study, for more details see Refs. [31,32]. Post-work PPTs were performed 15 min and 30 min into the recovery period following the 90 min computer work task.

2.3 Measurements

Pressure pain threshold, i.e. the pressure (kilopascals; kPa) when the sensation changed to a sensation of pain [34], was measured with an algometer (Somedic, Sollentuna, Sweden) with a tip size of 1 cm² covered with rubber. Pressure algometry provides a reliable and valid measure of PPT [35,36], reflecting pain sensitivity of deeper tissues [37]. The subjects were comfortable seated in a chair with the arms resting at a table, and the PPT recording sites were located and marked on the skin to ensure the same points to be used at all measurements. The pressure was applied perpendicular to the skin at a standardized rate of 50 kPa/s [30,37] in a fixed order; right and left medial upper trapezius, right and left muscle belly of extensor carpi radialis, and at sternum. The sequence was repeated three times and the mean value for each point was calculated and served as the PPT. The subjects reported the PPT by pressing a signal button attached to the algometer. All measurements were done by the same investigator. The subjects were explicitly instructed to respond to the threshold and not the tolerance.

2.4 Statistics

Statistical analyses were performed using the Statistical Package for the Social Sciences (release 18.0, SPSS Inc., Chicago, IL, USA). Mean ± 1standard deviation (SD) are reported, if not otherwise stated. For correlation analyses Spearman’s rho was used. A two-tailed significance level of 5% was adopted.

The pre-work pressure pain thresholds were analysed using a 2-way ANOVA, with pressure pain threshold as dependent variable with measure site (active/inactive side of both upper trapezius and extensor carpi radialis, and sternum) and group (pain group/reference group) as fixed within factors. Post hoc tests were made using Bonferroni adjustment. To test for between-group effects, an interaction term (measure site*group) was added to the model. Due to the significantly younger age in the reference group than in the pain group, age was included as a covariate.

To fit the best model for the time course, i.e. modeling the mean response of the PPT at each of the five locations (i.e. at the active and inactive side of the upper trapezius, the active and inactive side of the extensor carpi radialis and at the sternum), linear mixed model analysis for repeated measurements was used [38]. Separate analysis was performed for each of the measure sites, where the time was treated as a categorical variable in which the measures taken after the work task (post-work) were compared with the baseline (pre-task) measure. Normality of the data was checked with residual plots, and due to non-normality, the PPT data of the extensor carpi radialis in the pain group were logarithmically (log) transformed prior to the time course analysis.

Differences in the time course of the PPT between the groups and between men and women in each group were tested by separate analyses for each measure site with use of the mixed model analysis, as described above. Age was included as a covariate in the analyses of group differences.

Longitudinal data show that the variances are seldom constant over time [38]. With repeated-measures data the correlation between time-points decreases as the time increases [39]. Thus, the models were fitted either with an unstructured or a heterogeneous first-order autoregressive covariance structure based on Akaike’s information criterion for goodness of fit [38].

3.1 Pre-work pressure pain thresholds

The pre-work PPTs were not different between the groups at any of the five locations (Table 1). In the reference group, lower pre-work PPT at sternum was found in the women than in men (307 ± 72 kPa vs. 385 ± 116 kPa, p = 0.034).

3.2 Changes in pressure pain thresholds

The pressure pain thresholds in the active and inactive side of the upper trapezius and in the extensor carpi radialis of the active forearm were significantly reduced at post-work compared with the pre-work levels in both the pain and reference groups, as well as in the extensor carpi radialis of the inactive resting forearm in the pain group (Table 2).

During recovery, from 15 min post-work to 30 min post-work, a statistically significant increase in PPT was found for the inactive trapezius in the pain group (95% CI 1–39 kPa) and at sternum in the reference group (95% CI 10–46 kPa).

3.3 Group differences in pressure pain threshold changes

The changes in PPTs from pre-work to post-work (Fig. 1a and b) were significantly different between the groups only for the inactive extensor carpi radialis at 30 min post-work (p = 0.050).

Fig. 1

Changes in pressure pain thresholds (PPTs) from pre-work to post-work. Mean changes (kPa) in PPTs from pre-work to (a) 15 min post-work and (b) 30 min post-work at the different measure sites in subjects of the pain (n = 22) and reference (n = 26) groups after performing computer work for 90 min. Negative bars indicate reduction in the PPT and the ticks represent ± 1standard error of the mean. Asterisk (^*) indicates statistically significant difference between the groups (p < 0.05).

3.4 Pain intensity

The pain intensity results have been published previously [31,32]. In short, the pre-work ratings of current pain intensity (reported on a 100 mm visual analogue scale; VAS) prior to starting the computer task showed higher pain intensity in the pain group than in the reference group (Table 3). The increase in pain intensities during the 90 min work task was highly significant (p < 0.001) in both groups, but it was not significantly different between the groups (p > 0.15). The pain intensity did not return to baseline values in either group during the recovery period ([31,32]; Table 3).

3.5 Correlations between pressure pain threshold and pain intensity

The PPTs exhibited negative correlations with the pain intensity ratings in both groups (Table 4).

4.1 Pressure pain sensitivity after performing computer work

Performing the present computer office-work induced substantial pain in the shoulders and neck and in the forearm operating the computer mouse that lasted after the work (Table 3, and Refs. [31,32]). After performing the computer work, that was imposed with time pressure and high precision demands for 90 min without pauses, the PPTs in the shoulders and in the active side of the forearm were significantly reduced in subjects of both the pain and reference groups (Fig. 1, Table 2). This could indicate that the task produced muscular hyperalgesia to pressure stimuli.

Both peripheral and central mechanisms may be involved in muscular hyperalgesia [9]. The sensation of pressure and pain is the result of stimulation of nociceptive nerve endings in superficial and deeper tissues. Sensitization of muscle nociceptors may lead to a decrease of the mechanical threshold into the innocuous range, and thus represents a peripheral mechanism which may explain aspects of muscle hyperalgesia [28]. Experimental muscle pain and muscular hyperalgesia to mechanical stimuli have been reported after intramuscular injection of the combination of serotonin and bradykinin [40], but have, to our knowledge, not been reported previously after a session of experimental low-load computer office work. Muscle nociceptors may be activated by inflammatory substances such as bradykinin, serotonin and prostaglandins [28]. The present computer work task induced peripheral vasodilation in the trapezius muscles, similar in both groups, that was found to be associated with the pain intensity [31,32], a finding that may support the hypothesis that the blood vessel–nociceptor interaction is important to the activation of muscle nociceptors (see Refs. [41,42]). In the skeletal muscles nociceptors are located at the wall of arterioles and in the surrounding connective tissue [7]. The muscle pain may arise from interaction between the vessel and nerve of the connective tissue of the muscle, where putative mechanisms are vasodilation, vascular production and release of algogenic factors, and infiammation [41,42]. Both the nerves that innervate the blood vessels and the vessels themselves may produce factors that contribute to nociception, like prostaglandins, bradykinin, nitric oxide, and serotonin. In patients with chronic work-related trapezius myalgia increased levels of muscle serotonin, glutamate, [43], and bradykinin [44] have been reported at rest and during exercise compared to healthy controls.

The present study protocol exposed the subjects to several PPT measurements at the same site that possibly could result in local tissue responses brought on by the repeated pressure stimulation. Thus, the reduced pressure pain thresholds found after work could possibly also be due to a time effect of the repeated pressure stimulation, and not due to the work task performed. However, if the repeated pressure stimulation was to be the main causation of the hyperalgesia, it would be reasonable to assume that the 30 min post-work values should be further reduced from those at the 15 min post-work. Analyzing the change in PPT from 15 min post-work to 30 min post-work revealed no statistically significant change except for the inactive trapezius in the pain group and at sternum in the reference group, where the PPTs increased. In addition, the PPTs were negatively correlated with the perceived pain intensities (Table 4). Furthermore, test-retest reliability of PPT values has been found to be robust and highly consistent during repeated measurements [26,45]. In the study by Jones et al. [45] a lowering of PPTs with repeated measurements was however reported, but the decrease in PPT was mainly seen between measurement 1 and 2. Similarly, in the present study we found the pre-work PPTs to be significantly lower than that at the pretest (data not shown), thus indicating the importance of being familiarized with the measurements.

The reduced PPT’s after the computer task in the present study contrasts with findings of studies investigating PPT during work. Nakata et al. [17] found no significant trapezius PPT change during experimental light assembly work for 2 h in 13 healthy females, but found PPT to be negatively correlated with perceived muscular discomfort (mean VAS ca 20 mm). Mathiassen and Winkel [16] measured trapezius PPT repeatedly during a working day in eight healthy females during light assembly work and found the PPT to be lowest in the beginning of the work and thereafter steadily increasing. Increased PPT has also been found after static submaximal exercise of the trapezius [24,25,26], infraspinatus and quadriceps femoris muscles [22], possibly due to activation of central nervous antinociceptive mechanisms [46]. Performing 50 dynamic eccentric contractions of the shoulder muscles at a force equal to 100% maximum voluntary contraction (MVC) caused, however, the PPTs in the trapezius to be decreased [47]. In the present study the muscle activity levels (reflected by electromyography; EMG) were low both bilaterally in the upper trapezius muscles (<5% of EMG during MVC) and in the active forearm muscles (<10% of EMG during MVC), and similar in both groups [31,32]. Different types of work, exercise, and intensities thus seem to elicit different responses in pressure pain thresholds.

In the inactive resting forearm of the chronic pain subjects, the pressure pain thresholds were decreased after the work task as well, and tended to be at sternum (Table 2). This was in contrast to the lack of significant PPT changes at these measure sites in the reference group (Table 2). Lowered pressure pain thresholds in sites distant from pain areas have been reported in subjects with shoulder and neck pain after employment in low-load repetitive work for several months [15] or years [11,30], and suggests a generalized sensitivity to pressure [29] that may involve central sensitization [9]. Generalized musculoskeletal pain is one of the most important clinical characteristics of the fibromyalgia syndrome (FM) [33]. However, FM subjects were not included in the present study. During the present computer work task there was a modest increase in the pain intensity (approximately 20 mm) in the resting forearm of the subjects of both groups. The participants had on their third finger on this side mounted a cuff for measurement of heart rate and blood pressure (see Refs. [31,32]), and the rated pain may reflect unpleasantness with this cuff. Furthermore, the present study protocol exposed the subjects to several PPT measurements at the same site that possibly could result in local tissue responses brought on by the repeated pressure stimulation, as discussed above. Both groups were however exposed to the same protocol. Thus, a more generalized hyperalgesia and a possible involvement of central mechanisms cannot be excluded in the chronic pain subjects.

The substantial increase in pain during the present computer work, the lasting pain after the work and the reduced PPTs, may indicate need for frequent pauses during computer work in order to avoid pain to increase and sustain after work, and thus to prevent the possibility of pain to become chronic. This might especially be important when performed with time pressure and high precision demands. Further studies are needed to elucidate this.

4.2 Pain threshold levels

The pre-task PPTs (Table 1) appear to be in accordance with those reported in healthy and pain-free subjects of other studies [26,43]. Higher sensitivity to pressure pain in the shoulder and neck region is a more common finding in subjects with shoulder and neck complaints than in asymptomatic subjects [11,12,14,15,43]. The present results contrast with this in that we found similar PPT in all measurement sites in both the pain and reference groups. The subjects of the pain group reported the shoulder and neck pain complaints lasting from six months to more than ten years [32], hence they met customary criteria for chronicity. However, they were not severely pain afflicted since they were in fulltime work and not recruited from a patient population. In addition, the pain intensity change from pre-work to work task end was not significantly different from the reference group.

There were no significant differences between men and women in PPTs in the groups either before or after the office work task (except for at sternum in the reference group). This finding contradicts the majority of the literature reporting lower pressure pain thresholds in women (see Ref. [48]). Riley et al. [49] suggested that studies failing to identify sex differences in PPT have too few subjects included and thus lack of power. The present study was however not set up to investigate sex differences; hence a potential difference between men and women cannot be excluded.