Delayed onset muscle soreness: effect of an ischaemic block upon mechanical allodynia in humans

Hyperexcitability of muscle spindle afferents in jaw-closing muscles in experimental myalgia: Evidence for large primary afferents involvement in chronic pain

The goals of this review are to improve understanding of the aetiology of chronic muscle pain and identify new targets for treatments. Muscle pain is usually associated with trigger points in syndromes such as fibromyalgia and myofascial syndrome, and with small spots associated with spontaneous electrical activity that seems to emanate from fibers inside muscle spindles in EMG studies. These observations, added to the reports that large-diameter primary afferents, such as those innervating muscle spindles, become hyperexcitable and develop spontaneous ectopic firing in conditions leading to neuropathic pain, suggest that changes in excitability of these afferents might make an important contribution to the development of pathological pain. Here, we review evidence that the muscle spindle afferents (MSAs) of the jaw-closing muscles become hyperexcitable in a model of chronic orofacial myalgia. In these afferents, as in other large-diameter primary afferents in dorsal root ganglia, firing emerges from fast membrane potential oscillations that are supported by a persistent sodium current (INaP ) mediated by Na+ channels containing the α-subunit NaV 1.6. The current flowing through NaV 1.6 channels increases when the extracellular Ca2+ concentration decreases, and studies have shown that INaP -driven firing is increased by S100β, an astrocytic protein that chelates Ca2+ when released in the extracellular space. We review evidence of how astrocytes, which are known to be activated in pain conditions, might, through their regulation of extracellular Ca2+ , contribute to the generation of ectopic firing in MSAs. To explain how ectopic firing in MSAs might cause pain, we review evidence supporting the hypothesis that cross-talk between proprioceptive and nociceptive pathways might occur in the periphery, within the spindle capsule.

Susceptibility to movement-evoked pain following resistance exercise

Objective

To investigate the: (1) role of basic muscle pain sensitivity and psychological factors in the prediction of movement-evoked pain (MEP) following delayed onset muscle soreness (DOMS), and (2) association of MEP with changes in systemic muscle pain sensitivity following DOMS induction.

Methods

Fifty-one participants were assigned to either eccentric resistance exercise or control groups. They completed questionnaires evaluating psychological distress and underwent muscle pain sensitivity evaluation by the pressure pain threshold (PPT) test at the exercised and remote muscles, before and 24 hours following the intervention. MEP intensity was determined in response to lifting a 3kg canister using a visual analogue scale (VAS).

Results

The exercise group demonstrated MEP intensity of 5/10 on VAS and reduced PPTs at the main exercised muscle (p<0.001). A regression tree analyses revealed that the level of anxiety trait predicted a higher MEP intensity. A secondary analysis showed that 53% participants who were DOMS responders (MEP > mild intensity; ≥ 3/10 VAS) exhibited decreased PPTs in the exercised (p<0.001) and remote (p = 0.027) muscles following eccentric exercise. Characterization of DOMS responders revealed that, at baseline, they had lower PPTs in the exercised (p = 0.004) and remote (p = 0.001) muscles and reported higher psychological distress i.e., anxiety trait and depression symptoms (p<0.05), compared to non-responders. A regression analysis revealed that lower PPT or high levels of anxiety trait increased the probability to become a responder (p = 0.001).

Conclusions

Susceptibility to MEP following DOMS is determined by muscle pain hypersensitivity and high levels of anxiety trait. MEP at the early stage of DOMS is linked with an increase in systemic muscle pain sensitivity suggestive of central mechanisms. This knowledge is valuable in translating science into clinical musculoskeletal pain management.

Pain sensitivity after low-level clenching is influenced by preloading eccentric exercise

PURPOSE

To examine the effect of preloading eccentric exercise on pain sensitivity in healthy volunteers.

METHODS

In 20 healthy volunteers, pain-related sensations (6 items: pain, unpleasantness, fatigue, stiffness, tension, and soreness during maximum biting), and pain intensities induced by repeated electrical stimuli on the masseter and the hand palm were evaluated using a visual analog scale (VAS) of 0-100 mm. Eccentric exercise (6 min-test) or gum chewing (6 min-control) was used as preloading exercise to evaluate the effect on pain sensitivities before and after low-level clenching (15 min) performed 2 days after the preloading exercise.

RESULTS

Eccentric exercise induced only low levels of pain-related sensations 2 days later. However, the time course of temporal summation induced by four repeated electrical stimuli on the masseter was influenced by the type of preloading exercise, i.e., temporal summation increased after the low-level clenching (P = 0.016) when preloading was done by the eccentric exercise, while no significant change was observed when preloading was done by the gum chewing.

CONCLUSIONS

Eccentric exercise may facilitate pain sensitivity induced by subsequent low-level clenching via the central nervous system. In addition, it was demonstrated that pain sensitivity after the low-level clenching could be influenced by the type of preloading exercise. These experimental results may suggest that eccentric exercise could act as one of the triggering factors in the mechanism by which tooth clenching leads to a chronic pain condition in susceptible individuals.

The effect of eccentric exercise and delayed onset muscle soreness on the homologous muscle of the contralateral limb

High intensity eccentric exercise induces muscle fiber damage and associated delayed-onset muscle soreness (DOMS) resulting in an impaired ability of the muscle to generate voluntary force. This study investigates the extent to which DOMS, induced by high intensity eccentric exercise, can affect the activation and performance of the non-exercised homologous muscle of the contralateral limb. Healthy volunteers performed maximal voluntary contractions of knee extension and sustained isometric knee extension at 50% of maximal force until task failure on both the ipsilateral exercised limb and the contralateral limb. Surface electromyography (EMG) was recorded from the ipsilateral and contralateral knee extensor muscles (vastus medialis, rectus femoris, and vastus lateralis). Maximal isometric knee extension force (13.7% reduction) and time to task failure (38.1% reduction) of the contralateral non-exercised leg decreased immediately after eccentric exercise, and persisted 24 h and 48 h later (p < 0.05). Moreover, the amplitude of muscle activity recorded from the contralateral knee extensor muscles was significantly lower during the post exercise maximal and submaximal contractions following high intensity eccentric exercise of the opposite limb (p < 0.05). Unilateral high intensity eccentric exercise of the quadriceps can contribute to reduced neuromuscular activity and physical work capacity of the non-exercised homologous muscle in the contralateral limb.

Myelinated Afferents Are Involved in Pathology of the Spontaneous Electrical Activity and Mechanical Hyperalgesia of Myofascial Trigger Spots in Rats

Myofascial trigger points (MTrPs) are common causes for chronic pain. Myelinated afferents were considered to be related with muscular pain, and our clinical researches indicated they might participate in the pathology of MTrPs. Here, we applied myofascial trigger spots (MTrSs, equal to MTrPs in human) of rats to further investigate role of myelinated afferents. Modified pyridine-silver staining revealed more nerve endings at MTrSs than non-MTrSs (P < 0.01), and immunohistochemistry with Neurofilament 200 indicated more myelinated afferents existed in MTrSs (P < 0.01). Spontaneous electrical activity (SEA) recordings at MTrSs showed that specific block of myelinated afferents in sciatic nerve with tetrodotoxin (TTX) led to significantly decreased SEA (P < 0.05). Behavioral assessment showed that mechanical pain thresholds (MPTs) of MTrSs were lower than those of non-MTrSs (P < 0.01). Block of myelinated afferents by intramuscular TTX injection increased MPTs of MTrSs significantly (P < 0.01), while MPTs of non-MTrSs first decreased (P < 0.05) and then increased (P > 0.05). 30 min after the injection, MPTs at MTrSs were significantly lower than those of non-MTrSs (P < 0.01). Therefore, we concluded that proliferated myelinated afferents existed at MTrSs, which were closely related to pathology of SEA and mechanical hyperalgesia of MTrSs.

C-tactile fibers contribute to cutaneous allodynia after eccentric exercise

We recently showed that during acute muscle pain, C-tactile (CT) fibers mediate allodynia in healthy human subjects. In this study, we pursued the following questions: Do CTs contribute to allodynia observed in delayed onset muscle soreness (DOMS)? Is CT-mediated allodynia reproducible in a clinical pain state? In 30 healthy subjects, DOMS was induced in anterior compartment muscles of the leg by repeated eccentric contractions. DOMS was confirmed by mapping the emergence of tender points (decreased pressure pain thresholds). Furthermore, we measured pressure pain thresholds in a clinical subject who presented with activity-triggered heel pain but no resting pain. Cutaneous vibration (sinusoidal; 200 Hz-200 μm)--an otherwise innocuous stimulus--was applied to anterolateral leg before exercise, during DOMS, and following recovery from DOMS. The peripheral origin of allodynia was determined by employing conduction blocks of unmyelinated (intradermal anesthesia) and myelinated (nerve compression) fibers. In DOMS state, there was no resting pain, but vibration reproducibly evoked pain (allodynia). The blockade of cutaneous C fibers abolished this effect, whereas it persisted during blockade of myelinated fibers. In the clinical subject, without exposure to eccentric exercise, vibration (and brushing) produced a cognate expression of CT-mediated allodynia. These observations attest to a broader role of CTs in pain processing.

PERSPECTIVE

This is the first study to demonstrate the contribution of CT fibers to mechanical allodynia in exercise-induced as well as pathological pain states. These findings are of clinical significance, given the crippling effect of sensory impairments on the performance of competing athletes and patients with chronic pain and neurological disorders.

Pressure pain sensitivity mapping in experimentally induced lateral epicondylalgia

PURPOSE

The aim of this study was to apply topographical techniques to investigate changes in pressure pain sensitivity after induction of delayed onset muscle soreness (DOMS) in the elbow region in healthy subjects.

METHODS

Pressure pain thresholds (PPT) were assessed over 12 points forming a 3 x 4 matrix (4 points in the superior part, 4 points in the middle, and 4 points in the lower part around the lateral epicondyle) over the dominant elbow in 13 healthy men, and pressure sensitivity maps were calculated. DOMS, as a model for lateral epicondylalgia (LE), was induced by repetitive high-level eccentric contractions of the wrist extensor muscles. PPT maps were assessed before, immediately after, and 24 h after eccentric exercise (DOMS).

RESULTS

The two-way repeated-measure ANOVA detected significant differences in mean PPT for the measurement points (F = 5.96, P < 0.001), with lower PPT over the points located over the extensor carpi radialis brevis muscle. There was also a significant effect of time (F = 121.3, P < 0.001) but no time x point location interaction (F = 0.7, P = 0.8). PPT were lower 24 h after (P < 0.001) but not immediately after (P > 0.05) eccentric exercise.

CONCLUSIONS

The study provides new key information regarding mechanical pain hyperalgesia in experimentally induced LE. Topographical pressure pain sensitivity maps from the elbow region revealed heterogeneously distributed mechanical sensitivity before and during DOMS. The most sensitive localizations for PPT assessment correspond to the muscle belly of the extensor carpi radialis brevis. Our results support the implication of the extensor carpi radialis brevis muscle in LE.

Assessment of the potential role of muscle spindle mechanoreceptor afferents in chronic muscle pain in the rat masseter muscle

Background

The phenotype of large diameter sensory afferent neurons changes in several models of neuropathic pain. We asked if similar changes also occur in “functional” pain syndromes.

Methodology/Principal Findings

Acidic saline (AS, pH 4.0) injections into the masseter muscle were used to induce persistent myalgia. Controls received saline at pH 7.2. Nocifensive responses of Experimental rats to applications of Von Frey Filaments to the masseters were above control levels 1–38 days post-injection. This effect was bilateral. Expression of c-Fos in the Trigeminal Mesencephalic Nucleus (NVmes), which contains the somata of masseter muscle spindle afferents (MSA), was above baseline levels 1 and 4 days after AS. The resting membrane potentials of neurons exposed to AS (n = 167) were hyperpolarized when compared to their control counterparts (n = 141), as were their thresholds for firing, high frequency membrane oscillations (HFMO), bursting, inward and outward rectification. The amplitude of HFMO was increased and spontaneous ectopic firing occurred in 10% of acid-exposed neurons, but never in Controls. These changes appeared within the same time frame as the observed nocifensive behaviour. Ectopic action potentials can travel centrally, but also antidromically to the peripheral terminals of MSA where they could cause neurotransmitter release and activation of adjacent fibre terminals. Using immunohistochemistry, we confirmed that annulospiral endings of masseter MSA express the glutamate vesicular transporter VGLUT1, indicating that they can release glutamate. Many capsules also contained fine fibers that were labelled by markers associated with nociceptors (calcitonin gene-related peptide, Substance P, P2X3 receptors and TRPV1 receptors) and that expressed the metabotropic glutamate receptor, mGluR5. Antagonists of glutamatergic receptors given together with the 2^nd injection of AS prevented the hypersensitivity observed bilaterally but were ineffective if given contralaterally.

Conclusions/Significance

Low pH leads to changes in several electrical properties of MSA, including initiation of ectopic action potentials which could propagate centrally but could also invade the peripheral endings causing glutamate release and activation of nearby nociceptors within the spindle capsule. This peripheral drive could contribute both to the transition to, and maintenance of, persistent muscle pain as seen in some “functional” pain syndromes.

Ischemic compression block attenuates mechanical hyperalgesia evoked from latent myofascial trigger points

The aim of the present study is to test the hypothesis that large-diameter myelinated muscle afferents contribute to the pathophysiology of myofascial trigger points (MTrPs). The ischemic compression blockage (ICB) of large-diameter myelinated muscle afferents was obtained with a 7-cm-wide tourniquet applied around the upper arm proximal to the brachioradialis muscle in 20 healthy subjects. This study consisted of two randomized sessions with an interval of 1 week in between each session. In one session, pressure pain threshold (PPT) and pressure threshold for eliciting referred pain (PTRP) were measured at an MTrP region in the brachioradialis muscle in one forearm. In another session, PPT was measured at a non-MTrP region in the brachioradialis muscle of the contralateral forearm at the time of pre-compression, 20 min following compression, and 10 min after decompression. The results showed that ICB, which mainly blocks large-diameter myelinated muscle afferents, was associated with an increase in PPT and PTRP (all P < 0.001) at MTrP regions but not at non-MTrP regions. These results suggest that large-diameter muscle afferents may be involved in pain and mechanical hyperalgesia at MTrPs.

Nociceptive and non-nociceptive hypersensitivity at latent myofascial trigger points

OBJECTIVE

The aim of the study was to evaluate whether or not there exists nociceptive and non-nociceptive hypersensitivity at latent myofascial trigger points (MTrPs).

METHODS

Eleven healthy volunteers participated in this study, which consisted of 3 sessions of electromyography-guided intramuscular injection with a minimum of a week interval in between. In each session, a bolus of either hypertonic saline (6%, 0.1 mL, each), glutamate (0.1 mL, 0.5 M, each), or isotonic saline (0.9%, 0.1 mL, each) was randomly injected into a latent MTrP and a non-MTrP located in the right or left gastrocnemius medialis muscles. After each injection, participants were asked to rate the perceived pain intensity on an electronic visual analog scale (VAS) and to mark the pain areas on pain drawings. Maximal pain intensity (VAS(peak)), the area under the curve (VAS(auc)), and local and referred pain areas were extracted.

RESULTS

Injections of either hypertonic saline, glutamate, or isotonic saline into the latent MTrPs induced a higher VAS(peak) and larger VAS(auc) than the non-MTrPs (all, P<0.05). Furthermore, the MTrPs with referred pain after painful injections were found to show higher VAS(peak) and larger VAS(auc) than those without referred pain (both, P<0.001).

CONCLUSIONS

These results confirm the existence of nociceptive hypersensitivity at latent MTrPs and provide the first evidence that there exists non-nociceptive hypersensitivity (allodynia) at latent MTrPs. Finally, the occurrence of referred muscle pain is associated with higher pain sensitivity at latent MTrPs.

Appraisals of pain from controlled stimuli: relevance to quantitative sensory testing

OBJECTIVE

Sensory testing has been advocated for the diagnosis, prognosis, and outcome evaluation of pain patients, but responses to controlled stimuli have not been well correlated to clinical pain. As an initial step for improving the clinical relevance of sensory testing, this investigation compared appraisals of and responses to controlled pain stimuli.

DESIGN

A prospective within subjects design was used.

METHODS

Heat, ischaemic, and delayed-onset muscle pain were induced in the upper extremity of 44 participants (47.7% women) during four experimental sessions.

RESULTS

The threat of heat and ischaemic pain was higher than delayed-onset muscle pain (F(2,86) = 5.30, p<.01, eta(2) = .11). Threat, challenge, predictability, and controllability were related to heat pain most consistently. The affective-sensory ratios of ischaemic and delayed-onset muscle pain resembled those of clinical pain and were higher than heat pain (F(2,84) = 11.64, p<.01, eta(2) = .22). Delayed-onset muscle pain meaningfully affected daily activities, which correlated to delayed-onset muscle pain ratings (rs = .60-.68, ps <.001).

CONCLUSIONS

Heat stimuli may be well suited for instructional manipulations of appraisals to improve the clinical relevance of quantitative sensory testing and delayed-onset muscle pain's effects on daily activities are clinically relevant.

Gender effects on trapezius surface EMG during delayed onset muscle soreness due to eccentric shoulder exercise

The purpose of this study was to investigate gender-specific motor control strategies during eccentric exercise and delayed onset muscle soreness (DOMS) in the shoulder region. Twelve healthy males and females participated in the study. Eccentric shoulder exercises were conducted on the dominant shoulder while the other side served as control. The exerted force, range of shoulder elevation, rating of perceived exertion, pain intensity, and surface electromyography (EMG) from the trapezius muscles were recorded and analyzed. A significant decrease in exerted force during exercise was only found in males despite similar rating of perceived exertion among genders. During eccentric exercise: males showed increasing root mean square (RMS) of the EMG while a decrease occurred for females, no difference between genders in mean power frequency of the EMG were seen. During static and dynamic contractions: no differences between genders in pain intensity or RMS were observed; RMS of the exercised side were lower than that of the control side (P<0.05) at 24 h after exercise. The results indicated a more prominent muscle fatigue resistance in females compared with males and mobilization of different muscle activation strategies during eccentric exercise. A protective adaptation to DOMS, i.e. decrease in RMS values was found with no gender differences.

Changes in coordination of postural control during dynamic stance in chronic low back pain patients

The human postural system operates on the basis of integrated information from three independent sources: vestibular, visual and somatosensory. It is conceivable that a derangement of any of these systems will influence the overall output of the postural system. The peripheral proprioceptive system or the central processing of proprioceptive information may be altered in chronic low back pain (CLBP). We therefore investigated whether patients with CLBP exhibited an altered postural control during quiet standing. Dynamic posturography was performed by 12 CLBP patients and 12 age-matched controls. Subject's task was to stand quietly on a computer-controlled movable platform under six sensory conditions that altered the available visual and proprioceptive information. While the control of balance was comparable between the two groups across stabilized support surface conditions (1-3), CLBP patients oscillated much more than controls in the anterior-posterior (AP) direction in platform sway-referenced conditions (4-6). Control experiments ruled out that increased sway was due to pain interference. In CLBP patients, postural stability under challenging conditions is maintained by an increased sway in AP direction. This change in postural strategy may underlie a dysfunction of the peripheral proprioceptive system or the central integration of proprioceptive information.

Delayed onset muscle soreness at tendon-bone junction and muscle tissue is associated with facilitated referred pain

Delayed onset muscle soreness (DOMS) involves central and peripheral pain mechanisms. Referred pain patterns following stimulation of DOMS affected tissue have not been fully described. Referred pain may provide information on how central mechanisms are involved in DOMS, as referred pain is a central mechanism. Further, tendon tissue involvement in DOMS is not clear. This study assessed pressure pain threshold (PPT) sensitivity at the tendon, tendon-bone junction (TBJ) and muscle belly sites of tibialis anterior pre- and during DOMS in 45 subjects (34 males, 11 females). Furthermore, pain and referred pain areas at these three sites in response to hypertonic saline injection (n = 15 per injection site) were investigated pre- and during DOMS. DOMS was induced using controlled plantarflexion from a platform (bodyweight as resistance) causing eccentric contraction of the tibialis anterior muscle. DOMS induced PPT decrease was found at the TBJ and muscle belly sites only (P < 0.001). No mechanical effect was found in the unexercised limb. Maximal pain intensity induced by hypertonic saline given pre-DOMS was significantly higher for the tendon and TBJ injections compared to intramuscular injections (P < 0.05). Significantly higher referred pain frequency and enlarged pain areas were found at the muscle belly and TBJ sites following injection during DOMS compared to pre-DOMS. The results indicate that muscle belly and TBJ sites are sensitised while tendon tissue per se is unaffected by DOMS. Central sensitivity changes caused by DOMS may explain the increase in referred pain frequency and enlarged pain areas.

Rofecoxib and tramadol do not attenuate delayed-onset muscle soreness or ischaemic pain in human volunteers

We assessed the effect of rofecoxib, a cyclo-oxygenase-2 inhibitor, and tramadol, a centrally acting analgesic, on both delayed-onset muscle soreness (DOMS) and experimentally induced ischaemic pain. We induced DOMS in 10 male and 5 female healthy volunteers by downhill running for 30 min at a 12% decline and a speed of 9 km·h–1. We also induced ischaemic pain by finger movements with an arterial tourniquet around the arm. In a randomized, double-blind crossover format, we administered rofecoxib (50 mg, daily), tramadol (50 mg, 3 times per day), and a placebo (orally for 3 days), starting immediately after exercise. A 100 mm visual analogue scale (VAS) and McGill pain questionnaire were used to describe muscle soreness and ischaemic forearm pain 24 h after the exercise. The pressure pain threshold (PPT) in the thigh and ischaemic pain tolerance in the forearm were measured before exercise and 24 and 72 h after exercise. PPT decreased 24 h after exercise, compared with pre-exercise values (ANOVA, p < 0.05), but neither drug had any significant effect on the PPT. Neither rofecoxib nor tramadol had any effect on time of ischaemia tolerated or amount of finger activity during ischaemia. The VAS and pain-rating index, for both muscle soreness and experimental ischaemic pain, were not affected significantly by either drug. Both DOMS and ischaemic pain share peripheral and central mechanisms, yet neither are attenuated by rofecoxib or tramadol.

Enhanced temporal summation of pressure pain in the trapezius muscle after delayed onset muscle soreness

Temporal summation of muscle pain is an important factor in musculoskeletal pain as central integration of repetitive nociceptive input can be facilitated in musculoskeletal pain patients. The aim of this study is to evaluate changes in temporal summation of pressure pain after induction of delayed onset muscle soreness (DOMS) of the trapezius muscle. Sixteen healthy volunteers participated in the study. Temporal summation of pain was induced by sequential pressure stimulation by a computer-controlled algometer. Sequential stimulation consisting of ten stimuli (at pressure pain threshold intensity) was applied over the trapezius muscle. Stimulus duration was 1 s and inter-stimulus intervals (ISI) were 1, 5, 10, and 30 s, respectively. The pain was rated on a continuous visual analogue scale (VAS, 10 cm) after each stimulus and normalised to the VAS score from the first stimulus. DOMS was induced in the right trapezius muscle by eccentric shoulder exercises while the left trapezius muscle served as control. Temporal summation of pressure evoked pain was measured before and 24 h after the exercise. At 24 h after exercise, soreness intensity during shoulder elevation was 3.7+/-0.2 cm, while no soreness was observed on the control side. When sequential pressure stimulation was applied to the DOMS muscle, VAS scores for 1 s ISI progressively increased to a higher level than before exercise (VAS increase for the last stimulus: 0.8+/-0.2 cm vs. 0.6+/-0.1 cm, P<0.05), while VAS scores for ISI 5, 10, and 30 s were not increased. On the control side, significant increases in VAS scores was observed for all ISIs but not affected by contralateral DOMS. Facilitation of temporal summation for 1 s ISI indicated that DOMS may increase the central excitability besides involving peripheral sensitisation. During DOMS there was no potential for further nociceptor sensitisation by repeated noxious pressure stimuli, which may account for the diminishment of temporal summation evoked by pressure stimuli with ISI 5, 10, and 30 s. These data indicate that muscle soreness might facilitate the central components of temporal summation to mechanical stimulation.

An expansion of Simons' integrated hypothesis of trigger point formation

Simons' integrated hypothesis proposed a model of trigger point (TrP) activation to explain known TrP phenomena, particularly endplate noise. We propose an expansion of this hypothesis to account for new experimental data and established muscle pathophysiology.

A novel rodent neck pain model of facet-mediated behavioral hypersensitivity: implications for persistent pain and whiplash injury

Clinical, epidemiological, and biomechanical studies suggest involvement of cervical facet joint injuries in neck pain. While bony motions can cause injurious tensile facet joint loading, it remains speculative whether such injuries initiate pain. There is currently a paucity of data explicitly investigating the relationship between facet mechanics and pain physiology. A rodent model of tensile facet joint injury has been developed using a customized loading device to apply two separate tensile deformations (low, high; n = 5 each) across the C6/C7 joint, or sham (n = 6) with device attachment only. Microforceps were rigidly coupled to the vertebrae for distraction and joint motions tracked in vivo. Forepaw mechanical allodynia was measured postoperatively for 7 days as an indicator of behavioral sensitivity. Joint strains for high (33.6 +/- 3.1%) were significantly elevated (P < 0.005) over low (11.1 +/- 2.3%). Digitization errors (0.17 +/- 0.20%) in locating bony markers were small compared to measured strains. Allodynia was significantly elevated for high over low and sham for all postoperative days. However, allodynia for low injury was not different than sham. A greater than three-fold increase in total allodynia resulted for high compared to low, corresponding to the three-fold difference in injury strain. Findings demonstrate tensile facet joint loading produces behavioral sensitivity that varies in magnitude according to injury severity. These results suggest that a facet joint tensile strain threshold may exist above which pain symptoms result. Continued investigation into the relationship between injury mechanics and nociceptive physiology will strengthen insight into painful facet injury mechanisms.

Eccentric exercise alters muscle sensory motor control through the release of inflammatory mediators

Following downhill exercise, muscle damage and local inflammatory reactions, induced by lengthening contractions, are observed and voluntary muscle activation decreases. The hypothesis that feedback carried by the group IV muscle afferents could be involved has often been raised but never measured in vivo in these conditions. In this experiment, we tested the response of the group IV muscle afferents from the lower limb to injections of KCl and lactic acid in non-exercising rats and at 1, 2, and 8 days after one running session (-13 degrees, 16 m/min). At days 1 and 2, the baseline discharge of the group IV afferents increased, but further activation by test agents was absent. After 8 days, the afferent response was equivalent to the control response. Pretreatment with betamethasone before exercise abolished the effects of downhill exercise. In non-exercising rats, arachidonic acid evoked group IV afferent discharge and suppressed their further response to another stimulus. These results demonstrate that exhaustive downhill running highly activates, for at least 2 days, the sensory feedback carried by group IV afferents through the local release of inflammatory mediators. Such an altered sensori-motor control, accompanying the post-eccentric inflammatory syndrome, could play a key role in deterioration of muscle performance and of its voluntary activation.

Treatment of pain crisis at end of life from severe lower extremity venous outflow obstruction with hyperalgesia and allodynia

Pain crisis in metastatic disease can be a therapeutic dilemma when differing mechanisms contribute to severe hyperalgesia and neuropathic pain. We discuss clinical presentation and management of excruciating pain from severe lower limb venous outflow obstruction in an opioid tolerant, terminally ill patient with locally invasive and metastatic adenocarcinoma of the cervix. This case illustrates how pain crises at end of life can be successfully managed by using a rational, complementary, multimodal approach. Key points include possible mechanisms of pain from venous outflow obstruction, benefit of hyperbaric subarachnoid bupivacaine, opioid rotation to methadone, and use of ketamine by mouth.

Induction and assessment of muscle pain, referred pain, and muscular hyperalgesia

Muscle pain can be induced and assessed experimentally by a variety of methods. Ischemic and exercise-induced muscle pain are typical endogenous pain models; external stimulation with mechanical, electrical, and chemical modalities constitute the exogenous models. These models are a good basis to study the muscle sensitivity, muscle pain responses under normal and pathophysiologic conditions, and drug efficacy on specific muscle pain mechanisms. When evaluating muscle pain in clinical or experimental settings, it is important to assess parameters related to the pain intensity, pain quality, referred and local distribution, and the deep tissue sensitivity in local and referred areas. The experimental test paradigm must include different stimulation modalities (multimodal) to obtain sufficiently advanced and differentiated information about the human nociceptive system under normal and pathophysiologic conditions because the different stimuli activate different receptors, pathways, and mechanisms. This may be a useful approach in future mechanism-based classification and treatment of muscle pain. Similarly, the multimodal approach is important in clinical studies to provide evidence for which specific muscle pain modalities and mechanisms are affected and how they are modulated by pharmacologic approaches.

Experimental deep tissue pain in wrist extensors--a model of lateral epicondylalgia

The aim of this experimental study was to develop an in vivo model demonstrating sensory and motor interactions comparable to those seen in patients presenting with lateral epicondylalgia (i.e., deep tissue pain and hyperalgesia localised to specific sites in the wrist extensors, attenuation of wrist extension force). The effect of saline-induced deep pain combined with delayed onset muscle soreness (DOMS) on deep tissue sensitivity and motor function in wrist extensors was examined. Muscle pain intensity (visual analogue scale: VAS), distribution, and quality were assessed in 12 subjects. Pressure pain thresholds (PPTs) were recorded at five different sites around the elbow. Maximal wrist extension force was recorded. In the absence of DOMS, hypertonic saline administrated into different parts of the wrist extensors (extensor carpi radialis brevis, supinator, common extensor origin) induced significantly (P<0.05) higher VAS scores and larger pain areas compared with a control injection of isotonic saline. The typical quality of saline-induced pain was described as "drilling", "taut", "nagging" and "intense". In non-exercised wrist extensors, hyperalgesia to pressure was not detected during saline-induced pain but maximal wrist extensor force decreased significantly (P<0.05) compared with pre-pain recordings and recordings post isotonic saline. DOMS induced by eccentric wrist extension contractions generated moderate levels of soreness but no resting pain up to 24h post exercise. PPTs and maximal wrist extension force were significantly decreased (P<0.05) during DOMS compared with baseline and 7 days post exercise (P<0.05). VAS scores to injection of hypertonic saline into the DOMS arm were significantly increased (P<0.05) compared with injections into the unexercised arm. This is another manifestation of muscle hyperalgesia. Saline-induced pain combined with DOMS further decreased maximal wrist extension force (P<0.05). The simultaneous deep tissue pain and hyperalgesia linked with force attenuation support the use of the saline-induced deep tissue pain combined with DOMS as an experimental model simulating the clinical sensorimotor correlates of lateral epicondylalgia.

Botulinum toxin for the treatment of musculoskeletal pain and spasm

The impressive pain relief experienced by sufferers of dystonia and spasticity from intramuscular injections of botulinum toxin suggested that patients with other chronic, musculoskeletal pain conditions also may benefit. However, there have been relatively few placebo-controlled studies of botulinum toxin in such non-neurologic conditions as myofascial pain syndrome, chronic neck and low back pain, and fibromyalgia; the results of these studies have not been impressive. One explanation for the lack of positive findings may be the lack of clinically evident muscle spasms (overactivity), despite the presence of muscle tenderness, tightness, or trigger points. Clinical observations of pain relief from injections of botulinum toxin for dystonia and spasticity and its apparent efficacy in treating migraine suggest an anti-nociceptive action independent of its neuromuscular junction-blocking action. Evidence from animal experiments supports this notion, and other data provide plausible physiologic mechanisms in the periphery and central nervous systems. These involve modulation of the activity of the neurotransmitters glutamate, substance P, calcitonin gene-related peptide, enkephalins, and others. However, even if botulinum toxin is firmly established as an analgesic, there is insufficient clinical evidence of its efficacy in treating non-neurologic, chronic, musculoskeletal pain conditions.

Interferential therapy: lack of effect upon experimentally induced delayed onset muscle soreness

This study was designed to assess the analgesic effects of interferential therapy (IFT) on experimentally induced muscular pain under randomized, double-blind, placebo-controlled conditions. After ethical approval and written consent were obtained, 40 healthy human volunteers (20 males: 20 females) aged 18-25 years were recruited and randomly assigned to one of four experimental groups (n = 10 per group: male = female): IFT 1, IFT 2, control or placebo. Delayed onset muscle soreness (DOMS) was induced in the elbow flexors of the non-dominant arm of each subject using a single bout of eccentric exercises to exhaustion. Measurements of isometric peak torque, resting angle, mechanical pain threshold and visual analogue scales were performed at set time points. Treatment was applied for 30 min daily over the biceps brachii muscle, for five consecutive days, according to group allocation. IFT 1 received 10-20 Hz, whilst subjects in IFT 2 were treated with 80-100 Hz (bi-pole; carrier frequency: 4 kHz; pulse duration: 125 microseconds). For the placebo group, the procedure was identical to that in the active treatment groups; however, no interferential current was delivered. The control group received no treatment. No significant between group difference was identified at any time point (P > or = 0.14). However, some inconsistent, yet significant differences in daily treatment effects, interactive effects and effects over time were detected. Based on the results of this study it can be concluded that application of IFT at the parameters used here, had no overall beneficial effect on DOMS.

Peripheral and central sensitization in musculoskeletal pain disorders: an experimental approach

This report provides a brief introduction to the manifestations of peripheral and central sensitization involved in musculoskeletal pain disorders. It has become increasingly evident that muscle hyperalgesia, referred pain, referred hyperalgesia, and widespread hyperalgesia play an important role in chronic musculoskeletal pain. A better understanding of the involved basic mechanisms and better methods to assess muscle pain in the clinic may provide new possibilities for designing rational therapies and for targeting the pharmacologic intervention optimally. Peripheral sensitization plays an important role for increased sensitivity of deep tissue. However, central sensitization may be equally important but less addressed. Quantitative sensory testing provides the possibility to evaluate these manifestations in a standardized way in patients with musculoskeletal pain or in healthy volunteers (eg, experimentally induced referred pain can be used to assess the potential involvement of central sensitization in musculoskeletal pain conditions). Central sensitization may play a role in the persistence, amplification, and spread of pain. Interventions should take this aspect into consideration.

Effect of indomethacin on the development of eccentric exercise-induced localized sensitive region in the fascia of the rabbit

The effect of indomethacin on the development of delayed onset muscle soreness (DOMS) and localized sensitive region produced by eccentric exercise was examined in lightly anesthetized rabbits (n=12, 2.0-3.3 kg). Repeated eccentric contractions of the gastrocnemius (GS) muscle were made by manual extensions during the tetanic contractions induced by electrical stimulation of the tibial nerve. The development of DOMS was confirmed by evoked reflex EMG in the biceps femoris (BF) muscle elicited by a quantitative manual extension of the GS muscle. The distribution of thresholds for the evoked BF EMG was measured by focal electrical stimulations of the GS muscle. Indomethacin (5 mg/kg in 2% sodium bicarbonate) or a vehicle was injected subcutaneously before, during, and after the exercise (a total of 60 mg/kg in 12 doses). A clear ropy taut band was palpated at the GS muscle on the second day after the exercise and a localized sensitive region for evoked BF EMG was detected at the depth of the fascia of the band in the exercise and vehicle groups, whereas no such phenomena appeared in the control and indomethacin groups. The palpable band and sensitive region disappeared on the seventh day after the exercise. That indomethacin inhibits the development of DOMS and the localized sensitive region suggests that a sensitization of polymodal-type nociceptors in the fascia mediated by prostaglandins is a possible mechanism for the development of DOMS and the localized sensitive region.

Muscle damage from eccentric exercise: mechanism, mechanical signs, adaptation and clinical applications

In eccentric exercise the contracting muscle is forcibly lengthened; in concentric exercise it shortens. While concentric contractions initiate movements, eccentric contractions slow or stop them. A unique feature of eccentric exercise is that untrained subjects become stiff and sore the day afterwards because of damage to muscle fibres. This review considers two possible initial events as responsible for the subsequent damage, damage to the excitation-contraction coupling system and disruption at the level of the sarcomeres. Other changes seen after eccentric exercise, a fall in active tension, shift in optimum length for active tension, and rise in passive tension, are seen, on balance, to favour sarcomere disruption as the starting point for the damage. As well as damage to muscle fibres there is evidence of disturbance of muscle sense organs and of proprioception. A second period of exercise, a week after the first, produces much less damage. This is the result of an adaptation process. One proposed mechanism for the adaptation is an increase in sarcomere number in muscle fibres. This leads to a secondary shift in the muscle's optimum length for active tension. The ability of muscle to rapidly adapt following the damage from eccentric exercise raises the possibility of clinical applications of mild eccentric exercise, such as for protecting a muscle against more major injuries.

Large-fiber mechanoreceptors contribute to muscle soreness after eccentric exercise

Muscles subjected to eccentric exercise, in which the contracting muscle is forcibly lengthened, become sore the next day (delayed onset muscle soreness). In subjects who had their triceps surae of 1 leg exercised eccentrically by walking backwards on an inclined moving treadmill, mapping the muscle 48 hours later with a calibrated probe showed sensitive areas were localized but not restricted to the muscle-tendon junction. Injection of 5% sodium chloride into a sensitive site in the exercised leg did not produce more pain than injections into the unexercised leg, suggesting that nociceptor sensitization was not responsible. Applying controlled indentations to a sensitive area showed that the pain could be exacerbated by 20-Hz or 80-Hz vibration. In an unexercised muscle, vibration had the opposite effect; it reduced pain. Pain thresholds were measured before, during, and after a pressure block of the sciatic nerve. The block affected only large-diameter nerve fibers, as evidenced by disappearance of the H reflex and a weakened voluntary contraction, leaving painful heat and cold sensations unaltered. Pain thresholds increased significantly during the block. It is concluded that muscle mechanoreceptors, including muscle spindles, contribute to the soreness after eccentric exercise.