Research Review By Dr. Demetry Assimakopoulos©

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Date Posted:

April 2019

Study Title:

Clinical Outcomes and Central Pain Mechanisms are Improved After Upper Trapezius Eccentric Training in Female Computer Users with Chronic Neck/Shoulder Pain

Authors:

Heredia-Rizo AM, Petersen KK, Madeleine P & Arendt-Nielsen L

Author's Affiliations:

Department of Physiotherapy, Faculty of Nursing, Physiotherapy and Podiatry, University of Sevilla, Sevilla Spain; Department of Health Science and Technology, Center for Neuroplasticity and Pain (CNAP), SMI; Sport Sciences, Department of Health Science and Technology; Center for Sensory-Motor Interaction, School of Medicine, Aalborg University, Aalborg, Denmark.

Publication Information:

Clinical Journal of Pain 2019; 35(1): 65-76.

Background Information:

Neck and shoulder pain (NSP) are extremely common workplace musculoskeletal disorders (WMSDs), affecting approximately 50% of office workers on a weekly basis (1, 2)! It has been suggested by some researchers that central nervous system (CNS) excitability may be altered in specific subgroups of office workers, defined by higher clinical pain intensity and lowered mechanosensitivity thresholds over the neck/shoulder area (3).

Temporal summation of pain (TSP) and conditioned pain modulation (CPM) have been used to asses CNS pain amplification and descending pain inhibition respectively, in various MSK conditions (4). Changes in these neurophysiological variables have been demonstrated in a variety of chronic pain conditions, such as OA, fibromyalgia, WAD and shoulder pain (5-9). When measured, TSP and conditioned pain modulation may be prognostic factors, predicting success of surgery and pharmaceutical interventions (10-12).

Strengthening exercises are known to effectively manage neck and upper extremity WMSD’s in office workers (13, 14). The purported analgesic mechanisms are preventive dampening of widespread hyperalgesia and correction of dysfunctional descending inhibitory pain modulation, leading to an end-product of exercise-induced hypoalgesia (15, 16). Eccentric exercises are frequently prescribed in the context of pain management (for patellofemoral pain, for example), however the effect of eccentric training on clinical outcomes, local and widespread sensitization, and features of central pain mechanisms have not been investigated in NSP populations. As such, these authors sought to evaluate:
  1. the clinical impact of unilateral eccentric training in female computer users with chronic NSP;
  2. pressure pain sensitivity, temporal summation of pain (TSP) and conditioned pain modulation in female office workers with and without neck/shoulder pain (NSP); and
  3. sensitization and central pain responses after an eccentric training program.

Pertinent Results:

Nineteen participants were included in the treatment protocol. Fifteen reported bilateral symptoms, while 4 endorsed unilateral pain, mostly on the right side. The average pain duration was 120 months (range: 39-207 months). There were no baseline differences in work hours between groups. Participants with neck/shoulder pain (NSP) reported higher pain intensity (P < 0.001), poor levels of neck, shoulder and upper extremity functionality (P < 0.001) and had worked with computers statistically longer than controls (P = 0.030).

Pain intensities and neck/shoulder disability decreased significantly post-intervention (P < 0.05), with exception of the DASH work module. The reduction in worst pain in the previous 24-hours and average intensity within the previous work week remained significant at 3- and 6-month follow-ups (P = 0.002). Ten participants reported an important improvement in Global Rating of Change Score (GRCS > 5) after the training.

Patients with NSP reported significantly lower Pressure Pain Thresholds (PPTs) on both lower extremities compared with controls. Surprisingly, VAS scores during the 10 repeated cuff stimulations demonstrated a significant and progressive increase, illustrating a temporal summation effect in both the NSP and control groups. There was no difference in temporal summation between groups.

Participants with NSP had a 30% lower Conditioned Pain Modulation (CPM; central inhibition) response in their Pain Detection Threshold (PDTs) compared with controls, although conditioned pain modulation (CPM) efficiency (PDT during vs. before conditioning) were not statistically significant (Writer’s note: this result may be due to the methodology of testing temporal summation first, or because of the small group size).

Following eccentric training, PPTs increased significantly over the neck and forearm, indicating an improvement (P < 0.001). A significant improvement in PPT on the contralateral lower leg was also observed (P = 0.004), indicating that the eccentric training had a positive impact on widespread pain sensitivity (exercise induced hypoalgesia). No TSP within-subject differences were observed post-intervention. There were within-group improvements in conditioned pain modulation (i.e. descending pain modulation; P = 0.014) with eccentric training, but the difference between-groups was not statistically significant. Interestingly, a negative correlation between baseline CPM and upper limb disability (DASH) was identified (Writer’s note: this indicates that the lower the conditioned pain modulation value, the higher the DASH score/shoulder-related disability).

Clinical Application & Conclusions:

This study demonstrated, for the first time, that a 5-week unilateral upper trapezius eccentric training program improved pain intensity and disability in female computer users with chronic neck/shoulder pain (NSP). Conditioned pain modulation was negatively associated with pain levels and function in this population. Higher distal sensitization was observed in the NPS group compared to controls. Eccentric training improved conditioned pain modulation and reduced local and widespread pressure pain sensitivity.

This is the first study demonstrating positive effects of eccentric training in computer users with moderate intensity and long-lasting NSP. Eccentric training significantly decreased pain intensities and neck disability beyond the minimal clinically important change (2.5 points on the NPRS and 3.5 points on the NDI). However, eccentric training did not meaningfully improve DASH shoulder disability scores. Ten out of the 19 total participants endorsed an “important improvement” on the GRCS.

The authors also demonstrated that higher distal sensitization, as assessed by lower leg PPT, was found in the NSP group compared to controls. Differences in PPT at non-injured sites may reflect altered central pain processing in women with chronic NSP. However, it is unclear if differences in PPTs represent distal hyperalgesia, discrepancy in willingness to tolerate pain compared to controls, or if it reflects certain psychological traits in the NSP group. Interestingly, however, no differences in temporal summation were identified between groups. This is contrary to other studies, demonstrating that females with pelvic pain, TMD and WAD show enhanced TSP (17-19).

There were no significant differences between groups in conditioned pain modulation efficiency. However, the authors discovered that conditioned pain modulation was negatively correlated with self-reported disability, suggesting that ongoing pain intensity and disability may influence central pain processing. The eccentric training also augmented PPTs on the treated and non-treated sides above the 20% clinically meaningful threshold.

In summary, a 5-week unilateral eccentric training program resulted in clinically relevant improvements in pain and neck disability in female computer users with chronic pain. These changes in pain intensity remained significant at intermediate- and long-term follow-up. The study additionally revealed that female computer users with moderate NSP showed higher widespread sensitization, but no differences in temporal summation and conditioned pain modulation compared to controls. The eccentric training also reduced local and distal pressure pain sensitivity and improved conditioned pain modulation in participants with NSP.

These findings are important for two reasons. Firstly, multiple rehabilitation factions maintain that dysfunctional upper trapezius tone and altered scapular kinematics dominated by heightened upper trapezius activity (scapular dyskinesis) result in and promote neck and shoulder pains. This study contradicts those beliefs. Secondly, it is maintained by some clinicians that passive and/or pain aversive therapies are the only conservative methodologies necessary for the treatment of pain. While some passive and symptom control exercises can be helpful, controlled painful exercise may alter neurophysiological processing of nociceptive inputs, leading to an analgesic effect.

Study Methods:

There were two parts to this study protocol:

Part A was a cross-sectional study on female computer users. The experimental group included patients with unilateral or bilateral neck/shoulder pain (NSP), who reported pain intensity > 2 on an 11-point Numeric Pain Rating Scale (NPRS). Those reporting no pain or occasional pain (< 2) were added to the control group. All participants reported working on a computer for a minimum of 4-hours per day, spoke and understood English, and had not been involved in neck/upper extremity strength training within the previous year. The exclusion criteria were: pregnancy, previous whiplash, history of neurological disease or mental illness, consumption of pain killers in the last 24-hours, drug addiction (opioids, cannabis or recreational drugs), previous cervical spine or upper limb surgery, fibromyalgia, carpal tunnel syndrome, concomitant lumbar spine injury, heart disease and hypertension.

Part B introduced a 5-week eccentric training intervention to participants with chronic NSP, which began 3-7 days after the baseline testing (Part A).

Self-reported pain intensities were collected for both groups at 3- and 6-month follow-ups.

Patients were asked to rate their worst pain in the last 24-hours and average pain over the last week using an NPRS. Danish versions of the DASH (Disabilities of the Arm, Shoulder & Hand) questionnaire and NDI (Neck Disability Index) were used to identify self-reported function. Patients were also asked to rate their overall neck/shoulder functional condition post-intervention using the 15-point Global Rating of Change Scale (GRCS: scored from -7 [“a very great deal worse] to +7 [“a very great deal better”]) to assess the efficacy of intervention. A GRCS score of > 5 was considered an “important improvement.”

Pressure pain thresholds (PPTs) were measured twice bilaterally using a pressure algometer, at the middle point of the upper trapezius, extensor carpi radialis brevis and extensor carpi ulnaris. Participants were cued to press a handheld button when the exerted pressure turned into pain.

A computer-controlled cuff-algometer was used to evaluate distal sensitization at both lower legs by way of pain detection threshold (PDT) and pain tolerance threshold (PTT). During this test, participants rated their pain intensity on a 10-cm electronic visual analogue scale and asked to indicate when a sensation of pressure turned into a sensation of pain > 1 on the NPRS (PDT). The participants then pressed a button to release the cuff inflation when the pain became unbearable (PTT).

Measurement of Temporal Summation of Pain (TSP):

TSP was ascertained at the lower leg on the same side of the painful/most painful neck/shoulder area, and on the dominant side for controls. Ten consecutive cuff pressure stimuli (1-second duration and 1-second break interval) were delivered by inflating the cuff chamber at the same intensity as the Pressure Pain Threshold (PTT). TSP was defined as the difference between the mean VAS score from the 8th-10th stimulus (VAS-II) and the mean VAS value from the 1st-4th stimulus (VAS-I). A higher value indicates enhanced TSP. Writer’s note: A little background here - temporal summation is defined as an increase in perceived pain intensity in response to sequential stimuli of equal strength. For instance, when the cuff inflation leads to pain, a second, third and fourth inflation would lead to more pain with each sequential inflation. The researchers did this, because the TSP formula described above (TSP-I – TSP-II) is a surrogate measure to determine amplification of nociceptive signaling, related to central sensitization. Temporal summation is believed to occur due to increased firing of second-order dorsal horn neurons in response to repetitive peripheral c-fiber stimulation. Higher pain perception with each successive noxious stimulation from cuff inflation indicates enhanced TSP, and by extension, central sensitization.

Measurement of Conditioned Pain Modulation (CPM):

Writer’s note: Just a little bit of background on what this is measuring. Conditioned pain modulation is a surrogate measurement of descending pain inhibition, which is believed to be regulated through various brainstem nuclei. Procedurally, a painful threshold is first ascertained, such as pressure pain threshold (PPT) of the upper trapezius, as is the case in this study. Another controlled painful stimulus is then introduced, such as inflation of a blood pressure cuff in the contralateral leg. In a normally functioning nervous system, after the 2nd controlled painful stimulus (BP cuff), the measured PPT at the upper trapezius should increase (i.e. takes more pressure to elicit a painful stimulus). If PPT of the upper trapezius was at 10 units of measurement as an example, after the painful blood pressure cuff inflation, the PPT in in the upper trapezius should increase to 12 units of measurement, (i.e. harder to elicit a pain response or increased threshold to pain). The increase in threshold is mediated by an outpouring of endogenous neuropeptides from brainstem nuclei. This is important, because in some chronic pain sub-populations, conditioned pain modulation function is lost, meaning descending inhibition is lost. In this study, an experimental conditioning stimulus (after acquiring the PPT) was evoked on the contra-lateral calf, using a continuous cuff-induced painful stimulus at 70% of the PPT on that side, causing a pain intensity of 5-6/10. In this study, conditioned pain modulation was defined as follows: 1) The difference between PDT during vs. before conditioning; and 2) The percentage increase in PDT when comparing baseline with scores during conditioning pain. A lower conditioned pain modulation value indicates a less efficient central pain inhibitory system.

Eccentric Training Protocol:

The participants performed 10 sessions of 25-30 minutes of unilateral upper trapezius eccentric exercises over 5 consecutive weeks. Training was performed in a back-supported seated position, using a custom-built dynamic shoulder dynamometer. Three shoulder shrug maximum voluntary contractions were performed at baseline on the painful/most painful side. After a 5-minute warm-up and a 2-minute break, the range of shoulder elevation was measured on the affected side (highest and lowest vertical scapular position). Participants then performed 3-bouts of 10 repetitions of shoulder shrugging on the painful/most painful side at 60% MVC (sessions 1-3), 8 repetitions at 70% MVC (sessions 4-6) and 6 repetitions at 80% MVC (sessions 7-10). Three seconds of rest were allowed between contractions, while 2-minutes were allowed between sets.

Statistical Analysis:

For Part A of the study, mixed-model ANOVAs were used to compare the differences between groups on PPT, TSP and conditioned pain modulation. For Part B, a repeated measures ANOVA was utilized to assess changes in pain intensity, neck/shoulder function, sensitization and central pain responses from baseline to post-intervention. Bonferroni adjustments were used post hoc. For Part A, the Spearman rank test of person product-moment correlation coefficient analysis with Bonferroni’s correction were used to test for associations at baseline between clinical data and outcome measures.

Study Strengths / Weaknesses:

Weaknesses:

This study had a small sample size and lacked assessor blinding, which limits widespread generalizability. There was also no control group for part B of the study, so learning effects may have influenced the results. The subjects’ baseline physical activity levels were also not assessed.

Interestingly, the authors mixed bilateral and unilateral pain groups. It is understood that more diffuse pain patterns are associated with greater levels of central sensitization. However, the participants generally endorsed mild disability levels at baseline, potentially indicating a lower overall level of sensitization. Psychosocial variables were not measured in this study. The authors also performed their temporal summation measurement procedure prior to measuring conditioned pain modulation. Performing the testing procedure in this order may have affected the results.

It is uncertain whether patients in the general public would be able to complete a 30-minute eccentric upper trapezius training program because of significant delayed onset muscle soreness. This may also limit the generalizability of these findings to clinical practice. Experimentally, however, it might have been necessary to induce a painful sensation to the nervous system in a controlled environment. The pain induced from exercise is one potential trigger for an improvement in CPM.

Strengths:

The authors correlated changes in pain intensity and disability with neurophysiological changes in descending inhibitory pain modulation in a clinical environment. Additionally, this was the first study to demonstrate positive changes from eccentric training in computer users with moderate intensity and long-lasting NSP.

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