Search

Objectives: Tension-type headache (TTH) is a common condition but the underlying etiology is not understood. Episodic TTH may develop into chronic TTH, and some possible triggers may be involved in generation and maintenance. Nociceptive generators and hyperexcitable spots in neck and shoulder regions may to some degree contribute to TTH. The current paper highlights some of the possible triggers and associated pain mechanisms involved in TTH and discusses whether inhibition of these possible triggers may provide new treatment options., Results: This paper presents possible pathophysiological factors in TTH, the role of muscle pain, and how referred pain from triggers can contribute to development, maintenance of sensitization, or both. Referred pain patterns from trigger points and associated muscle hyperalgesia seem to be clinically important factors. Damping the nociceptive peripheral drive may not only reduce the number of TTH attacks but may also prevent, delay the transition from episodic into more chronic TTH, or both. The role of muscle triggers in driving TTH is debated as the pathogenesis of such triggers is not fully understood. Furthermore, inhibiting the drive from the triggers does not consistently modulate TTH., Discussion: Understanding the possible triggers in TTH, muscle hyperalgesia, and widespread pain sensitization, may help to develop better management regimes and possibly prevent TTH from developing into more chronic conditions. Currently, there is a striking difference between the clinical observational studies favoring the role of muscle triggers in TTH and the intervention studies generally not supporting the role of muscle triggers in TTH.

Objective: Cutaneous hyperalgesia is prominent in the ultraviolet-B (UVB) model of inflammatory pain. This study investigated possible interactions between cutaneous and deep tissues hyperalgesia., Methods: A total of 16 healthy volunteers participated in the study. Skin inflammation was induced unilaterally by UVB irradiation (three times of the individual minimal erythema dose) in square-shaped areas on the upper-trapezius and lower-back. Moderate delayed onset muscle soreness (DOMS) was induced bilaterally in the low-back by eccentric exercise. Cutaneous blood flow, mechanical thresholds, pressure pain thresholds (PPTs), temporal summation to repetitive pressure stimulation, and stimulus-response functions (SR curve) relating graded pressure stimulations and pain intensity were measured within and outside the irradiated areas, before and 24 hours after irradiation and eccentric muscle exercise., Results: Compared with baseline (P < 0.05): the assessments 24 hours after irradiation demonstrated: 1) increased superficial blood flow inside and outside the irradiated areas and in the DOMS site; 2) reduced mechanical thresholds within the irradiated areas; 3) left-shifted SR curve function within and outside the irradiated areas; and 4) facilitated temporal summation of pain inside the irradiated areas and in the DOMS site. There was no significant influence of muscle hyperalgesia on skin measures in normal or UVB-inflamed skin., Conclusions: Moderate degrees of muscle sensitization could not facilitate UVB-induced cutaneous mechanical sensitivity, whereas UVB-induced neurogenic inflammation is enhanced when the DOMS is present., (Wiley Periodicals, Inc.)

Purpose: This study aimed to investigate the hypothesis that a repeated bout of eccentric exercise (ECC2) would result in smaller increase in the sensitivity of spinal nociceptive system, and smaller decrease in the local muscle blood oxygenation response in both the ipsilateral and the contralateral tibialis anterior muscle (TA) when compared with the initial bout (ECC1). It was hypothesized that the magnitude of the repeated bout effect (RBE) would be greater for the ipsilateral side than the contralateral side., Methods: Twenty-six healthy young men performed two bouts of high-intensity eccentric exercise of TA separated by 2 weeks. Half of the participants used the same leg for both bouts (IPSI) and the other half used the contralateral leg for ECC2 (CONTRA). Nociceptive withdrawal reflex threshold (NWRT) and local muscle blood oxygenation were assessed for the exercised TA muscle before, immediately after, and one day after exercise., Results: Significant decreases in NWRT and muscle oxygenation were observed after ECC1 (p < 0.05), but NWRT did not change after ECC2 in both groups. Smaller decreases in muscle oxygenation were observed after ECC2 than ECC1 in both groups with a similar magnitude of the difference between bouts, but an increase in muscle oxygen re-perfusion before ECC2 was only observed in the IPSI group., Conclusion: These results suggest that contralateral RBE was associated with spinal facilitation of the neuronal pathways situated at a homologous innervation level, and it is unlikely that oxygen re-perfusion improvement plays a major role in the contralateral RBE.

The aim of this randomized controlled crossover study was to investigate the effect of a bout of unaccustomed eccentric exercise (ECC) followed by a consecutive bout of the same intensity on local muscle blood flow, amplitude, and frequency of the electromyographic (EMG) signal from the exercised tibialis anterior muscle. Sixteen healthy male participants (age, 25.7 (0.6) years; body mass index 24.8 (1) kg·m(-2) participated in this study. Two identical bouts of high-intensity ECC were performed on the tibialis anterior muscle 7 days apart. Control sessions involving no exercise were performed 4 weeks either before or after the exercise sessions. Changes in local total blood flow [ΔtHb], EMG root mean square, and median power frequency were recorded during isometric maximum voluntary contraction of ankle dorsiflexion. Measurements were performed before, immediately after, and the day after both ECCs (ECC1 and ECC2). The participants rested quietly in a chair in the control session. Eccentric exercise 1 led to a significant decrease in [ΔtHb] on the day after (p ≤ 0.05), whereas ECC2 did not. Median power frequency decreased significantly in ECC2 compared with ECC1 (p < 0.01). Root mean square was unchanged in all the instants. The present study showed that adaptation is depicted in the local muscle blood flow and the frequency contents of the EMG after an unaccustomed ECC inducing muscle soreness. These alterations provide a potential mechanism for a rapid adaptation, which decreases susceptibility of the muscle to develop further soreness in the subsequent ECC bout.

Background: Musculoskeletal pain is often associated with a nonhomogeneous distribution of mechanical hyperalgesia. Consequently, new methods able to detect this distribution are needed., Objective: To develop and test a new method for assessing muscle hyperalgesia with high temporal and spatial resolution that provides complementary information compared with information obtained by traditional static pressure algometry., Methods: The dynamic pressure algometer was tested bilaterally on the tibialis anterior muscle in 15 healthy subjects and compared with static pressure algometry. The device consisted of a wheel that was rolled over the muscle tissue with a fixed velocity and different predefined forces. The pain threshold force was determined and pain intensity to a fixed-force stimulation was continuously rated on a visual analogue scale while the wheel was rolling over the muscle. The pressure pain sensitivity was evaluated before, during, and after muscle pain and hyperalgesia induced unilaterally by either injection of hypertonic saline (0.5 mL, 6%) into the tibialis anterior or eccentric exercise evoking delayed-onset muscle soreness (DOMS)., Results: The intraclass correlation coefficient was >0.88 for the dynamic thresholds; thus, the method was reliable. Compared with baseline, both techniques detected hyperalgesia at the saline injection site and during DOMS (P<0.05). The dynamic algometer also detected the widespread, patchy distribution of sensitive loci during DOMS, which was difficult to evaluate using static pressure algometry., Discussion and Conclusion: The present study showed that dynamic pressure algometry is a reliable tool for evaluating muscle hyperalgesia (threshold and pain rating) with high temporal and spatial resolution. It can be applied as a simple clinical bed-side test and as a quantitative tool in pharmacological profiling studies.

The effect of botulinum neurotoxin type A (BoNTA) on glutamate-evoked temporalis muscle pain and vasomotor responses was investigated in healthy men and women over a 60 day time course. Subjects participated in a pre-BoNTA session where their responses to injection of glutamate (1 M, 0.2 mL) and saline (0.2 mL) into the temporalis muscles were assessed. On Day 1, BoNTA (5 U) was injected into one temporalis muscle and saline into the contralateral temporalis muscle, in a randomized order. Subjects then received intramuscular injections of glutamate (1 M, 0.2 mL) into the left and right temporalis muscles at 3 h and subsequently 7, 30 and 60 days post-injection of BoNTA. Pain intensity, pain area, and neurogenic inflammation (skin temperature and skin blood perfusion) were recorded. Prior to BoNTA treatment, glutamate evoked significantly greater pain and vasomotor reactions (P < 0.001) than saline. BoNTA significantly reduced glutamate-evoked pain intensity (P < 0.05), pain area (P < 0.01), skin blood perfusion (P < 0.05), and skin temperature (P < 0.001). The inhibitory effect of BoNTA was present at 3 h after injection, peaked after 7 days and returned to baseline by 60 days. Findings from the present study demonstrated a rapid action of BoNTA on glutamate-evoked pain and neurogenic inflammation, which is in line with animal studies.

Purpose: The purpose of this study was to investigate the effect of repeated bouts of eccentric exercise on the nociceptive withdrawal reflex (NWR) threshold, a measure of sensitivity in the spinal nociceptive system., Methods: Sixteen healthy students (age 25.7 ± 0.6 years, BMI 24.8 ± 1 kg m(-2)) participated in this randomized, controlled, crossover study. Two identical bouts of high-intensity eccentric exercises were performed on the tibialis anterior muscle 7 days apart. Control sessions involving no exercise were performed 4 weeks apart the exercise sessions. Pressure pain thresholds (PPT) and the NWR threshold were recorded before, immediately after, and 1 day after both bouts of exercise., Results: Pressure pain thresholds decreased significantly at two of the muscle belly sites on the day after initial bout compared with baseline. NWR threshold decreased by 25 ± 4 % immediately after initial bout and by 30 ± 5 % the next day (p < 0.05) as an indication of generalized pain hypersensitivity. On the contrary, no changes were found in both pain thresholds after second bout of eccentric exercise indicating that both localized and generalized pain sensitivity were normalized., Conclusion: In conclusion, this study for the first time documented that an initial bout of unaccustomed high-intensity eccentric exercise, which results in muscle soreness can induce central sensitization. A repeated bout of exercise, however, facilitates inherent protective spinal mechanisms against the development of muscle soreness.

Objective: Muscle fatigue is prevalent in acute and chronic musculoskeletal pain conditions in which myofascial trigger points (MTPs) are involved. The aim of this study was to investigate the association of latent MTPs with muscle fatigue., Design: Intramuscular electromyographic (EMG) recordings were obtained from latent MTPs and non-MTPs together with surface EMG recordings from the upper trapezius muscles during sustained isometric muscle contractions in 12 healthy subjects., Outcome Measures: Normalized root mean square (RMS) EMG amplitude and mean power frequency (MNF) were analyzed. The rate of perceived exertion and pain intensity from MTP side and non-MTP side were recorded., Results: Pain intensity on the MTP side was significantly higher than the non-MTP side (P < 0.05). Intramuscular EMG from latent MTPs showed an early onset of decrease in MNF and a significant decrease at the end of fatiguing contraction as compared with non-MTPs (P < 0.05). Surface EMG from muscle fibers close to latent MTPs presented with an early onset of the increase in RMS amplitude and the increase was significantly higher than that from non-MTPs at the end of sustained isometric contraction (P < 0.05)., Conclusions: A latent MTP is associated with an accelerated development of muscle fatigue and simultaneously overloading active motor units close to an MTP. Elimination of latent MTPs and inactivation of active MTPs may effectively reduce accelerated muscle fatigue and prevent overload spreading within a muscle., (Wiley Periodicals, Inc.)

Unlabelled: The aim of this present study was to test the hypothesis that tonic nociceptive stimulation of latent myofascial trigger points (MTPs) may induce a spatially enlarged area of pressure pain hyperalgesia. Painful glutamate (.2 mL, 1M) stimulation of latent MTPs and non-MTPs in the forearm was achieved by an electromyography-guided procedure. Pain intensity (as rated on the visual analog scale [VAS]) and referred pain area following glutamate injections were recorded. Pressure pain threshold (PPT) was measured over 12 points in the forearm muscles and at the mid-point of tibialis anterior muscle before and at .5 hour, 1 hour, and 24 hours after glutamate injections. The results showed that maximal pain intensity, the area under the VAS curve, and referred pain area were significantly higher and larger following glutamate injection into latent MTPs than non-MTPs (all, P < .05). A significantly lower PPT level was detected over time after glutamate injection into latent MTPs at .5 hour (at 4 points), 1 hour (at 7 points), and 24 hours (at 6 points) in the forearm muscles. However, a significantly lower PPT was observed only at 24 hours after glutamate injection into non-MTPs in the forearm muscles (at 4 points, P < .05) when compared to the pre-injection PPT. PPT at the mid-point of the tibialis anterior was significantly decreased at 1 hour only as compared to the pre-injection PPT in both groups (< .05). The results of the present study indicate that nociceptive stimulation of latent MTPs is associated with an early onset of locally enlarged area of mechanical hyperalgesia., Perspective: This study shows that MTPs are associated with an early occurrence of a locally enlarged area of pressure hyperalgesia associated with spreading central sensitization. Inactivation of MTPs may prevent spatial pain propagation., (Copyright © 2012 American Pain Society. Published by Elsevier Inc. All rights reserved.)

Unlabelled: The aim of this study was to evaluate motor unit activity from a latent myofascial trigger point (MTP) in an antagonist muscle during isometric agonist muscle contraction. Intramuscular activity was recorded with an intramuscular electromyographic (EMG) needle inserted into a latent MTP or a non-MTP in the posterior deltoid muscle at rest and during isometric shoulder flexion performed at 25% of maximum voluntary contraction in 14 healthy subjects. Surface EMGs were recorded from the anterior and posterior deltoid muscles. Maximal pain intensity and referred pain induced by EMG needle insertion were recorded on a visual analogue scale. The results showed that higher local pain was observed following needle insertion into latent MTPs (4.64 ± .48 cm) than non-MTPs (2.35 ± .43 cm, P < .005). Referred pain was reported in 6/14 subjects following needle insertion into latent MTPs, but none into the non-MTPs. The intramuscular EMG activity, but not surface EMG activity, in the antagonist muscle was significantly higher at rest and during shoulder flexion at latent MTPs than non-MTPs (P < .05). The current study provides the first evidence that increased motor unit excitability is associated with reduced antagonist reciprocal inhibition., Perspective: This study shows that MTPs are associated with reduced efficiency of reciprocal linhibition, which may contribute to the delayed and incomplete muscle relaxation following exercise, disordered fine movement control, and unbalanced muscle activation. Elimination of latent MTPs and/or prevention of latent MTPs from becoming active may improve motor functions., (Copyright © 2011 American Pain Society. Published by Elsevier Inc. All rights reserved.)

Introduction: Eccentric contractions induce muscle fiber damage that is associated with delayed-onset muscle soreness and an impaired ability of the muscle to generate voluntary force. Pain and pathophysiological changes within the damaged muscle can delay or inhibit neuromuscular responses at the injured site, which is expected to have an effect on reflex activity of the muscle., Purpose: The aim of the study was to investigate the reflex activity of knee muscles to rapid destabilizing perturbations, before, immediately after, and 24 and 48 h after eccentric exercise., Methods: Bipolar surface EMG signals were recorded from 10 healthy men with seven pairs of electrodes located on the knee extensor muscles (vastus medialis, rectus femoris, and vastus lateralis) and knee flexor muscles (the medial and lateral heads of the hamstring and the medial and lateral heads of gastrocnemius) of the right leg during rapid perturbations., Results: The maximal voluntary contraction force decreased by 24% ± 4.9% immediately after exercise and remained reduced by 21.4% ± 4.1% at 24 h and by 21.6% ± 9.9% at 48 h after exercise with respect to baseline. During the postexercise postural perturbations, the EMG average rectified value of the knee extensor muscles was significantly lower than baseline (P < 0.001). Moreover, the decrease in average rectified value over time during postexercise sustained contractions was greatest compared with the session before exercise (P < 0.0001)., Conclusions: Reflex activity in leg muscles elicited by rapid destabilizing perturbations is reduced after exercise-induced muscle soreness.

Unlabelled: Diffuse Noxious Inhibitory Controls (DNIC) involves application of a noxious stimulus outside the testing site to produce analgesia. In human subjects with a variety of chronic pain conditions, DNIC is less effective; however, in animal studies, DNIC is more effective after tissue injury. While opioids are involved in DNIC analgesia, the pathways involved in this opioid-induced analgesia are not clear. The aim of the present study was to test the effectiveness of DNIC in inflammatory muscle pain, and to study which brainstem sites mediate DNIC- analgesia. Rats were injected with 3% carrageenan into their gastrocnemius muscle and responses to cutaneous and muscle stimuli were assessed before and after inflammation, and before and after DNIC induced by noxious heat applied to the tail (45 °C and 47 °C). Naloxone was administered systemically, into rostral ventromedial medulla (RVM), or bilaterally into the medullary reticularis nucleus dorsalis (MdD) prior to the DNIC-conditioning stimuli. DNIC produced a similar analgesic effect in both acute and the chronic phases of inflammation reducing both cutaneous and muscle sensitivity in a dose-dependent manner. Naloxone systemically or microinjected into the MdD prevented DNIC-analgesia, while naloxone into the RVM had no effect on DNIC analgesia. Thus, DNIC analgesia involves activation of opioid receptors in the MdD., Perspective: The current study shows that DNIC activates opioid receptors in the MdD, but not the RVM, to produce analgesia. These data are important for understanding clinical studies on DNIC as well as for potential treatment of chronic pain patients., (Copyright © 2011 American Pain Society. Published by Elsevier Inc. All rights reserved.)

Objective: Pressure algometry is used for assessment of pain sensitivity. In this study the relation between tissue characteristics and pressure pain thresholds was investigated., Methods: Three-dimensional finite-element computer-models were developed to simulate the tissue stress and strain distribution during pressure stimulation on muscles with different hardness (I, II, III, IV (hardest)) and subcutaneous adipose tissue thickness (normal and thicker). The computer model was validated based on data recorded by computer-controlled pressure-induced muscle pain in 8 and 16 partecipants, respectively., Results: The experimental pressure-indentation curve fitted the outcome of the FE model (R>0.73). Stress and strain were extracted from the models at a known painful pressure stimulation level. PPT and PPTO were not significantly different in subjects with normal and thick adipose tissue in accordance with the simulation model where the strain in muscle tissue was comparable in the two conditions. The strain in adipose tissue was larger in subjects with thick adipose tissue compared with normal adipose thickness. In relaxed muscle (hardness I) the principal strain peaked at 0.12 in the adipose tissue, was reduced to 0.07 in the muscle tissue and 0.05 in the harder muscle. Significantly higher PPT and PPTO were recorded in harder compared with softer muscles (P<0.02)., Discussion: The pressure pain sensitivity of the deep layer is related to the amount of muscle strain, which is affected by the muscle hardness and the thickness of adipose tissue. This is clinically relevant as these two factors are not taken into consideration when pressure pain assessments are performed in clinical routine.

Introduction: It has previously been reported that local and referred pain from active myofascial trigger points (MTPs) in the neck and shoulder region contribute to fibromyalgia (FM) pain and that the pain pattern induced from active MTPs can reproduce parts of the spontaneous clinical FM pain pattern. The current study investigated whether the overall spontaneous FM pain pattern can be reproduced by local and referred pain from active MTPs located in different muscles., Methods: A spontaneous pain pattern in FM was recorded in 30 FM patients and 30 healthy subjects served as controls. Local and referred pain patterns induced from active (patients) and latent (controls) MTPs were recorded following manual stimulation. The existence of MTPs was confirmed by intramuscular electromyographical registration of spontaneous electrical activity., Results: Local and referred pain areas induced from key active MTPs in FM were larger than pain areas from latent MTPs in healthy controls (P < 0.001), but were similar to the overall spontaneous FM pain area in FM (P > 0.05). The induced pain area was positively associated with current spontaneous pain intensity in FM (P < 0.01). The locations of key active MTPs in FM patients were found to have latent MTPs in healthy subjects. The muscles containing key active MTPs in FM are often observed in the muscles of extensor digitorum, trapezius, infraspinatus in the upper part of the body and of quadratus lumborum, gluteus medius in the lower part of the body., Conclusions: The overall spontaneous FM pain pattern can be reproduced by mechanical stimulation of active MTPs located in different muscles, suggesting that fibromyalgia pain is largely composed of pain arising from muscle pain and spasm. Targeting active MTPs and related perpetuating factors may be an important strategy in FM pain control., Trial Registration: ISRCTN ISRCTN43167547.

Pressure algometry is widely used to assess deep tissue sensitivity. In this study the relation between pressure-induced pain in humans and stress/strain distribution within the deep tissue is evaluated. A three-dimensional finite-element computer model was used to describe the stress/strain distribution in tissues of the lower leg during pressure stimulation. The computer model was validated based on data recorded by computer-controlled pressure-induced muscle pain in eight subjects. An indentation of 7 mm was painful for all subjects and at this level data were extracted from each simulation. Simulations were performed with different stimulation sites (muscle, near-bone), probe diameters (5, 10, 15 mm), and probe designs (flat, rounded). The principal stress peaked in the skin and was reduced to about 10% in the underlying muscle tissue. The principal strain peaked in adipose tissue and was reduced in muscle tissue to 80% with the 15 mm probe and 66% with the 5mm probe. The large diameter probe evoked a strain peak in adipose tissue at 0.12 (flat probe) and 0.24 (rounded probe); in muscle tissue 0.10 and 0.20 respectively. The human pressure pain thresholds with the rounded probe were significantly lower compared with the flat probe (p<0.05). The results suggest that pressure-induced muscle pain is mainly related to muscle strain and most efficiently induced by large rounded probes, while smaller and flat ones mainly activate superficial structures. The relatively low stress in the deep tissue suggests that the mechanosensitivity of nociceptors in the deep tissue is lower compared with nociceptors in the superficial tissue., (Copyright © 2010 European Federation of International Association for the Study of Pain Chapters. Published by Elsevier Ltd. All rights reserved.)

The aim of this study was to investigate and present a new mapping method to describe muscle pain sensitivity based on the assessment of pressure pain threshold (PPT) over the trapezius muscle. PPT data were recorded from 36 points in 20 healthy males using a standardised grid. Points were clustered using the K-means algorithm with a fixed initialisation procedure. The total number of clusters was determined on the basis of (1) R(2) evaluation of the clustering outcome compared against a desired 95% reduction in variance criterion and (2) the number of empty clusters. A minimum of three clusters were found which fulfilled the criteria. The proposed method enables the identification of a relation between the muscle subdivisions and pressure pain sensitivity within the trapezius. Further, the cluster analysis will enable the study of differences in pain sensitivity distributions between patients and controls and quantify the effect of intervention (physical or pharmacological treatments).

Objective: To describe the presence of widespread pressure pain hyperalgesia and myofascial trigger points (TrPs) in neck and shoulder muscles in patients with postmastectomy pain., Methods: Twenty-nine women (mean age: 50±8 y) with postmastectomy pain and 23 matched healthy controls (mean age: 50±9 y) participated. Pressure pain thresholds (PPT) were bilaterally assessed over the C5-C6 zygapophyseal joint, the deltoid muscle, the second metacarpal, and the tibialis anterior muscle. TrPs in the upper trapezius, suboccipital, levator scapulae, sternocleidomastoid, scalene, infraspinatus, and pectoralis major muscles were explored. TrPs were considered active if the local and referred pain reproduced symptoms and the patient recognized the pain as familiar., Results: Twenty-five (86%) patients reported neck pain whereas 20 (69%) patients showed shoulder/axillary pain. The results showed that PPT levels were significantly decreased bilaterally over the C5-C6 zygapophyseal joint, deltoid muscle, second metacarpal, and tibialis anterior muscle in patients with postmastectomy pain as compared with controls (all sites, P<0.001). No significant differences in the magnitude of PPT decrease between sites were found (P=0.222). The mean number of active TrPs for each woman with postmastectomy pain was 5.4±1.8. Healthy controls only had latent TrPs (0.5±0.6). Patients with postmastectomy pain showed a greater number of TrPs than controls (P<0.001). In all muscles, there was significantly more active TrPs in patients with postmastectomy pain as compared with controls (P<0.001). Active TrPs in the pectoralis major (n=27, 93%), infraspinatus (n=23, 79%), and upper trapezius (n=19, 65%) muscles were the most prevalent in the affected side in the postmastectomy group. The number of active TrPs was positively correlated with neck (rs=0.392, P=0.036) and shoulder/axillary (rs=0.437, P=0.018) pain intensity., Conclusions: Our findings revealed bilateral widespread pressure pain hypersensitivity in patients with postmastectomy pain. In addition, the local and referred pain elicited by active TrPs reproduced neck and shoulder/axillary complaints in these patients. These results suggest peripheral and central sensitization in patients with postmastectomy pain.

In conscious rats, we investigated the change of nociceptive paw withdrawal reflexes elicited by mechanical and heat stimuli during intramuscular (i.m.) 5.8% hypertonic (HT) saline elicited muscle nociception. i.m. injection of HT saline caused rapid onset, long lasting (around 7 days), bilateral mechanical hyperalgesia, while it induced bilateral, slower onset (1 day after the HT saline injection), long-term (about 1-2 weeks) heat hypoalgesia. Ipsilateral topical pre-treatment of the sciatic nerve with 1% capsaicin significantly prevented the occurrence of both the bilateral mechanical hyperalgesia and the contralateral heat hypoalgesia. Intrathecal administration of either 6-hydroxydopamine hydrobromide (6-OHDA) or 5,7-dihydroxytryptamine (5,7-DHT), and intraperitoneal injection of naloxone all markedly attenuated the HT saline induced bilateral heat hypoalgesia, but not the mechanical hyperalgesia. Combined with experiments with lesioning of the rostroventral medulla with kainic acid, the present data indicate that unilateral i.m. injection of HT saline elicits time-dependent bilateral long-term mechanical hyperalgesia and heat hypoalgesia, which were modulated by descending facilitatory and inhibitory controls, respectively. We hypothesize that supraspinal structures may function to discriminate between afferent noxious inputs mediated by Aδ- and C-fibres, either facilitating Aδ-fibre mediated responses or inhibiting C-fibre mediated activities. However, this discriminative function is physiologically silent or inactive, and can be triggered by stimulation of peripheral C-fibre afferents. Importantly, in contrast to the rapid onset of descending facilitation, the late occurrence of descending inhibition suggests a requirement of continuous C-fibre input and temporal summation. Thus, a reduction of C-fibre input using exogenous analgesic agents, i.e. opioids, may counteract the endogenous descending inhibition.

Background: The purpose of this study was to evaluate how acute pain changes the postural control and stability during quiet standing and after unexpected perturbations., Methods: Nine subjects stood as quiet as possible on a movable force platform that recorded the centre of pressure position and provided unexpected floor perturbations, before, during and after experimental calf muscle pain. Bilateral surface electromyography from the tibialis anterior and medial gastrocnemius muscles was recorded. The foot pressure distributions were measured using pressure insoles. Intramuscular injections of hypertonic saline were administrated (right leg) to induce acute pain in the tibialis anterior and/or medial gastrocnemius muscles, and an isotonic injection was used as control., Findings: Simultaneous pain in tibialis anterior and medial gastrocnemius altered the postural control. During quiet standing: higher medial-lateral centre of pressure speed and increased total sway displacement (P<0.05), weight moved to the non-painful side, (P<0.05) and plantar centre of pressure of the left foot was shifted towards the heel's direction (P<0.05). During forward perturbation: higher mean displacement in the medial-lateral direction (P<0.05). After the perturbation: larger sway area (P<0.05). Pain only in the medial gastrocnemius muscle increased medial-lateral centre of pressure speed (P<0.05) during the quiet standing. Pain only in the tibialis anterior muscle increased peak pressure on the contralateral foot (P<0.05)., Interpretation: These findings suggest that large acute painful areas on the calf muscles impair the postural control and potentially increase the risk factors for falls. Further strategies aiming to reduce pain in patients may lead to improvement in balance., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Summary: Our aim was to analyze the differences in the referred pain patterns and size of the areas of those myofascial trigger points (TrPs) involved in chronic tension type headache (CTTH) including a number of muscles not investigated in previous studies. Thirteen right handed women with CTTH (mean age: 38 ± 6 years) were included. TrPs were bilaterally searched in upper trapezius, sternocleidomastoid, splenius capitis, masseter, levator scapulae, superior oblique (extra-ocular), and suboccipital muscles. TrPs were considered active when both local and referred pain evoked by manual palpation reproduced total or partial pattern similar to a headache attack. The size of the referred pain area of TrPs of each muscle was calculated. The mean number of active TrPs within each CTTH patient was 7 (95% CI 6.2-8.0). A greater number (T = 2.79; p = 0.016) of active TrPs was found at the right side (4.2 ± 1.5) when compared to the left side (2.9 ± 1.0). TrPs in the suboccipital muscles were most prevalent (n = 12; 92%), followed by the superior oblique muscle (n =11/n = 9 right/left side), the upper trapezius muscle (n = 11/n = 6) and the masseter muscle (n = 9/n=7). The ANOVA showed significant differences in the size of the referred pain area between muscles (F = 4.7, p = 0.001), but not between sides (F = 1.1; p = 0.3): as determined by a Bonferroni post hoc analysis the referred pain area elicited by levator scapulae TrPs was significantly greater than the area from the sternocleidomastoid (p = 0.02), masseter (p = 0.003) and superior oblique (p = 0.001) muscles. Multiple active TrPs exist in head, neck and shoulder muscles in women with CTTH. The referred pain areas of TrPs located in neck muscles were larger than the referred pain areas of head muscles. Spatial summation of nociceptive inputs from multiple active TrPs may contribute to clinical manifestations of CTTH., (Copyright © 2009 Elsevier Ltd. All rights reserved.)

This study aimed at investigating in details the spatial characteristics of muscular hyperalgesia after development of delayed onset muscle soreness (DOMS) in the trapezius muscle. High density pressure pain mapping consisting of 36 pain pressure threshold (PPT) recording points were assessed over the trapezius muscle from 20 subjects. PPT were recorded before, immediately after and 24h after eccentric exercise/rest for the exercise group (N=10) and the control group (N=10). A 36 points geometric grid was used on both the exercise and control groups. The eccentric exercise used to elicit DOMS consisted of 50 contractions against a downward pressing force at 100% maximum voluntary contraction in bouts of 10 contractions followed by 2 min break. For the exercise group, PPT values decreased significantly over time for all points (P<0.001) but not for the control group. At baseline, both muscle belly sites and upper part of the trapezius were more sensitive than muscle belly sites and middle and lower parts (P<0.001 for both). The hyperalgesia was also mostly developed in the muscle belly sites (P<0.001), further enhancing its position as the most sensitive part of the muscle. The present results showed the topographical distribution of pressure pain sensitivity over the trapezius muscle and also that hyperalgesia developed in a heterogeneous manner over the trapezius muscle in response to eccentric exercise underlining sensory partitioning of the muscle. The technique of high density pressure pain topographical mappings can be helpful in characterizing muscle hyperalgesia and its heterogeneity., (Copyright (c) 2009 European Federation of International Association for the Study of Pain Chapters. Published by Elsevier Ltd. All rights reserved.)

The study compared the distribution of electromyographic (EMG) signal amplitude in the upper trapezius muscle in 10 women with fibromyalgia and in 10 healthy women before and after experimentally-induced muscle pain. Surface EMG signals were recorded over the right upper trapezius muscle with a 10 x 5 grid of electrodes during 90 degrees shoulder abduction sustained for 60s. The control subjects repeated the abduction task following injections of isotonic and hypertonic (painful) saline into the upper trapezius muscle. The EMG amplitude was computed for each electrode pair and provided a topographical map of the distribution of muscle activity. The pain level rated by the patients at the beginning of the sustained contraction was 5.9+/-1.5. The peak pain intensity for the control group following the injection of hypertonic saline was 6.0+/-1.6. During the sustained contractions, the EMG amplitude increased relatively more in the cranial than caudal region of the upper trapezius muscle for the control subjects (shift in the distribution of EMG amplitude: 2.3+/-1.3mm; P<0.01). The patient group showed lower average EMG amplitude than the controls during the contraction (P<0.05) and did not show different changes in EMG amplitude between different regions of the upper trapezius. A similar behavior was observed for the control group following injection of hypertonic saline. The results indicate that muscle pain prevents the adaptation of upper trapezius activity during sustained contractions as observed in non-painful conditions, which may induce overuse of similar muscle compartments with fatigue., ((c) 2009 Elsevier Ltd. All rights reserved.)

Our aim was to describe the differences in the presence of trigger points (TrPs) in the shoulder muscles and to investigate the presence of mechanical hypersensitivity in patients with unilateral shoulder impingement and healthy controls. Twelve patients with strictly unilateral shoulder impingement and 10 matched controls were recruited. TrPs in the levator scapula, supraspinatus, infraspinatus, subscapularis, pectoralis major, and biceps brachii muscles were explored. TrPs were considered active if the local and referred pain reproduced the pain symptoms and the patient recognized the pain as a familiar pain. Pressure pain thresholds (PPT) were assessed over the levator scapulae, supraspinatus, infraspinatus, pectoralis major, biceps brachii, and tibialis anterior muscles. Both explorations were randomly done by an assessor blinded to the subjects' condition. Patients with shoulder impingement have a greater number of active (mean +/- SD: 2.5 +/- 1; P < 0.001) and latent (mean +/- SD: 2 +/- 1; P = 0.003) TrPs when compared to controls (only latent TrPs, mean +/- SD: 1 +/- 1). Active TrPs in the supraspinatus (67%), infraspinatus (42%), and subscapularis (42%) muscles were the most prevalent in the patient group. Patients showed a significant lower PPT in all muscles when compared to controls (P < 0.001). Within the patient group a significant positive correlation between the number of TrPs and pain intensity (r (s) = 0.578; P = 0.045) was found. Active TrPs in some muscles were associated to greater pain intensity and lower PPTs when compared to those with latent TrPs in the same muscles (P < 0.05). Significant negative correlations between pain intensity and PPT levels were found. Patients with shoulder impingement showed widespread pressure hypersensitivity and active TrPs in the shoulder muscles, which reproduce their clinical pain symptoms. Our results suggest both peripheral and central sensitisation mechanisms in patients with shoulder impingement syndrome.

Unlabelled: Fatiguing exercise can affect muscle pain sensitivity and muscle hardness, as seen with work-related neck and shoulder pain. Objective methods to assess muscle pain sensitivity are important because the reliability of manual assessment is generally poor. The aim of this study was (1) to compare coexistence of tender points identified by manual palpation and pressure algometry or hardness assessments and (2) to examine the influence of exercise on muscle pain sensitivity and hardness. Fourteen sites in the upper trapezius muscle were selected for assessments in 12 healthy subjects. Pressure pain thresholds and muscle hardness were examined by computer-controlled pressure algometry at baseline, immediately after static or dynamic exercise, and 20 minutes after static or dynamic exercise. Before recording of pressure pain thresholds, the trapezius muscle was examined for tender points by manual palpation. Two sites with low pressure pain thresholds were typical locations for tender points, and these were the least hard sites. However, manually detected tender points were often (29%) not colocalized with most sensitive sites according to the pressure algometry. A heterogeneous distribution of pressure pain sensitivity and muscle hardness was found in the upper trapezius. The short duration of exercise until exhaustion did not change muscle sensitivity or muscle hardness in asymptomatic muscles., Perspective: This study confirms clinical findings with heterogeniosity in pain sensitivity and hardness across the upper trapezius muscle. Developments of new techniques that objectively can identify tender points are important, but thus far, manual palpation is best clinical practice., (Copyright 2010 American Pain Society. Published by Elsevier Inc. All rights reserved.)

Objective: To investigate the changes in surface and intramuscular electromyographic (EMG) activity at latent trigger points (TrPs) in the extensor carpi radialis brevis muscle after injection of either glutamate or isotonic saline into latent TrPs in the infraspinatus muscle., Method: Nociceptive muscle stimulation was obtained by a bolus injection of glutamate (0.2 mL, 0.5 M) into a latent TrP located in the right infraspinatus muscle in 12 healthy volunteers. A bolus of isotonic saline (0.9%, 0.2 mL) injection served as control. Injections were guided by intramuscular EMG showing resting spontaneous electrical activity at the latent myofascial TrP in the infraspinatus muscle. Intramuscular (at the TrP) and surface EMG activities of both infraspinatus and extensor carpi radialis brevis muscles were recorded before, during, and after injection for a period of 6 minutes to monitor changes produced in EMG activity., Results: Glutamate injection into latent TrPs induced higher pain intensity than isotonic saline injection (P<0.001). The analysis of variance showed a significant increase in root mean square score of intramuscular EMG activity at TrP in the extensor carpi radialis brevis after glutamate (mean+/-SD: 212.0+/-215.6 microV) but not isotonic saline (mean+/-SD: 74.2+/-72.2 microV) injections (P<0.001). No changes in surface EMG activity were found. No significant changes in root mean square of intramuscular and surface EMG activity in the infraspinatus muscle were found., Conclusions: Our results show that an increased nociceptive activity at latent TrPs in the infraspinatus muscle may increase motor activity and sensitivity of a TrP in distant muscles at a same segmental level.

Background: Myofascial trigger points (MTrPs) present with mechanical hyperalgesia and allodynia. No electrophysiological evidence exists as to the excitability of muscle spindle afferents at myofascial trigger points MTrPs. The purpose of this current study was to explore whether an H-reflex response could be elicited from intramuscular electrical stimulation. If so, to assess the possibility of increased reflex response at MTrPs., Methods: The H-reflex latency and the conduction velocity were first determined from electrical stimulation of the tibial nerve in 13 healthy subjects. Then an intramuscular monopolar needle electrode was inserted randomly into a latent MTrP or a non-MTrP in the gastrocnemius muscle. Electrical stimuli at different intensities were delivered via the intramuscular recording electrode to the MTrP or non-MTrP., Results: The average conduction velocity (44.3 +/- 1.5 m/s) of the electrical stimulation of tibial nerve was similar (p>0.05) with the conduction velocity (43.9 +/- 1.4 m/s) of intramuscular electrical stimulation. The intramuscular H-reflex at MTrPs was higher in amplitude than non-MTrPs (p<0.001). The reflex threshold was lower for MTrPs than non-MTrPs (p<0.001)., Conclusion: The current study provides first electrophysiological evidence that intramuscular electrical stimulation can evoke H-reflex, and that higher H-reflex amplitude and lower H-reflex threshold exist at MTrPs than non-MTrPs respectively, suggesting that muscle spindle afferents may be involved in the pathophysiology of MTrPs.

Background: Eccentric contractions induce muscle fiber damage that is associated with a decreased capacity to generate voluntary force and increased fiber membrane permeability. Changes in fiber membrane permeability results in cell depolarization that is expected to have an effect on the action potential propagation velocity of the muscle fibers., Purpose: The aim of the study was to investigate the action potential propagation velocity in individual motor units before and 24 and 48 h after eccentric exercise., Methods: Multichannel surface and fine-wire intramuscular EMG signals were concurrently recorded from two locations of the right vastus medialis muscle of 10 healthy men during 60-s isometric contractions at 10% and 30% of the maximal force., Results: The maximal force decreased by 26.1 ± 16.1% (P < 0.0001) at 24 h and remained reduced by 23.6 ± 14.5% (P < 0.0001) 48 h after exercise with respect to baseline. With respect to baseline, motor unit conduction velocity decreased (P < 0.05) by (average over 24 and 48 h after exercise) 7.7 ± 2.7% (10% maximal voluntary contraction (MVC), proximal), 7.2 ± 2.8% (10% MVC, distal), 8.6 ± 3.8% (30% MVC, proximal), and 6.2 ± 1.5% (30% MVC, distal). Moreover, motor unit conduction velocity decreased over time during the sustained contractions at faster rates when assessed 24 and 48 h after exercise with respect to baseline for both contraction forces and locations (P < 0.05)., Conclusions: These results indicate that the electrophysiological membrane properties of muscle fibers are altered by exercise-induced muscle fiber damage.

Mechanisms and structures which are involved in eccentric exercise-induced delayed onset muscle soreness (DOMS) are not yet clarified. Tissue and site specificity may be important considerations in afferent sensitisation following eccentric exercise. This study investigated the nociceptive response to hypertonic sodium solution applied to fascial/epimysium tissue and mechanically sensitised sites in muscle by assessing (1) afferent recordings in animals and (2) psychophysical assessment in humans. Seventeen male rats underwent eccentric contraction of extensor digitorum longus muscle, while 11 rats served as an unexercised naïve group. Two days post-exercise, group IV afferent fibre activity was recorded in response to superfusion of hypertonic Krebs solution on the mechanically sensitised muscle/epimysium site. Mechanical sensitisation was confirmed with significant increases in afferent response and decreases in threshold to mechanical stimulation in the eccentrically exercised rats compared to naïve rats. There was no difference in afferent response magnitude to hypertonic Krebs solution between exercise and naïve groups. In the human study, 13 volunteers participated. After bilateral assessment of pressure pain thresholds (PPT) along the tibialis anterior muscles, eccentric exercise was performed to induce DOMS in m. tibialis anterior of one leg. Site of maximal mechanical sensitivity was identified 24 h later and injected with hypertonic saline at fascial and deep muscle levels. The corresponding site on the opposite unexercised leg served as a control. Fascial injection of the exercised muscle caused significantly higher pain intensity compared to all other injections. Response to deep muscle stimulation was not different between sides. This suggests that fascia rather than muscle tissue is important in DOMS associated sensitisation.

A method for quantifying nociceptive withdrawal reflex receptive fields in human volunteers and patients is described. The reflex receptive field (RRF) for a specific muscle denotes the cutaneous area from which a muscle contraction can be evoked by a nociceptive stimulus. The method is based on random stimulations presented in a blinded sequence to 10 stimulation sites. The sensitivity map is derived by interpolating the reflex responses evoked from the 10 sites. A set of features describing the size and location of the RRF is presented based on statistical analysis of the sensitivity map within every subject. The features include RRF area, volume, peak location and center of gravity. The method was applied to 30 healthy volunteers. Electrical stimuli were applied to the sole of the foot evoking reflexes in the ankle flexor tibialis anterior. The RRF area covered a fraction of 0.57+/-0.06 (S.E.M.) of the foot and was located on the medial, distal part of the sole of the foot. An intramuscular injection into flexor digitorum brevis of capsaicin was performed in one spinal cord injured subject to attempt modulation of the reflex receptive field. The RRF area, RRF volume and location of the peak reflex response appear to be the most sensitive measures for detecting modulation of spinal nociceptive processing. This new method has important potential applications for exploring aspects of central plasticity in volunteers and patients. It may be utilized as a new diagnostic tool for central hypersensitivity and quantification of therapeutic interventions.

A reorganized motor control system is a key factor in musculoskeletal pain conditions, and its relevance in the transition from acute pain to chronic pain is most likely underestimated. The interaction between muscle pain and motor control depends on the specific motor task. Muscle pain causes no increase in electromyographic activity at rest and reduces maximal voluntary contraction and endurance time during submaximal contractions. Furthermore, muscle pain causes an adaptive change in the coordination during dynamic exercises. Increased muscle activity reflecting reorganized muscle coordination and strategy is also a component of the functional adaption to muscle pain. In general, the "vicious cycle" hypothesis is not supported by these findings. Instead, they support an adaptive model predicting reduced agonistic muscle activity eventually advanced by changed antagonistic muscle activity. The motor control assessment procedures provide complementary clinical information and give further support for optimizing treatment regimens and prevention procedures for musculoskeletal pain.

Intramuscular injection of nerve growth factor (NGF) has been shown to induce long-term sensitisation and time-dependent hyperalgesia indicating potential involvement of both central and peripheral pain mechanisms. This double-blind placebo-controlled study was designed to describe the spatial distribution of muscle hyperalgesia over time (immediately after, 3 h, 1, 4, 7 and 21 days) after injecting NGF (5 mug) into the tibialis anterior (TA) muscle, to explore possibly involved central pain mechanisms and to investigate the effect of gender on development of hyperalgesia. Totally 20 healthy volunteers (10 men and 10 women) participated in the study. An isotonic saline injection into the contralateral TA muscle served as a control condition for the NGF injection. Pressure pain thresholds (PPT) were used to test for muscle hyperalgesia along the TA (seven sites) muscle at the extensor digitorum longus and at the web between 1st and 2nd metatarsal (central involvement). One day after the NGF/control injections, hypertonic saline (0.5 ml, 5.8%) was injected into the left and right TA to study the pain response to chemical stimulation of the hyperalgesic muscle tissue. Scores on a modified Likert scale were used to assess soreness during muscle function. An area of hyperalgesia was observed locally at the injected site 3 h after injection of NGF, which expanded both proximally and distally on day 1; this effect subsided on day 4. Decreased PPT was also found between 1st and 2nd metatarsal on day 1. Hypertonic saline evoked more pain in men when injected in the NGF treated TA compared to the control leg. Injection of NGF increased muscle soreness during muscle activity for 7 days. In this material there was no gender effect of NGF-induced muscle hyperalgesia. The expansion of muscle hyperalgesia to distant areas indicates that central mechanisms are involved.

Objectives: To screen for the presence of latent and active myofascial trigger points (MTrPs) in patients with unilateral shoulder and arm pain and perform topographical mapping of mechanical pain sensitivity bilaterally in the infraspinatus muscles., Methods: Nineteen patients with unilateral musculoskeletal shoulder pain participated in the study. The area overlying the infraspinatus on each side was divided into 10 adjacent sub-areas of 1cm(2), corresponding to the area of a pressure algometer probe. Pressure pain threshold (PPT) was measured in each sub-area bilaterally in the infraspinatus muscles. Following PPT measurement, an acupuncture needle was inserted into each sub-area five times in different directions in order to induce local twitch response and/or referred pain., Results: A significantly lower PPT level in the infraspinatus muscle was detected on the painful side compared with the non-painful side (P=0.001). PPT at midfiber region of the infraspinatus muscles was lower than that at other muscle parts (P<0.05). Multiple, but not single, active MTrPs were found in the infraspinatus muscle on the painful side and there were also multiple latent MTrPs bilaterally in the infraspinatus muscles. PPT at active MTrPs was much lower than the latent MTrPs and again lower than the non-MTrPs., Conclusions: There exists bilateral mechanical hyperalgesia in patients with unilateral shoulder pain. Further, the association of multiple active MTrPs with unilateral shoulder pain and the heterogeneity of mechanical pain sensitivity distribution suggest a crucial role of peripheral sensitization in chronic myofascial pain conditions. Additionally, the locations of MTrPs identified with dry needling correspond well to PPT topographical mapping, suggesting that dry needling and PPT topographical mapping are sensitive techniques in the identification of MTrPs.

The aim of the present study was to explore variation in skin blood flow and temperature following experimental muscle pain. In 14 male human subjects, 2 ml and 4.8 ml of hypertonic (5.8%) saline were injected into the left tibialis anterior (TA) muscle to induce muscle pain. The subjects rated the pain intensity on a 10 cm visual analogue scale (VAS). Using laser-Doppler flowmetry and thermography, the skin blood flow and temperature were assessed at four different skin areas: ipsilateral muscle pain area (5x5 cm), ipsilateral referred pain area (5x10 cm), and two corresponding mirror areas on the contralateral non-injected leg. Compared with 2 ml hypertonic saline injection, significantly longer pain duration (1599+/-119 s) and stronger pain intensity (VAS peak: 6.9+/-0.6 cm) were found after the injection of 4.8 ml hypertonic saline (P<0.001, respectively). In addition, 4.8 ml hypertonic saline evoked a significant increase in skin blood flow and higher skin temperature around the injection site, the referred pain area, and the contralateral area to the injection site (P<0.05). By contrast, 2 ml hypertonic saline injection only elicited an increase in skin blood flow, but not temperature, at the injection area and the contralateral mirror area to the injection site (P<0.05). These results suggested that the vasodilation in different skin areas following intramuscular injection of hypertonic saline was dose-dependent. Injection of 4.8 ml hypertonic saline after local intramuscular anesthesia (2% lidocaine) did not evoke any significant changes in skin blood flow or skin temperature in any of the four observation areas. This suggested that both homotopic and heterotopic vascular reactions triggered by hypertonic saline stimulation of thin muscle afferent fibers were a neurogenically associated reaction.

This study examined gender differences in the effect of experimental muscle pain on changes in the relative activation of regions of the upper trapezius muscle during a sustained contraction. Surface electromyographic (EMG) signals were recorded from multiple locations over the upper trapezius muscle with a 10 x 5 grid of electrodes from nine women and nine men during 90 degrees shoulder abduction sustained for 60s. Measurements were performed before and after the injection of 0.4 ml hypertonic (painful) and isotonic (control) saline into the cranial region of the upper trapezius muscle. The EMG root mean square (RMS) was computed for each location of the grid to form a map of the EMG amplitude distribution. The peak pain intensity following the injection of hypertonic saline was greater for women (numerical rating scale 0-10: women 6.0+/-2.1, men 4.2+/-0.9; P<0.01). For both genders, upper trapezius RMS averaged across the grid decreased following the injection of hypertonic saline (P<0.0001). Moreover, there was a relatively larger pain-induced decrease in RMS in the cranial region compared to the caudal region of the muscle for both genders. During the non-painful sustained contractions, the EMG RMS progressively increased more in the cranial than the caudal region, for both men and women, due to fatigue. This mechanism was maintained in men but not in women during the painful condition. The results demonstrate that muscle pain alters the normal adaptation of upper trapezius muscle activity to fatigue in women but not in men.

The aim of the study was to investigate EMG signal features during fatigue and recovery at three locations of the vastus medialis and lateralis muscles. Surface EMG signals were detected from 10 healthy male subjects with six 8-electrode arrays located at 10%, 20%, and 30% of the distance from the medial (for vastus medialis) and lateral (vastus lateralis) border of the patella to the anterior superior spine of the pelvic. Subjects performed contractions at 40% and 80% of the maximal force (MVC) until failure to maintain the target force, followed by 20 2-s contractions at the same force levels every minute for 20 min (recovery). Average rectified value, mean power spectral frequency, and muscle fiber conduction velocity were estimated from the EMG signals in 10 epochs from the beginning of the contraction to task failure (time to task failure, mean+/-SD, 70.7+/-25.8s for 40% MVC; 27.4+/-16.8s for 80% MVC) and from the 20 2s time intervals during recovery. During the fatiguing contraction, the trend over time of EMG average rectified value depended on location for both muscles (P<0.05). After 20-min recovery, mean frequency and conduction velocity of both muscles were larger than in the beginning of the fatigue task (P<0.05) (supernormal values). Moreover, the trend over time of mean frequency during recovery was affected by location and conduction velocity values depended on location for both muscles (P<0.05). The results indicate spatial dependency of EMG variables during fatigue and recovery and thus the necessity of EMG spatial sampling for global muscle assessment.

The aim of the study was to confirm the hypothesis that the longer a contraction is sustained, the larger are the changes in the spatial distribution of muscle activity. For this purpose, surface electromyographic (EMG) signals were recorded with a 13 x 5 grid of electrodes from the upper trapezius muscle of 11 healthy male subjects during static contractions with shoulders 90 degrees abducted until endurance. The entropy (degree of uniformity) and center of gravity of the EMG root mean square map were computed to assess spatial inhomogeneity in muscle activation and changes over time in EMG amplitude spatial distribution. At the endurance time, entropy decreased (mean+/-SD, percent change 2.0+/-1.6%; P<0.0001) and the center of gravity moved in the cranial direction (shift 11.2+/-6.1mm; P<0.0001) with respect to the beginning of the contraction. The shift in the center of gravity was positively correlated with endurance time (R(2)=0.46, P<0.05), thus subjects with larger shift in the activity map showed longer endurance time. The percent variation in average (over the grid) root mean square was positively correlated with the shift in the center of gravity (R(2)=0.51, P<0.05). Moreover, the shift in the center of gravity was negatively correlated to both initial and final (at the endurance) entropy (R(2)=0.54 and R(2)=0.56, respectively; P<0.01 in both cases), indicating that subjects with less uniform root mean square maps had larger shift of the center of gravity over time. The spatial changes in root mean square EMG were likely due to spatially-dependent changes in motor unit activation during the sustained contraction. It was concluded that the changes in spatial muscle activity distribution play a role in the ability to maintain a static contraction.

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.

Objective: To investigate the effects of intramuscular electrical conditioning in the modulation of nociceptive withdrawal reflex (NWR) and further to determine what muscle afferents are involved in the modulation of the nociceptive withdrawal reflex and the sites along the reflex pathway where the NWR modulation occurs in healthy humans., Methods: The NWR elicited by a cutaneous test stimulus to the dorsal foot was modulated by a short (21 ms) intramuscular conditioning electrical stimulus at two times the pain threshold. At varying conditioning-test stimulus intervals, segmental conditioning stimulus was applied in the tibialis anterior muscle ipsilateral and contralateral to the test stimulus, and heterosegmental conditioning stimulus was applied in the contralateral trapezius muscle to modulate the NWR. Non-painful and painful intramuscular conditioning stimuli were also used to modulate the NWR and the soleus H-reflex., Results: The NWR was depressed by preceding intramuscular conditioning stimuli, with a degree that depended on the conditioning-test stimulus intervals and on the conditioning site. Segmental conditioning depressed the NWR more quickly and gave a longer duration (15-1500 ms), and larger magnitude than heterosegmental conditioning, which depressed the NWR in a short temporal window (80-100 ms). No difference was seen in the magnitude of the NWR depression between the painful and non-painful intramuscular stimuli, and the soleus H-reflex was not affected., Conclusions: Our results suggest that segmental and heterosegmental conditionings of NWR are mediated by myelinated muscle afferents engaging central inhibitory mechanisms rather than direct changes in the excitability of motor neurons., Significance: The therapeutic effects of electrotherapy could involve these mechanisms in the treatment of muscle pain syndromes.

The purpose of this study was to investigate the force and electromyographic (EMG) signal recorded from the muscles immediately after a sustained fatiguing contraction with or without muscle pain. Ten subjects performed sustained dorsi- and plantarflexions at two contraction levels (50 and 80% of maximum voluntary contraction) until exhaustion with or without muscle pain induced by injection of 6% hypertonic saline in one of the torque producing muscles. The muscle pain intensity was scored on a visual analogue scale (VAS, 0-10 cm). The root mean square (RMS) of the surface EMG signal from plantarflexors and dorsiflexors were estimated during maximum voluntary contractions (MVC) and ramp contractions before and after the fatiguing task at 0, 5, 10 and 15 min during the recovery phase. VAS scores immediately after the contractions with hypertonic saline (on average 3.2 +/- 1.1 cm) progressively decreased during recovery and no pain was experienced 15 min after the contraction. After the painful contraction the RMS-EMG during MVC was on average decreased (23.4 +/- 7.4%) compared to the non-painful condition both in muscles where pain was previously induced and in non-painful synergists. During recovery, the slope of the torque-EMG curve during ramp contraction was significantly decreased (28.4 +/- 8.1%) after the painful contraction compared to the control contraction both for the muscle previously exposed to pain and also the other active synergists. The decreased EMG during recovery after painful contractions compared with control was not accompanied by significant reductions in force during MVC indicating a change in the strategy for motor unit recruitment. This study shows that localized muscle pain inhibits muscle activation and increases the effects of fatigue on EMG recovery curves both for painful and non-painful synergists probably by a central effect. These effects can modify the normal patterns of synergistic activation and can also generate overload problems in muscle pain patients if compensatory motor control strategies are applied.

The aim of this study was to compare mechanomyogram (MMG) recorded by a condenser microphone (MIC) and an accelerometer (ACC) during submaximal isometric, concentric and eccentric contractions in 14 males. The maximal voluntary force (MVC) of the biceps brachii was measured. The subjects were asked to do short duration isometric, concentric and eccentric contraction at 10%, 30%, 50%, 70% MVC twice. For the concentric and eccentric contraction, the subject bent his arm for 3s (concentric) then held it for 3s and extended (eccentric) during 3s. The normalized root mean square (RMS) and mean power frequency (MPF) increased linearly with increased force for both transducers. There was a correlation between MIC MPF and ACC MPF at 10%, 30%, 50% MVC, and between MIC RMS and ACC RMS at 30% MVC during isometric contractions. There was significantly higher MPF for the ACC than for the MIC in concentric and eccentric modes, while the RMS did not differ among transducers in the three contraction modes. The RMS and MPF values coefficient of variations were significantly larger during anisometric contractions compared with isometric contractions and were lower for the accelerometer than for the microphone. The present results obtained during isometric, concentric and eccentric contractions of increased intensity showed that the information contained in microphone- and accelerometer-based MMG signals is different despite similar trends. It can be concluded that at low-moderate movement velocity, concentric contractions can be investigated by means of accelerometer and microphone.

Objective: Referred pain and pain characteristics evoked from the extensor carpi radialis brevis, extensor carpi radialis longus, extensor digitorum communis, and brachioradialis muscles was investigated in 20 patients with lateral epicondylalgia (LE) and 20-matched controls., Methods: Both groups were examined for the presence of myofascial trigger points (TrPs) in a blinded fashion. The quality and location of the evoked referred pain, and the pressure pain threshold (PPT) at the lateral epicondyle on the right upper extremity (symptomatic side in patients, and dominant-side on controls) were recorded. Several lateral elbow pain parameters were also evaluated., Results: Within the patient group, the elicited referred pain by manual exploration of 13 out of 20 (65%) extensor carpi radialis brevis muscles, 12/20 (70%) extensor carpi radialis longus muscles, 10/20 (50%) brachioradialis muscles, and 5/20 (25%) extensor digitorum communis muscles, shares similar pain patterns as their habitual lateral elbow and forearm pain. The mean number of muscles with TrPs for each patient was 2.9 [95% confidence interval (CI) 1,4] of which 2 (95% CI 1,3) were active, and 0.9 (95% CI 0,2) were latent TrPs. Control participants only had latent TrPs (mean: 0.4; 95% CI 0,2). TrP occurrence between the 2 groups was significantly different for active TrPs (P<0.001), but not for latent TrPs (P>0.05). The referred pain pattern was larger in patients than in controls, with pain referral to the lateral epicondyle (proximally) and to the dorso-lateral aspect of the forearm in the patients, and confined to the dorso-lateral aspect of the forearm in the controls. Patients with LE showed a significant (P<0.001) lower PPT (mean: 2.1 kg/cm; 95% CI 0.8, 4 kg/cm) as compared with controls (mean: 4.5 kg/cm; 95% CI 3, 7 kg/cm). Within the patient group, PPT at the lateral epicondyle was negatively correlated with both the total number of TrPs (rs=-0.63; P=0.003) and the number of active TrPs (rs=-0.5; P=0.02): the greater the number of active TrPs, the lower the PPT at the lateral epicondyle., Discussion: Our results suggest that in patients with LE, the evoked referred pain and its sensory characteristics shared similar patterns as their habitual elbow and forearm pain, consistent with active TrPs. Lower PPT and larger referred pain patterns suggest that peripheral and central sensitization exists in LE.

Referred pain and pain characteristics evoked from the upper trapezius muscle was investigated in 20 patients with chronic tension-type headache (CTTH) and 20 age- and gender-matched controls. A headache diary was kept for 4 weeks in order to confirm the diagnosis and record the pain history. Both upper trapezius muscles were examined for the presence of myofascial trigger points (TrPs) in a blinded fashion. The local and referred pain intensities, referred pain pattern, and pressure pain threshold (PPT) were recorded. The results show that referred pain was evoked in 85% and 50% on the dominant and non-dominant sides in CTTH patients, much higher than 55% and 25% in controls (P<0.01). Referred pain spread to the posterior-lateral aspect of the neck ipsi-lateral to the stimulated muscle in both patients and controls, with additional referral to the temple in most patients, but none in controls. Nearly half of the CTTH patients (45%) recognized the referred pain as their usual headache sensation, i.e. active TrPs. CTTH patients with active TrPs in the right upper trapezius muscle showed greater headache intensity and frequency, and longer headache duration than those with latent TrPs. CTTH patients with bilateral TrPs reported significantly decreased PPT than those with unilateral TrP (P<0.01). Our results showed that manual exploration of TrPs in the upper trapezius muscle elicited referred pain patterns in both CTTH patients and healthy subjects. In CTTH patients, the evoked referred pain and its sensory characteristics shared similar patterns as their habitual headache pain, consistent with active TrPs. Our results suggest that spatial summation of perceived pain and mechanical pain sensitivity exists in CTTH patients.

Spatial dependency of paraspinal muscle activity was assessed using a new two-dimensional MMG recording system. MMG signals were detected over the left and right paraspinal muscles of 10 volunteers using a grid of 12 accelerometers. During two separate trials subjects maintained a 20 degrees flexed position and held loads that ranged from 0 to 15 kg (in 2.5 kg increments) for 20s; and 7.5 kg for 6 min. Maps of absolute and normalised (with respect to initial values) average rectified value, mean power frequency, variance and skewness of the power spectral density were obtained from the two-dimensional MMG recordings. For both the short duration and sustained contractions, the MMG absolute average rectified value, mean power frequency, variance and skewness depended on accelerometer location (P<0.05), while, with the exception of the skewness (P<0.05), normalised values did not. These results demonstrate both inhomogeneous MMG absolute activity and homogeneous MMG normalised activity in paraspinal muscles for short duration and sustained contractions. Moreover, the effect of accelerometer location on spectral variables confirmed the limited validity of general relationships between MMG spectral changes and motor unit recruitment strategies. This study underlines the importance of using multiple recording sites when assessing back muscle activity.

Objective: To investigate the effect of local excitation of nociceptive muscle afferents on the spatial distribution of muscle activity., Methods: Surface electromyographic (EMG) signals were recorded from the upper trapezius muscle of 10 healthy volunteers with a 5 x 13 electrode grid during 90-s isometric contractions before, during, 15 and 30 min after intramuscular injection of hypertonic (painful) or isotonic (non-painful) saline. From the multi-channel EMG recordings, two-dimensional maps of root mean square and mean power frequency were obtained. The centre of gravity of the root mean square map was used to quantify global changes in the spatial distribution of muscle activity., Results: During sustained contractions, average root mean square increased, average mean frequency decreased and the centre of gravity moved cranially. During experimental muscle pain, compared to before injection, the average root mean square decreased and there was a caudal shift of the centre of gravity. Fifteen minutes after the painful injection the centre of gravity returned to its original position., Conclusions: Short-term dynamic reorganization of the spatial distribution of muscle activity occurred in response to nociceptive afferent input., Significance: The study furnishes an extension of the pain adaptation model indicating heterogeneous inhibition of muscle activity.

Convergence between cutaneous heat nociceptors and muscles afferents was investigated by applying a phasic, conditioning electrical stimulus to the tibialis anterior muscle (a train of five 1 ms pulses over 21 ms) at varying time intervals relative to a thermal test stimulus used for evoking the withdrawal reflex in humans. The 200 ms thermal stimulus was applied on the dorsum of the foot at an intensity of two times the pain threshold. The conditioning electrical stimulus was applied at an intensity of two times the pain threshold via a set of intramuscular needle electrodes. The conditioning-test interval was varied between -400 ms and 8,000 ms at 17 different intervals. The mean reflex onset latency of reflexes evoked by thermal stimuli alone was 354 +/- 9 ms. A facilitation of the reflex was seen when the conditioning stimulus was applied 275 ms (174 +/- 30% compared to control) and 300 ms (162 +/- 32% compared to control) after the test stimulus onset indicating sensory convergence between muscle afferents (group I-III) and cutaneous Adelta heat nociceptors arriving simultaneously at the spinal cord.

The spatial distribution of pressure sensitivity and muscle hardness was examined on normal muscle tissue and muscle tissue after induction of delayed onset muscle soreness (DOMS). The pressure sensitivity and muscle hardness were assessed at nine sites on the tibialis muscle from the proximal to distal tendon on two separate days. In total 37 healthy volunteers participated in three experiments. In the first experiment pressure pain threshold (PPT) and pressure pain tolerance (PPTO) were assessed. Decreased PPT and PPTO were found on day 2, 7 days after day 1. Proximal and distal stimulation sites were harder compared to muscle belly sites. In a second experiment two different probe sizes were used. Variation in PPT between the nine sites was found for the large probe with muscle belly being less sensitive to pressure stimulation compared to proximal and distal sites. The most proximal stimulation site was harder compared to muscle belly sites. In a third experiment PPT and muscle hardness were assessed before and 48 h after eccentric exercise. PPT at two muscle belly sites was significantly decreased during DOMS. No specific sites were harder during DOMS, the average muscle hardness across sites was however significantly increased. Decreased PPT and increased muscle hardness did not correlate. In conclusion, within subjects the pressure sensitivity varies along the musculoskeletal unit. In DOMS, specific muscle belly sites were more sensitive to pressure stimulation. Muscle-tendon sites were harder compared to muscle belly sites.

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.

In patients with lateral epicondylalgia, mobilization-with-movement (MWM) is used as an intervention aimed at achieving analgesia and enhancing grip force, although the mechanisms underlying these effects are unclear. The present study investigated the acute sensory and motor effects of an MWM intervention in healthy controls with experimentally induced lateral epicondylalgia. Twenty-four subjects were randomly allocated to either a MWM or a placebo group (n=12). In both groups, to generate the model of lateral epicondylalgia, delayed onset muscle soreness (DOMS) was provoked in one arm 24h prior (Day 0) to hypertonic saline-induced pain in the extensor carpi radialis brevis muscle (Day 1). Either a MWM or placebo intervention was applied during the saline-induced pain period. Saline-induced pain intensity (visual analogue scale: VAS), pain distribution and pain quality were assessed quantitatively. Pressure pain thresholds (PPTs) were recorded at the common extensor origin and the extensor carpi radialis brevis muscle. Maximal measures of grip and wrist extension force were recorded. In both groups (pooled data), DOMS was efficiently induced as demonstrated by a significant decrease in pre-exercise to pre-injection PPT at the common extensor origin (-45+/-19%) and at the extensor carpi radialis brevis (-61+/-23%; P<0.05), and a significant decrease in maximal grip force (-25+/-6%) and maximal wrist extension force (-40+/-12%; P<0.001). Moreover, both groups experienced a significant increase in muscle soreness (3.9+/-0.2; P<0.0001) at Day 1 compared to pre-exercise. During saline-induced pain and in response to intervention, there were no significant between-group differences in VAS profiles, pain distributions, induced deep tissue hyperalgesia or force attenuation. These data suggest that the lateral glide-MWM does not activate mechanisms associated with analgesia or force augmentation in subjects with experimentally induced features simulating lateral epicondylalgia.

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.