Impact of isometric and concentric resistance exercise on pain and fatigue in fibromyalgia.

PURPOSE: The aim of this study was to determine the local and systemic effects of isometric and concentric muscle contractions on experimental pain and performance fatigability in people with and without fibromyalgia. METHODS: Forty-seven fibromyalgia (FM: 51.3 ± 12.3 year) and 47 control (CON: 52.5 ± 14.7 year) participants performed submaximal isometric and concentric exercise for 10 min with the right elbow flexors. Assessments before and after exercise included pressure pain thresholds (PPT) of the biceps and quadriceps, central pain summation, self-reported exercising arm and whole-body pain, and maximal voluntary isometric contraction (MVIC) of the right elbow flexors and left handgrip. RESULTS: People with FM experienced greater reductions in local fatigue (right elbow flexor MVIC: CON: - 4.0 ± 6.7%, FM: - 9.8 ± 13.8%; p = 0.013) and similar reductions in systemic fatigue (left handgrip MVIC: - 6.5 ± 10.2%; p < 0.001) as CON participants, which were not different by contraction type nor related to baseline clinical pain, perceived fatigue, or reported pain with exercise. Following exercise both groups reported an increase in PPTs at the biceps (pre: 205.5 ± 100.3 kPa, post: 219.0 ± 109.3 kPa, p = 0.004) only and a decrease in central pain summation (pre: 6.8 ± 2.9, post: 6.5 ± 2.9; p = 0.013). FM reported greater exercising arm pain following exercise (CON: 0.7 ± 1.3, FM: 2.9 ± 2.3; p < 0.001), and both groups reported greater arm pain following concentric (isometric: 1.4 ± 2.0, concentric: 2.2 ± 2.9; p = 0.001) than isometric exercise. Neither group reported an increase in whole-body pain following exercise. CONCLUSION: People with FM experienced greater performance fatigability in the exercising muscle compared to CON that was not related to central mechanisms of fatigue or pain. These results suggest changes in performance fatigability in FM may be due to differences occurring at the muscular level. TRIAL REGISTRATION #: NCT #: NCT03778385, December 19, 2018, retrospectively registered. IRB#: HR-3035.

Exploring the Pain in Patellofemoral Pain: A Systematic Review and Meta-Analysis Examining Signs of Central Sensitization.

BACKGROUND: Patellofemoral pain (PFP) has high recurrence rates and minimal long-term treatment success. Central sensitization refers to dysfunctional pain modulation that occurs when nociceptive neurons become hyperresponsive. Researchers in this area of PFP have been increasingly productive in the past decade. OBJECTIVE: To determine whether evidence supports manifestations of central sensitization in individuals with PFP. DATA SOURCES: We searched MeSH terms for quantitative sensory testing (QST) pressure pain thresholds (PPTs), conditioned pain modulation (CPM), temporal summation, sensitization, hyperalgesia, and anterior knee pain or PFP in PubMed, SPORTDiscus, CINAHL, Academic Search Complete, and EBSCOhost. STUDY SELECTION: Peer-reviewed studies that were written in English and published between 2005 and 2020 and investigated QST or pain mapping in a sample with PFP were included in this review. DATA EXTRACTION: The initial search yielded 140 articles. After duplicates were removed, 78 abstracts were reviewed. The full text of 21 studies was examined, and we included 15 studies in our evaluation: 6 in the meta-analysis, 4 in the systematic review, and 5 in both the meta-analysis and systematic review. DATA SYNTHESIS: A random-effects meta-analysis was conducted for 4 QST variables (local PPTs, remote PPTs, CPM, temporal summation). Strong evidence supported lower local and remote PPTs, impaired CPM, and facilitated temporal summation in individuals with PFP compared with pain-free individuals. Evidence for heat and cold pain thresholds was conflicting. Pain mapping demonstrated expanding pain patterns associated with long duration of PFP symptoms. CONCLUSIONS: Signs of central sensitization were present in individuals with PFP, indicating altered pain modulation. The etiologic and treatment models of PFP should reflect the current body of evidence regarding central sensitization. Signs of central sensitization should be monitored clinically, and treatments with central effects should be considered as part of a multimodal plan of care.

Conditioned pain modulation predicts exercise-induced hypoalgesia in healthy adults.

INTRODUCTION: Conditioned pain modulation (CPM) is the concept that pain inhibits pain and has potential rehabilitation implications for exercise prescription. The purpose of this study was to determine whether changes in pressure pain perception after a thermal conditioning stimulus (i.e., CPM) was attenuated with aging and whether CPM predicted pain relief after exercise (exercise-induced hypoalgesia (EIH)) in healthy young and older adults. METHODS: Twenty young (21.9 ± 3.3 yr, 10 men) and 19 older (72.0 ± 4.5 yr, 10 men) adults participated in three sessions: one familiarization and two experimental (EIH and CPM) sessions. Pressure pain perception was assessed using a weighted Lucite edge placed on the right index finger for 1 min. EIH was determined by measuring pressure pain perception before and after prolonged submaximal isometric contraction of the elbow flexors. CPM was assessed by measuring pressure pain perception at the finger while the foot was immersed in neutral water versus painful ice water. RESULTS: Young, but not older, adults reported a decrease in pressure pain at the finger while their foot was immersed in the ice water bath compared with the neutral bath (i.e., CPM, trial-age: P = 0.001). Pressure pain ratings decreased after exercise (P = 0.03) that was perceived as painful (peak arm pain, 7.0 ± 3.3) for both young and older adults. Regression analysis showed that after controlling for age and baseline pain, CPM predicted EIH (model adjusted R = 0.23, P = 0.007). CONCLUSIONS: CPM was attenuated in older adults, as measured with a noxious pressure stimulus after a thermal conditioning stimulus, and adults with greater CPM were more likely to report greater EIH.

Co-localization of p-CREB and p-NR1 in spinothalamic neurons in a chronic muscle pain model.

Activation of the cAMP pathway is an important mediator of chronic muscle pain. This study examined phosphorylation of the transcription factor cAMP-response-element-binding protein (p-CREB) and the NR1 subunit of the NMDA receptor (p-NR1) in the spinal cord. Bilateral mechanical hyperalgesia of the paw was induced by administering two injections of acidic saline, 5 days apart, into the gastrocnemius muscle of male Sprague-Dawley rats. The proportion of spinothalamic neurons that expressed p-NR1 or p-CREB did not change in the dorsal horn 24h after the second intramuscular acid injection compared with animals that received pH 7.2 injections. This lack of change in spinothalamic neurons in the dorsal horn may be due to increases in individual spinothalamic neurons or increases in non-spinothalamic neurons. There was an increase in the proportion of spinothalamic neurons expressing p-NR1 in lamina X. These findings suggest that there are region-specific changes in spinothalamic neurons that express p-NR1 and lamina X may play an important role in the modulation of chronic muscle pain.

Low-intensity exercise reverses chronic muscle pain in the rat in a naloxone-dependent manner.

OBJECTIVE: To determine the effects of low-intensity exercise on chronic muscle pain and potential activation of the endogenous opioid system. DESIGN: Randomized placebo-controlled trial. SETTING: Animal laboratory. ANIMALS: Sixty-three male Sprague-Dawley rats. INTERVENTIONS: Rats performed a low-intensity exercise protocol for 5 consecutive days after the induction of chronic muscle pain. In a separate experiment, naloxone or saline was administered systemically before 5 low-intensity exercise sessions. MAIN OUTCOME MEASURE: Mechanical hyperalgesia was measured using von Frey filaments to determine the mechanical withdrawal threshold. RESULTS: Low-intensity exercise increased mechanical withdrawal threshold in the chronic muscle pain model. Naloxone attenuated the antihyperalgesic effects of low-intensity exercise. CONCLUSIONS: Low-intensity exercise reversed mechanical hyperalgesia in the chronic muscle pain model through activation of the endogenous opioid system.

Models of muscle pain: carrageenan model and acidic saline model.

Carrageenan or acidic saline injected unilaterally into the gastrocnemius muscle or triceps muscle produces a robust and long-lasting hyperalgesia in rats and mice, which is reversible with systemic administration of opioid or anti-inflammatory drugs. This unit describes detailed protocols for inducing and measuring hyperalgesia, and provides information on validation of these models. These models are useful for assessing new compounds for their analgesic activity in muscular pain.