The Interaction Between Psychosocial Factors And Exercise-Induced Hypoalgesia in Pain-Free Nurses Part 1

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【Contact】Email: george.deng@wecistanche.com / WhatsApp:008613632399501/Wechat:13632399501

Purpose: This cross-sectional study aimed to investigate whether psychosocial factors were predictive of exercise-induced hypoalgesia (EIH) in pain-free adults.

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Methods: A sample of 38 pain-free nurses with a mean (SD) age of 26 (6) years were included in this study. Participants completed psychosocial questionnaires before physical tests. Pressure pain threshold (PPT) was assessed bilaterally at the calves (local), lower back (semi-local), and forearm (remote) before and immediately after a maximal graded cycling exercise test. Separate linear mixed-effects models were used to determine the change in PPT before and after cycling exercise (EIH). Multiple linear regression for all psychosocial variables and best subset regression were used to identify predictors of EIH at all locations.

Results: The relative mean increase in PPT at the forearm, lumbar, calf, and globally (all sites pooled) was 6.0% (p<0.001), 10.1% (p<0.001), 13.9% (p<0.001), and 10.2% (p=0.013), respectively. Separate best subset multiple linear regression models at the forearm (predictors; Multidimensional Scale of Perceived Social Support (MSPSS) total), lumbar (predictors; MSPSS total, Pain  Catastrophizing Scale (PCS) total, Depression Anxiety Stress Scale (DASS) depression), calf (predictors; MSPSS friends, PCS total), and global (predictors; MSPSS friends, PCS total) accounted for 7.5% (p=0.053), 13% (p=0.052), 24% (p=0.003), and 17% (p=0.015) of the variance, respectively.

Conclusion: These findings confirm that cycling exercise produced EIH in young nurses and provided preliminary evidence to support the interaction between perceived social support, pain catastrophizing, and EIH. Further investigation is required to better understand psychological and social factors that mediate EIH in a larger sample of adults at high risk of developing chronic musculoskeletal pain.

Keywords: aerobic exercise, acute exercise, pain pressure threshold, perceived social support, kinesiophobia

Introduction

Acute exercise can lead to decreased pain sensitivity, which is referred to as exercise-induced hypoalgesia (EIH).1–3 This form of endogenous pain modulation, which occurs predictably in healthy pain-free populations, is characterized by a reduction in pain sensitivity, expressed as a reduction in self-reported pain intensity and a heightened pain threshold and tolerance to a noxious stimulus (such as pressure, thermal or electro-cutaneous stimuli).1–3 EIH can persist for up to 30 minutes after aerobic, isometric or isotonic exercise.1–3 The hypoalgesic response appears smaller and less consistent in non-exercised versus exercised body parts,4,5 and varies between individuals.1–3 Although mechanisms of EIH are not completely understood, endogenous opioids and endocannabinoids likely activate endogenous pain inhibitory pathways to mediate EIH.1,6–8 These physical factors can be released at peripheral, spinal, and supraspinal brain sites,7 which attenuate ascending sensory information relayed towards pain-processing brain regions.6,9–11 However, pain is more complex than physical factors alone,12–15, and the pain experiences are shown to be moderated by psychological and social factors.12,14–19

Psychosocial factors may play a mediating role in EIH for both pain and pain-free populations. Negative psychological features, such as depression, anxiety, and pain catastrophizing, influence endogenous pain modulation via descending pain inhibitory pathways.20 For instance, conditioned pain modulation, a paradigm that measures endogenous pain modulation, may be reduced by the presence of these psychological features.18,20 There is also evidence that social support in the form of verbal support and touching or viewing of an intimate other can moderate pain.19 However, research investigating the relationship between psychosocial factors and EIH is limited and conflicting.21–23 This is important because psychosocial factors such as depression, kinesiophobia, and pain catastrophizing may influence endogenous pain modulation, and increase the risk of pain onset, development of chronic pain, and increased pain intensity.21 A better understanding of the association between psychosocial factors with endogenous pain modulation may therefore assist with identifying individuals at greater risk of developing musculoskeletal pain conditions, or predict which individuals develop persistent pain.

Physically and psychologically demanding occupations such as nursing have a high prevalence of both mental illness and musculoskeletal injury.24 Nurses confront potential exposure to infectious diseases, toxic substances, back injuries due to patient lifting and challenging postures, and radiation. They are also subject to hazards such as stress, shift work, working long hours and overtime, and violence in the workplace.25,26 Nursing, a female-dominant occupation, has the second highest incidence of non-fatal occupational injury in the United States,27, and of these, 25% become chronic.28  Chronic pain is associated with the development of dysfunctional pain processing pathways29 and comorbid psychosocial health impairment.24 However, it is not clear whether endogenous pain modulation is already impaired in the nursing population when there is no current or previous history of serious musculoskeletal pain. Although considered important,1,21 few studies have investigated the influence of psychosocial factors on EIH in pain-free populations, and none have been conducted in a population known to be at high risk of developing musculoskeletal pain. Therefore, this study aimed to determine the effect of aerobic exercise on EIH, and the association of psychosocial factors with EIH in nurses without a history of chronic pain.

Methods

Study Design

This cross-sectional study was performed as a secondary analysis of a prospective cohort study investigating causative factors in the development of low back pain from August 2018 to August 2019 at Deakin University (Victoria, Australia). This project complies with the Declaration of Helsinki, and ethical approval was provided by the Deakin University  Human Research Ethics Committee (URBAN; project number 2018–221). This study followed the Strengthening the  Reporting of Observational Studies in Epidemiology (STROBE) guidelines.30

Participant Recruitment and Eligibility

Pain-free female and male adult nurses between 18 and 55 years of age were recruited from the greater Melbourne area (Victoria, Australia). This age range was selected to ensure a representative selection of nurses without a history of low back pain. Evidence shows that the first incidence of low back pain typically occurs before or during mid-life and the peak burden of disease is in the age bracket below 55 years.31 The recruitment process occurred within the months between October 2018 and August 2019. Due to time and cost restraints, and the impact of the COVID-19 pandemic and extensive government-imposed restrictions, this exploratory study ceased recruitment at the end of 2019. Primary recruitment strategies included word-of-mouth, social media (Facebook), and printed flyers distributed around hospitals and clinics. Screening against the eligibility criteria took place via phone by research personnel after participants expressed interest via the study-specific email address. The pain-free status of the participants was confirmed upon attendance at the initial testing session to reconfirm eligibility into the study. Exclusion criteria included the following: <18 or >55 years of age; traumatic spinal injury (eg, fracture); spinal surgery; low back pain history; scoliosis; prior acute spinal pain; any current spinal pain episode; treatment for low back pain (eg, allied health professional); other forms of chronic pain conditions (eg, fibromyalgia); any history of cardiovascular disease; unable to communicate in English; planning a pregnancy or currently pregnant; moving overseas or interstate; unable to abstain from smoking for 8 hours before testing; and prior or current elite athlete (defined as a member of Australian Institute of Sport, State Institutes or Academies of Sport or the national squad of any sport). All participants provided signed and written informed consent before data collection.

Maximal Graded Aerobic Exercise Test

Participants undertook a maximal graded exercise test on an electronically braked cycle ergometer (Lode Excalibur  Sport, Lode, GR) at Deakin University’s clinical exercise laboratories. The protocol was consistent with previous EIH studies by attaining high-intensity exercise (>75% VO2 max) for approximately 10 minutes of test duration.1,2 Pressure pain thresholds (PPTs) were measured immediately before cycling to attain pre-exercise pain sensitivity. The progressive protocol commenced with a 7-minute warm-up at 50 watts and increased by 25 watts every 3 minutes until the mean respiratory exchange ratio (RER) was greater than 1.0 for an entire stage. Once mean RER surpassed 1.0, power output increased by 25 watts per minute until voluntary exhaustion (ie, cadence falls below 50 revolutions per minute). A 5-minute active cool-down was followed by a 10-minute quiet rest in which PPTs were obtained. A confirmation test was conducted after subsequent post-exercise PPTs32 and involved cycling at 110% peak power output achieved on the graded exercise test for up to 10 minutes or volitional fatigue.

Assessment of Pain Sensitivity

PPTs were quantified using a validated measure of pain sensitivity to pressure pain.33 A hand-held digital algometer  (Commander Echo Console, J Tech Medical Industries, UT) with a stimulation area of 1 cm2 was used to experimentally induce and assess pain.34 PPTs were determined with the participants laying in prone on a plinth while the researcher applied pressure perpendicular to the test sites at a gradual rate of approximately 10 N/cm2 /s.35 Participants were instructed to say “stop” once pressure first turned to pain.34 After a familiarisation trial, PPTs were determined bilaterally by taking two measures per test site with a 20-second interval between consecutive measures.36 Test sites at the upper quadrant (forearms), axial (low back), lower quadrant (calves), and body sites were chosen and defined as pain sensitivity remote, semi-remote, and close to the exercising body parts.36 Test sites included the calves (gastrocnemius muscle; between the two heads one-third distal from the proximal muscle attachment), low back (lumbar paraspinal muscle; 4-cm lateral from mid-line), and forearms (extensor carpi radialis longus muscle; 3-cm posterior and distal from lateral epicondyle). These muscles were selected based on previous protocols.36–39 Measurements were applied in a nonrandomized fashion whereby testing was completed at the forearm on the left-hand side, followed by the right-hand side. The same test order was completed at the lower back and then the calves. The PPT per body site was calculated as the mean of the two measures per test site by averaging the measures from the left and right test sites. Assessment of global PPT was calculated as a pooled average of all sites. PPTs have an excellent intraclass correlation coefficient in multiple bodily sites (wrist=0.81–0.97; leg=0.96–0.98; neck=0.92–0.98; back=0.94–0.99).40

Assessment of Psychosocial Factors

Self-reported questionnaires were implemented using Qualtrics survey software (Qualtrics, Provo, UT). Participants completed questionnaires using an iPad (Apple, Cupertino, CA) with the assistance of researchers for clarification purposes. All questionnaires were completed before the physical tests.

Pain Catastrophizing Scale

The Pain Catastrophizing Scale (PCS) is a valid measurement to assess pain-related catastrophic thinking in both nonclinical and clinical populations.41–43 This questionnaire is a 13-item self-reported measure with each question measured on a 5-point Likert scale, ranging from 0 (not at all) to 4 (all the time) for a maximum score of 52. Higher scores indicate more severe catastrophic thoughts about pain. A threshold of 30 points is considered to be clinically relevant.42 The overall score has been shown to have high internal consistency, with a Cronbach’s α of 0.87.43

Tampa Scale for Kinesiophobia

The Tampa Scale for Kinesiophobia (TSK) is a valid measurement to evaluate the fear of movement or reinjury44,45 and consists of 13 questions measured on a 4-point Likert scale ranging from 1 (strongly disagree) to 4 (strongly agree) for a maximum score of 52. Higher scores represent higher levels of kinesiophobia. Severity levels were determined as sub-clinical (score = 13–22), mild (23–32), moderate (33–42), and severe (43–52)46. The TSK is valid in both non-clinical and clinical populations.44,45 The TSK-13 was utilized over the original TSK-17 due to improved psychometrics by removing the four reversed items.46 The TSK-  13 has shown acceptable internal consistency, with a Cronbach’s α of 0.772.47

Depression Anxiety Stress Scale

The Depression Anxiety Stress Scale (DASS) is a 21-item instrument consisting of three subscales to measure stress, depression, and anxiety.48,49 This questionnaire has been validated in both non-clinical and clinical populations.48,50,51 Each item is measured on a 4-point Likert scale with scores of 0 (did not apply at all) to 3 (applied to me very much or most of the time),52 with a maximum score of 21 in each subscale. Subscale total scores are multiplied by two and categorized as follows: mild (depression = 10–13; anxiety = 8–9; stress = 15–18), moderate (depression = 14–20; anxiety = 10–14; stress = 19–25), severe and extremely severe (depression >20; anxiety >14; stress >25). This scale has high internal consistency, with a Cronbach’s α of 0.93–0.95.52,53

Stamps Index of Work Satisfaction

The Stamps Index of Work Satisfaction (IWS) is a valid tool to measure nursing satisfaction levels.54 The questionnaire is a 45-item self-reported form measured on a 7-point Likert scale.55 The questionnaire has a maximum total score of  270, where scores under the 25th percentile are considered extremely low work satisfaction, under the 50th percentile are low work satisfaction, greater than the 50th percentile are deemed moderate work satisfaction, and greater than the 75th percentile are high work satisfaction.56 The overall questionnaire has high internal consistency, with a Cronbach’s α ranging from 0.82 to 0.91.54

Multidimensional Scale of Perceived Social Support

The Multidimensional Scale of Perceived Social Support (MSPSS) is a valid questionnaire to measure social support.57,58 The questionnaire is a 12-item measure assessing three subscales: family, friends, and significant others.57 The subscales are measured on a 7-point Likert scale, ranging from 1 (very strongly disagree) to 7 (very strongly agree), with a maximum total score of 28 for each subscale and 84 for the sum score. Scores were scaled by calculating the mean score for the total and subscales by dividing by the number of items for each score. Higher values represent greater perceived social support. The questionnaire has high internal consistency, with a Cronbach’s α of 0.88.57

Statistical Analysis

Descriptive statistics were calculated for demographics, psychosocial variables, and medical history and presented as mean (SD) for continuous data and frequency (%) for categorical data.

Separate linear mixed-effects models were completed to evaluate changes in pain sensitivity within individuals before and following aerobic exercise. The model included time and PPT as fixed effects and participants as random effects.

To evaluate associations between psychosocial factors and EIH, correlation and regression analyses were performed.  As TSK-13 data were missing for eight participants, missing values were imputed via random forest with the number of trees at 100 per random forest. No limitations were placed on the maximum number of iterations59 (R statistical package “missForest”, version 1.4). EIH was calculated by subtracting PPT post-exercise by PPT pre-exercise (PPTpost-exercise – PPTpre-exercise) as per standard protocol.60–62 Spearman rho correlation coefficients were calculated and classified as negligible (0.00–0.10), weak (0.10–0.39), moderate (0.40–0.69), strong (0.70–0.89), or very strong (0.90–1.00).63 Total scores for all psychosocial questionnaires were entered into separate multiple linear regression models as possible predictors for each EIH location.64 Assessment of multicollinearity was performed by evaluating the variance inflation factor for predictor variables. A variance inflation factor threshold of ≥10 was used to define excessive multicollinearity.65 A secondary exploratory analysis by best subset regression was performed to identify the best subset of psychosocial predictors for models with 1 through 10 predictors for each EIH location. The model with the lowest Bayesian information criterion (BIC) and highest adjusted r-squared values was chosen as the optimal unbiased model.66–68 Where two models were of similar adjusted r-squared the most simple model was selected. All analyses were performed in the “R” statistical environment (version 4.1.1, http://www.r-project.org). An alpha level of 0.05 was used to determine the statistical significance. This study was an exploratory pilot study on the association between EIH and psychosocial factors using existing data. Due to the early cessation of participant recruitment, a calculation of the design sensitivity was completed. The sample size of 38 participants would be sensitive to effects of r=0.44 with 80% power (alpha: 0.05, two-tailed) between each psychosocial variable and EIH. The design sensitivity power calculation was undertaken using G*Power (version 3.1.9.4, Dusseldorf, Germany).69

Results

Participants

In total, 38 pain-free adults with a mean age of 28 ± 6 years (range: 20–53 years) participated in the study with participant sample characteristics presented in Table 1. DASS scores showed most participants to have normal anxiety levels (73.6%), with mild (5.3%) and moderate (21.1%) levels of anxiety being less common. Similarly, DASS scores showed mild (n=4; 10.5%) to moderate (n=2; 5.2%) levels of symptoms of depression in a small subset of the sample, while the majority of the participants (84.3%) did not present with depressive symptoms. DASS scores were generally low with three participants showing mild stress (7.9%), one participant having moderate stress (2.6%), and one having severe stress (2.6%). Most PCS scores were considered normal (97.4%); only one participant (2.6%) was considered to have clinically important levels of pain catastrophizing. The TSK-13 scores showed 18 participants (47.4%) to have mild kinesiophobia levels, while the remaining 52.6% had no clinical kinesiophobia.

Exercise-Induced Hypoalgesia

Table 2 presents the mean and standard deviation for pre and post-exercise PPT for all regions. All sites except for the left forearm reached statistical significance (β=1.2 (2.06), p=0.564) (Table 2). The relative mean increase in PPT at the forearm, lumbar, calf, and globally was 6.0%, 10.1%, 13.9%, and 10.2%, respectively, when the left and right sides were pooled.

Correlations Between Exercise-Induced Hypoalgesia and Psychosocial Factors

Multiple imputations for missing values for TSK (n=8) were completed using random forest with a resultant imputation estimated error (normalized root mean squared error) of 0.0144. There was evidence for a weak negative correlation between MSPSS (total) (rs=−0.380, p=0.019), MPSS significant other subscale (rs=−0.357, p=0.028), and MPSS friends subscale (rs=0.329, p=0.044), with global EIH. There was also some evidence for a weak negative correlation between MSPSS significant other (rs=−0.333; P=0.041), MSPSS family (rs=−0.372; p =0.021), and MSPSS total (rs=−0.397; p=0.014), with the pooled regional EIH located at the forearm. Negligible to weak, nonstatistically significant correlations were observed for TSK and all EIH locations when analyzed with (n=38) and without (n=30) imputed data for missing TSK values (see supplementary Tables S1 and S2, respectively).

【Contact】Email: george.deng@wecistanche.com / WhatsApp:008613632399501/Wechat:13632399501