Exercise induced hypoalgesia during different intensities of a dynamic resistance exercise: A randomized controlled trial

INTRODUCTION

Exercise produces an immediate lessening of pain sensitivity (Exercise-Induced Hypoalgesia (EIH)) in healthy individuals at local and distant sites, possibly through a shared mechanism with conditioned pain modulation (CPM). Dynamic resistance exercise is a recommended type of exercise to reduce pain, yet limited research has examined the effects of intensity on EIH during this type of exercise. Therefore, the primary purpose of this study is to compare changes in PPT at a local and distant site during a leg extension exercise at a high intensity, a low intensity, or a quiet rest condition. A secondary purpose is to examine if CPM changes after each intervention. The final purpose is to examine if baseline pain sensitivity measures are correlated with response to each intervention.

METHODS

In a randomized controlled trial of 60 healthy participants, participants completed baseline pain sensitivity testing (heat pain threshold, temporal summation, a cold pressor test as measure of CPM) and were randomly assigned to complete a knee extension exercise at: 1) high intensity (75% of a 1 Repetition Maximum (RM), 2) low intensity (30% 1RM), or 3) Quiet Rest. PPT was measured between each set at a local (quadriceps) and distant (trapezius) site during the intervention. CPM was then repeated after the intervention. To test the first purpose of the study, a three-way ANOVA examined for time x site x intervention interaction effects. To examine for changes in CPM by group, a mixed-model ANOVA was performed. Finally, a Pearson Correlation examined the association between baseline pain sensitivity and response to each intervention.

RESULTS

Time x site x intervention interaction effects were not significant (F(5.3, 150.97) = 0.87, p = 0.51, partial eta2 = 0.03). CPM did not significantly change after the interventions (time x intervention F(1,38) = 0.81, p = 0.37, partial eta2 = 0.02. EIH effects at the quadriceps displayed a significant, positive moderate association with baseline HPT applied over the trapezius (r = 0.61, p<0.01) and TS (r = 0.46, p = 0.04).

DISCUSSION

In healthy participants, PPT and CPM did not significantly differ after a leg extension exercise performed at a high intensity, low intensity, or quiet rest condition. It is possible pre-intervention CPM testing with a noxious stimuli may have impaired inhibitory effects frequently observed during exercise but future research would need to examine this hypothesis.

Exercise-Induced Hypoalgesia in Patients with Chronic Whiplash-Associated Disorders: Differences between Subgroups Based on the Central Sensitization Inventory

BACKGROUND

Physical exercise is an important element in the rehabilitation of chronic whiplash-associated disorders, with the physiological process underlying pain reduction called exercise-induced hypoalgesia. In chronic whiplash-associated disorders, exercise-induced hypoalgesia appears impaired, and the research suggests a relationship with symptoms of dysfunctional nociceptive processing, such as central sensitization. This study improves our understanding of exercise-induced hypoalgesia in chronic whiplash-associated disorders by examining the differences between the extent of exercise-induced hypoalgesia in subgroups based on scores on the central sensitization inventory (CSI).

METHODS

Data were collected from 135 participants with chronic whiplash-associated disorders who completed a set of questionnaires. Pain pressure thresholds and temporal summations were assessed before and after a submaximal aerobic bicycle exercise test.

RESULTS

We observed no interaction effect between exercise-induced hypoalgesia and the CSI scores for both pain pressure threshold and temporal summation. No overall statistical effect was measured in the analysis of the effect of time. The pain pressure threshold significantly related to the CSI. The temporal summation showed no correlation.

CONCLUSIONS

During this study, we did not find evidence for a difference in the presence of exercise-induced hypoalgesia when the subgroups were created based on the central sensitization cluster calculator. Limited evidence was found for the influence of CSI scores on the delta pain pressure threshold.

Can we improve exercise-induced hypoalgesia with exercise training? An overview and suggestions for future studies

Exercise-induced hypoalgesia refers to a reduction in pain sensitivity following a single bout of exercise, which has been shown to be diminished or impaired with aging and chronic pain. Exercise training (repeated bouts of exercise over time) is often recommended as a non-pharmacological treatment for chronic pain and age-related functional declines. However, whether exercise training can augment the exercise-induced hypoalgesia has not been well studied. The purpose of this paper is to 1) provide an overview of the existing literature investigating the effect of exercise training on the magnitude of exercise-induced hypoalgesia, and 2) discuss potential underlying mechanisms as well as considerations for future research. Given the paucity of randomized controlled trials in this area, the effects of exercise training on exercise-induced hypoalgesia are still unclear. Several potential mechanisms have been proposed to explain the impaired exercise-induced hypoalgesia in chronic pain and older individuals (e.g., endogenous opioid, cardiovascular, and immune system). Exercise training appears to induce physiological changes in those systems, however, further investigations are necessary to test whether this will lead to improved exercise-induced hypoalgesia. Future research should consider including a time- and age-matched non-training group and utilizing the same exercise protocol for testing exercise-induced hypoalgesia across intervention groups.

The role of spontaneous vs. experimentally induced attentional strategies for the pain response to a single bout of exercise in healthy individuals

OBJECTIVES

Exercise-induced pain and exercise-induced hypoalgesia (EIH) are well described phenomena involving physiological and cognitive mechanisms. Two experiments explored whether spontaneous and instructed mindful monitoring (MM) were associated with reduced exercise-induced pain and unpleasantness, and increased EIH compared with spontaneous and instructed thought suppression (TS) in pain-free individuals.

METHODS

Eighty pain-free individuals participated in one of two randomized crossover experiments. Pressure pain thresholds (PPTs) were assessed at the leg, back and hand before and after 15 min of moderate-to-high intensity bicycling and a non-exercise control condition. Exercise-induced pain and unpleasantness were rated after bicycling. In experiment 1 (n=40), spontaneous attentional strategies were assessed with questionnaires. In experiment 2, participants (n=40) were randomly allocated to use either a TS or MM strategy during bicycling.

RESULTS

In experiment 1, the change in PPTs was significantly larger after exercise compared with quiet rest (p<0.05). Higher spontaneous MM was associated with less exercise-induced unpleasantness (r=-0.41, p<0.001), whereas higher spontaneous TS was associated with higher ratings of exercise-induced unpleasantness (r=0.35, p<0.05), but not with pain intensity or EIH. In experiment 2, EIH at the back was increased in participants using instructed TS compared with participants using instructed MM (p<0.05).

CONCLUSIONS

These findings suggest that spontaneous and presumably habitual (or dispositional) attentional strategies may primarily affect cognitive-evaluative aspects of exercise, such as feelings of exercise-induced unpleasantness. MM was related to less unpleasantness, whereas TS was related to higher unpleasantness. In terms of brief experimentally-induced instructions, TS seems to have an impact on physiological aspects of EIH; however, these preliminary findings need further research.

A Clinically-Reasoned Approach to Manual Therapy in Sports Physical Therapy

Symptom modification techniques have been recently dichotomously labeled as either passive or active therapies. Active therapy such as exercise has been rightfully advocated for while "passive" therapies, mainly manual therapy have been regarded as low value within the physical therapy treatment spectrum. In sporting environments where physical activity and exercise are inherent to the athletic experience, the utilization of exercise-only strategies to manage pain and injury can be challenging when considering the demands and qualities of a sporting career which include chronically high internal and external workloads. Participation may be impacted by pain and its influence on related factors such as training and competition performance, career length, financial earning potential, educational opportunity, social pressures, influence of family, friends, and other key stakeholders of their athletic activity. Though highly polarizing viewpoints regarding different therapies create black and white "sides," a pragmatic gray area regarding manual therapy exists in which proper clinical reasoning can serve to improve athlete pain and injury management. This gray area includes both historic positive reported short-term outcomes and negative historical biomechanical underpinnings that have created unfounded dogma and inappropriate overutilization. Applying symptom modification strategies to safely allow the continuation of sport and exercise requires critical thinking utilizing not only the evidence-base, but also the multi-factorial nature of sports participation and pain management. Given the risks associated with pharmacological pain management, the cost of passive modalities like biophysical agents (electrical stimulation, photobiomodulation, ultrasound, etc), and the indications from the evidence-base when combined with active therapies, manual therapy can be a safe and effective treatment strategy to keep athletes active.

Level of Evidence

5.

Training-induced hypoalgesia and its potential underlying mechanisms

It is well-established that a single bout of exercise can reduce pain sensitivity (i.e., exercise-induced hypoalgesia) in healthy individuals. However, exercise-induced hypoalgesia is often impaired in individuals with chronic pain. This might suggest that repeated bouts of exercise (i.e., exercise training) are needed in order to induce a reduction in pain sensitivity (i.e., training-induced hypoalgesia) among individuals with chronic pain, given that a single bout of exercise seems to be insufficient to alter pain. However, the effect of repeated bouts of exercise on pain sensitivity and its underlying mechanisms remain poorly understood. Therefore, the purpose of this review was to provide an overview of the existing literature on training-induced hypoalgesia, as well as discuss potential mechanisms of training-induced hypoalgesia and offer considerations for future research. Existing literature suggests that training interventions may induce hypoalgesic adaptations potentially driven by central nervous system and immune system factors. However, the limited number of randomized controlled trials available, along with the lack of understanding of underlying mechanisms, provides a rationale for future research.

Hypoalgesia after exercises with painful vs. non-painful muscles in healthy subjects - a randomized cross-over study

OBJECTIVES

Exercise-induced hypoalgesia (EIH) is a decrease in the pain sensitivity after exercise. Individuals with chronic pain show less EIH after one exercise session compared with pain-free individuals possibly due to pain in exercising muscles. The primary aim of this randomized controlled cross-over study was to compare the EIH response at the exercising thigh muscle following exercises performed with painful vs. non-painful muscles. Secondary aims were to explore if a reduced EIH response was confined to the painful muscle, and whether the muscle pain intensity and the EIH responses were negatively associated.

METHODS

In two sessions, 34 pain-free participants received a painful (hypertonic saline, 5.8%) injection and a control (isotonic saline, 0.9%) injection in the right thigh muscle before performing a 3 min isometric wall squat exercise. Pressure pain thresholds (PPTs) were assessed at both thighs and the left neck/shoulder at baseline, after injections and after exercise. Pain intensities in the thighs were rated on numerical rating scales (NRS: 0-10).

RESULTS

Hypertonic saline induced moderate thigh pain at rest (NRS: 4.6 ± 2.1) compared to the control injection (NRS: 0.3 ± 0.4; p<0.001). EIH at the thighs and neck/shoulder were not different between sessions (Injected thigh: 0 kPa; 95% CI: -51 to 52; Contralateral thigh: -6 kPa; 95% CI: -42 to 30; neck/shoulder: 19 kPa; 95% CI: -6 to 44). No significant associations between pain intensity ratings immediately after the Painful injection and EIH responses at any assessment sites were found (right thigh: β=0.08, 95% CI: -12.95 to 20.64, p=0.64, left thigh: β=-0.33, 95% CI: -27.86 to 0.44, p=0.06; neck/shoulder: β=-0.18, 95% CI: -15.11 to 4.96, p=0.31).

CONCLUSIONS

Pain in the area of an exercising muscle did not reduce local or systemic EIH responses.

TRIAL REGISTRATION NUMBER

NCT04354948.

Impaired conditioned pain modulation was restored after a single exercise session in individuals with and without fibromyalgia

Supplemental Digital Content is Available in the Text.

Submaximal isometric exercise improved impaired conditioned pain modulation acutely in individuals with and without fibromyalgia, regardless of health status.

Introduction:

Exercise is an effective nonpharmacological intervention for individuals with fibromyalgia syndrome (FMS); however, considerable variability is observed in their pain response after a single exercise session that could be due to differences in baseline central pain inhibition (ie, conditioned pain modulation [CPM]).

Objectives:

This study examined the effect of isometric exercise on CPM in people with FMS and control participants. A subaim was to identify whether pain inhibition after exercise was due to differences in baseline CPM.

Methods:

Twenty-one individuals with FMS (50.5 ± 14.9 years) and 22 age-matched and sex-matched controls (49.2 ± 13.3 years) participated in a familiarization session and 2 randomized experimental sessions: (1) low-intensity isometric exercise and (2) quiet rest control. Conditioned pain modulation was measured before and after each experimental session. In addition, body composition and physical activity levels were collected to determine potential group differences.

Results:

Both groups had comparable body composition and physical activity levels and reported similar exercise-induced hypoalgesia (increase in pressure pain thresholds) at the exercising muscle (quadriceps muscle) and systemically (deltoid muscle). Both groups had a decrease in CPM after exercise and quiet rest; however, in both FMS and control participants with impaired baseline CPM, there was an increase in CPM at the deltoid muscle after exercise.

Conclusion:

In persons with low CPM, irrespective of health status, isometric exercise enhanced CPM at a site distal from the exercising muscle. Our results support the use of isometric exercise when initiating an exercise program especially for individuals with impaired CPM.

[Pain modulation through exercise : Exercise-induced hypoalgesia in physiotherapy]

Exercise prescription is a central tenet of physiotherapy. One of the numerous benefits of exercise is its influence on endogenous pain modulation. Exercise-induced hypoalgesia (EIH) refers to a short-term change in pain sensitivity following an acute bout of exercise. Interest in this phenomenon has grown considerably with over 150 articles published, including four systematic reviews in 2020 alone.This narrative review provides an overview of EIH including a definition and summary of the underlying mechanisms and mediating factors. Recent systematic reviews assessing EIH in people with and without musculoskeletal complaints were evaluated using AMSTAR2. Review findings confirm the presence of EIH. For asymptomatic people, confidence in the evidence was low to very low due to high heterogeneity of included studies, risk of bias, and study eligibility. For people with pain, there is very low confidence, at best, that subgroups or isometric exercise show altered EIH. Despite the growing body of evidence, challenges within the available evidence due to its complex nature are highlighted. Recommendations regarding outcome measures and exercise parameters are required, and further understanding of reliability and validity of EIH is needed. There is a demand to further elucidate these parameters and contextual factors to advance our understanding of EIH. Additional clinical research, especially in patient populations, is required to then provide implications for rehabilitation.

Pain and executive function: no association between remote exercise-induced hypoalgesia and cognitive inhibition in pain-free participants

OBJECTIVES

Cognitive inhibition, which denotes the ability to suppress predominant or automatic responses, has been associated with lower pain sensitivity and larger conditioned pain modulation in humans. Studies exploring the association between cognitive inhibition and other pain inhibitory phenomena, like exercise-induced hypoalgesia (EIH), are scarce. The primary aim was to explore the association between cognitive inhibition and EIH at exercising (local) and non-exercising (remote) muscles after isometric exercise. The secondary aim was to explore the association between cognitive inhibition and pressure pain sensitivity.

METHODS

Sixty-six pain-free participants (28.3 ± 8.9 years old, 34 women) completed two cognitive inhibition tasks (stop-signal task and Stroop Colour-Word task), a 3-min isometric wall squat exercise, and a quiet rest control condition with pre- and post-assessments of manual pressure pain thresholds at a local (thigh) and a remote site (shoulder). In addition, cuff pressure pain thresholds, pain tolerance and temporal summation of pain were assessed at baseline.

RESULTS

No association was found between remote EIH and cognitive inhibition (Stroop interference score: r=0.12, [-0.15; 0.37], p=0.405, BF01=6.70; stop-signal reaction time: r=-0.08, [-0.32; 0.17], p=0.524, BF01=8.32). Unexpectedly, individuals with worse performance on the Stroop task, as indicated by a higher Stroop interference score, showed higher local EIH (r=0.33; [0.10; 0.53], p=0.007, BF01=0.29). No associations were observed between pain sensitivity and any of the cognitive inhibition performance parameters.

CONCLUSIONS

The present findings do not support previous evidence on positive associations between exercise-induced hypoalgesia and cognitive inhibition, as well as baseline pain sensitivity and cognitive inhibition.

Hypoalgesia following isometric handgrip exercise with and without blood flow restriction is not mediated by discomfort nor changes in systolic blood pressure

The purpose was to examine the effect of isometric handgrip exercise with and without blood flow restriction on exercise-induced hypoalgesia at a local and non-local site, and its underlying mechanisms. Sixty participants (21 males & 39 females, 18-35 years old) completed 3 trials: four sets of 2-minute isometric handgrip exercise at 30% of maximum handgrip strength; isometric handgrip exercise with blood flow restriction at 50% of arterial occlusion pressure; and a non-exercise time-matched control. Pain thresholds increased similarly in both exercise conditions at a local (exercise conditions: ~0.45 kg/cm², control: ~-0.04 kg/cm²) and non-local site (exercise conditions: ~0.37 kg/cm², control: ~-0.16 kg/cm²). Blood flow restriction induced greater feelings of discomfort compared to exercise alone [median difference (95% credible interval) of 4.5 (0.5, 8.6) arbitrary units]. Blood pressure increased immediately after exercise (systolic: 10.3 mmHg, diastolic: 7.7 mmHg) and decreased in recovery. There was no within participant correlation between changes in discomfort and pressure pain threshold. A bout of isometric handgrip exercise with or without blood flow restriction can provide exercise-induced hypoalgesia at a local and non-local site. However, discomfort and changes in systolic blood pressure do not explain this response.

Endogenous Modulation of Pain: The Role of Exercise, Stress, and Cognitions in Humans

BACKGROUND

Pain is a complex and highly subjective phenomenon that can be modulated by several factors. On the basis of results from experimental and clinical studies, the existence of endogenous pain modulatory mechanisms that can increase or diminish the experience of pain is now accepted.

METHODS

In this narrative review, the pain modulatory effects of exercise, stress, and cognitions in humans are assessed.

RESULTS

Experimental studies on the effect of exercise have revealed that pain-free participants show a hypoalgesic response after exercise. However, in some patients with chronic pain, this response is reduced or even hyperalgesic in nature. These findings will be discussed from a mechanistic point of view. Stress is another modulator of the pain experience. Although acute stress may induce hypoalgesia, ongoing clinical stress has detrimental effects on pain in many patients with chronic pain conditions, which have implications for the understanding, assessment, and treatment of stress in patients with pain. Finally, cognitive strategies play differing roles in pain inhibition. Two intuitive strategies, thought suppression and focused distraction, will be reviewed as regards experimental, acute, and chronic pain.

CONCLUSION

On the basis of current knowledge on the role of exercise, stress, and cognitive pain control strategies on the modulation of pain, implications for treatment will be discussed.

Reduction in Pain Inhibitory Modulation and Cognitive-Behavioral Changes in Patients With Chronic Low Back Pain: A Case-Control Study

BACKGROUND

Recent studies indicate that the assessment of conditioned pain modulation (CPM) responses and emotional factors can provide important information regarding chronification mechanisms, choices for more efficient therapeutic strategies, and clinical variables supporting a therapeutic prognosis.

AIMS

To investigate the impact of nonspecific chronic low back pain (NSCLBP) on CPM and psychosocial factors in individuals with NSCLBP compared with healthy controls.

DESIGN

Case-control study.

METHODS

Eighteen patients with NSCLBP and 18 healthy controls were recruited for this study. Pain intensity and affective-emotional aspects of pain, functional disability, kinesiophobia, depression, anxiety, and catastrophizing pain were obtained using a questionnaire. A CPM protocol was established to assess the functioning of the descending inhibitory system, with a cold pressor test as a conditioning stimulus and pressure pain threshold as a test stimulus. The maximal isometric strength of the trunk extensors also was evaluated.

RESULTS

Healthy participants demonstrated a greater CPM response than those with NSCLBP. Patients with NSCLBP exhibited significantly lower pressure pain threshold than healthy subjects. Moreover, patients with NSCLBP presented with a considerable exacerbation of cognitive-behavioral changes. NSCLBP patients showed diminished maximal isometric strength of the trunk extensor compared to healthy subjects.

CONCLUSIONS

The endogenous pain inhibition system is reduced in patients with NSCLBP, with significant cognitive-behavioral changes indicated by high levels of anxiety and moderate pain intensity.

SETTING

Clinical School of Physical Therapy of the Federal University of Delta of Parnaíba, Piauí - Brazil.

PARTICIPANTS/SUBJECTS

36 subjects with and without NSCLBP.

Exercise-induced hypoalgesia and pain reduction following blood flow restriction: A brief review

OBJECTIVE

To review past literature regarding exercise-induced hypoalgesia and pain reduction following blood flow restriction interventions, and to discuss potential mechanisms as well as future considerations towards the efficacy of blood flow restriction in pain reduction following exercise.

METHODS

To be eligible for inclusion, studies had to include acute exercise, or long-term training interventions, with blood flow restriction, along with including pre and post intervention pain measurements.

RESULTS

A total of 13 studies met the inclusion criteria. Among these 13 studies, 3 studies examined exercise-induced hypoalgesia after an acute bout of resistance exercise with blood flow restriction, and 10 studies investigated pain reduction following long-term blood flow restriction training.

CONCLUSIONS

Existing literature suggests that low load resistance exercise with blood flow restriction may serve as an effective pain management method for those who are unable or unwilling to train with high loads. Several potential mechanisms have been suggested, however, the roles of these mechanisms are still unclear and require further clarification. Future research should consider implementing different methods of blood flow restriction application, and research study design to clarify the utility and efficacy of blood flow restriction as a pain management tool, by itself or in combination with exercise.

Interrelated hypoalgesia, creep, and muscle fatigue following a repetitive trunk flexion exposure

Repetitive trunk flexion can damage spinal tissues, however its association with low back pain in the workplace may be confounded by factors related to pain sensitivity. Muscle fatigue, exercise-induced hypoalgesia, and creep-induced neuromuscular changes following repetitive trunk flexion may all affect this assumed exposure-pain relationship. This study's purpose was to determine how mechanical pain sensitivity in the low back is affected by a repetitive trunk flexion exposure and identify factors associated with changes in low back pain sensitivity. Pressure pain thresholds, perceptions of sub-threshold stimuli, and muscle fatigue in the trunk and tibia, as well as lumbar spine creep were tracked in 37 young healthy adults before and up to 40 min after a 10-min repetitive trunk flexion exposure. Pressure pain thresholds (p = 0.033), but not perceptions of sub-threshold stimuli (p > 0.102) were associated with approximately a 12.5% reduction in pain sensitivity 10 min after completing the exposure, while creep and local muscle fatigue effects were only observed immediately following the exposure. Creep and fatigue interactions and the corresponding tibial measure co-varied with individual low back pressure pain thresholds. The net hypoalgesic effects of repetitive trunk flexion have the potential to partially mask possibly injurious loads, which could contribute to the severity or incidence of lower back injuries related to these exposures.

Impaired exercise-induced hypoalgesia in individuals reporting an increase in low back pain during acute exercise

OBJECTIVES

Exercise therapy is recommended for low back pain (LBP) although the immediate effects on pain are highly variable. In 96 individuals with LBP this cross-sectional study explored (a) the magnitude of exercise-induced hypoalgesia (EIH) and (b) measures of pain sensitivity and clinical pain manifestations in individuals reporting a clinical relevant increase in back pain during physical activity compared with individuals reporting low or no increase in back pain during physical activity.

METHODS

Cuff algometry was performed at baseline on the leg to assess pressure pain threshold (cPPT), tolerance (cPTT) and temporal summation of pain (cTSP). Manual PPTs were assessed on the back and leg before and after a 6-min walk test (6MWT). Back pain was scored on a numerical rating scale (NRS) after each minute of walking. The EIH-effect was estimated as the increase in PPTs after the walk exercise.

RESULTS

Twenty-seven individuals reported an increase of ≥2/10 in pain NRS scores during walking and compared with the individuals with <2/10 NRS scores: cPPT and EIH-effects were lower whereas cTSP, pain intensity and disability were increased (p < 0.03). Baseline NRS scores, EIH and pain thresholds were associated with the likelihood of an increase of ≥2/10 in back pain intensity during walking (p < 0.05).

CONCLUSIONS

Pain flares in response to physical activity in individuals with LBP seem to be linked with baseline pain sensitivity and pain intensity, and impair the beneficial EIH. Such information may better inform when individuals with LBP will have a beneficial effect of physical activity.

SIGNIFICANCE

Pain flares in response to physical activity in individuals with LBP seem to be linked with baseline pain sensitivity and pain intensity, and impair the beneficial exercise-induced hypoalgesia. Such information may better inform when individuals with LBP will have a beneficial effect of physical activity.

Assessing Endogenous Pain Inhibition: Test-Retest Reliability of Exercise-Induced Hypoalgesia in Local and Remote Body Parts After Aerobic Cycling

OBJECTIVE

Acute exercise can trigger a hypoalgesic response (exercise-induced hypoalgesia [EIH]) in healthy subjects. Despite promising application possibilities of EIH in the clinical context, its reliability has not been sufficiently examined. This study therefore investigated the between-session and within-subject test-retest reliability of EIH at local and remote body parts after aerobic cycling at a heart rate-controlled intensity.

METHODS

Thirty healthy adults (15 women) performed 15 minutes of aerobic cycling in two sessions. Pressure pain thresholds (PPTs) were assessed at the leg (local), the back (semilocal), and the hand (remote) before, immediately after, and 15 minutes after exercise. Intraclass correlation coefficients (ICCs) were calculated for absolute and percent changes in PPT from baseline to immediately postexercise, and between-session agreement of EIH responders was examined.

RESULTS

PPTs significantly increased at the leg during both sessions (all P < 0.001) and at the back during session 2 (P < 0.001), indicating EIH. Fair between-session reliability was shown for absolute changes at the leg (ICC = 0.54) and the back (ICC = 0.40), whereas the reliability of percent changes was poor (ICC < 0.33). Reliability at the hand was poor for both absolute and percent changes (ICC < 0.33). Agreement in EIH responders was not significant for EIH at the leg or the back (all P > 0.05).

CONCLUSIONS

Our results suggest fair test-retest reliability of EIH after aerobic cycling for local and semilocal body parts, but only in men, demonstrating the need for more standardized methodological approaches to improve EIH as a clinical parameter.

Exercise-induced hypoalgesia after acute and regular exercise: experimental and clinical manifestations and possible mechanisms in individuals with and without pain

This review describes methodology used in the assessment of the manifestations of exercise-induced hypoalgesia in humans and previous findings in individuals with and without pain. Possible mechanisms and future directions are discussed.

Exercise and physical activity is recommended treatment for a wide range of chronic pain conditions. In addition to several well-documented effects on physical and mental health, 8 to 12 weeks of exercise therapy can induce clinically relevant reductions in pain. However, exercise can also induce hypoalgesia after as little as 1 session, which is commonly referred to as exercise-induced hypoalgesia (EIH). In this review, we give a brief introduction to the methodology used in the assessment of EIH in humans followed by an overview of the findings from previous experimental studies investigating the pain response after acute and regular exercise in pain-free individuals and in individuals with different chronic pain conditions. Finally, we discuss potential mechanisms underlying the change in pain after exercise in pain-free individuals and in individuals with different chronic pain conditions, and how this may have implications for clinical exercise prescription as well as for future studies on EIH.

Pain can be conditioned to voluntary movements through associative learning: an experimental study in healthy participants

Experimental data suggest that associative learning can influence defensive avoidance behavior and pain perception in humans. However, whether voluntary movements can become conditioned stimuli (CSs) and influence pain responses is yet to be evaluated. Forty healthy volunteers participated in this study. Electrocutaneous stimuli applied to the shoulder at pain threshold level (US) and at pain tolerance level (US) were determined before a movement-conditioning paradigm. First, reaching movements to visual cues shown on one side of a computer screen were associated with the US (CS+ movements) on 80% of trials, whereas reaching movements to visual stimuli shown on the other side were never associated with the nociceptive-US (CS- movements). Next, participants underwent a test phase in which movements to visual cues on both sides were paired with the US on 50% of trials. During the test phase, participants were asked to evaluate whether the movement was painful (yes/no) and to rate pain intensity after each trial. Movement onset and duration as well as skin conductance responses were collected. The US stimuli were more likely to be perceived as painful and were also rated as more painful during CS+ movements. Movement onset latency and skin conductance responses were significantly higher in anticipation of the CS+ movement as compared to the CS- movement. These findings suggest that pain can be conditioned to voluntary movements.

The addition of blood flow restriction to resistance exercise in individuals with knee pain: a systematic review and meta-analysis

BACKGROUND

Blood flow restriction (BFR) is an effective clinical intervention used to increase strength in healthy individuals. However, its effects on pain and function in individuals with knee pain are unknown.

OBJECTIVE

To determine the effectiveness of adding BFR to resistance exercise for pain relief and improvement of function in patients with knee pain.

METHODS

Systematic review with meta-analysis of randomized clinical trials. Medline, Central, Embase, PEDro, Lilacs, CINAHL, SPORTDiscus, and Web of Science databases were searched from inception to May 2019. Randomized clinical trials that compared resistance exercise with or without BFR to treat knee pain and function in individuals older than 18 years of age with knee pain were included.

RESULTS

Eight randomized clinical trials met the eligibility criteria and for the quantitative synthesis, five studies were included. The pooled standardized mean difference (SMD) estimate showed that resistance exercises with BFR was not more effective than resistance exercises for reducing pain (SMD: -0.37cm, 95% CI=-0.93, 0.19) and improving knee function (SMD=-0.23 points, 95% CI=-0.71, 0.26) in patients with knee pain.

CONCLUSION

In the short term, there is low quality of evidence that resistance exercise with BFR does not provide significant differences in pain relief and knee function compared to resistance exercises in patients with knee pain. PROSPERO registration number: CRD42018102839.

Walking increases pain tolerance in humans: an experimental cross-over study

Background and aims

Exercise is commonly used as treatment for chronic pain with positive long-term effects on pain and pain-related disability. In pain-free subjects, hypoalgesia following an acute bout of exercise compared with a control condition has consistently been demonstrated also known as exercise-induced hypoalgesia (EIH). Walking exercise, a low intensity aerobic exercise, is frequently used in clinical practice as an easily applicable intervention for patients with chronic pain. Walking exercise is furthermore recommended as an effective treatment for patients with chronic musculoskeletal pain conditions to alleviate pain and reduce disability, however, the effect of walking on pain sensitivity is currently unknown. The aims of the present study were to investigate (1) the acute effect of walking on pain sensitivity, and (2) the relative (between-subjects) and absolute (within-subject) test-retest reliability of the hypoalgesic response across two sessions separated by 1 week.

Methods

In this randomised experimental cross-over study including two identical sessions, 35 pain-free subjects performed a standardized 6 min walking test and a duration-matched quiet rest condition in a randomized and counterbalanced order in each session. Before and after both conditions, handheld pressure pain thresholds (PPTs) were assessed at the thigh and shoulder, and pressure pain thresholds (cPPT) and pain tolerance (cPTT) were assessed with computer-controlled cuff algometry at the lower leg. Change in the pain sensitivity measures were analysed with repeated-measures ANOVAs, and test-retest reliability with intraclass correlation coefficients (ICC) and agreements in classification of EIH responders/non-responders between the two sessions.

Results

All subjects completed the walking conditions in both session 1 and session 2. The perceived intensity of walking assessed with rating of perceived exertion (RPE) and walking distance did not differ significantly between session 1 (distance: 632.5 ± 75.2 meters, RPE: 10.9 ± 1.9) and session 2 (distance: 642.1 ± 80.2 meters, RPE: 11.0 ± 2.4) (p > 0.11). Moreover, RPE showed excellent relative reliability with an ICC value of 0.95 [95%CI: 0.90–0.97]. Walking increased pain tolerance (mean difference: 2.6 kPa [95%CI: 0.5–4.9 kPa; p = 0.02]), but not pain thresholds compared with rest in both sessions. Hypoalgesia after walking demonstrated fair to good relative reliability (ICC = 0.61), however the agreement in classification of EIH responders/non-responders (absolute reliability) across sessions was low and not significant (κ = 0.19, p = 0.30).

Conclusions

Walking consistently increased pain tolerance but not pain thresholds compared with a duration-matched control condition with fair to good relative reliability between sessions. Based on classification of EIH responders/non-responders the absolute reliability between the two sessions was low indicating individual variance in the EIH response. Future studies should investigate the hypoalgesic effect of a walking exercise in a clinical pain population.