Effects of naloxone on dental pain threshold following muscle exercise and low frequency transcutaneous nerve stimulation: a comparative study in man

Pharmacological and non-pharmacological therapeutic interventions for the treatment of spinal cord injury-induced pain

Spinal cord injury (SCI) is a complex neurophysiological disorder, which can result in many long-term complications including changes in mobility, bowel and bladder function, cardiovascular function, and metabolism. In addition, most individuals with SCI experience some form of chronic pain, with one-third of these individuals rating their pain as severe and unrelenting. SCI-induced chronic pain is considered to be "high impact" and broadly affects a number of outcome measures, including daily activity, physical and cognitive function, mood, sleep, and overall quality of life. The majority of SCI pain patients suffer from pain that emanates from regions located below the level of injury. This pain is often rated as the most severe and the underlying mechanisms involve injury-induced plasticity along the entire neuraxis and within the peripheral nervous system. Unfortunately, current therapies for SCI-induced chronic pain lack universal efficacy. Pharmacological treatments, such as opioids, anticonvulsants, and antidepressants, have been shown to have limited success in promoting pain relief. In addition, these treatments are accompanied by many adverse events and safety issues that compound existing functional deficits in the spinally injured, such as gastrointestinal motility and respiration. Non-pharmacological treatments are safer alternatives that can be specifically tailored to the individual and used in tandem with pharmacological therapies if needed. This review describes existing non-pharmacological therapies that have been used to treat SCI-induced pain in both preclinical models and clinical populations. These include physical (i.e., exercise, acupuncture, and hyper- or hypothermia treatments), psychological (i.e., meditation and cognitive behavioral therapy), and dietary interventions (i.e., ketogenic and anti-inflammatory diet). Findings on the effectiveness of these interventions in reducing SCI-induced pain and improving quality of life are discussed. Overall, although studies suggest non-pharmacological treatments could be beneficial in reducing SCI-induced chronic pain, further research is needed. Additionally, because chronic pain, including SCI pain, is complex and has both emotional and physiological components, treatment should be multidisciplinary in nature and ideally tailored specifically to the patient.

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

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

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

Does exercise increase or decrease pain? Central mechanisms underlying these two phenomena

Exercise is an integral part of the rehabilitation of patients suffering a variety of chronic musculoskeletal conditions, such as fibromyalgia, chronic low back pain and myofascial pain. Regular physical activity is recommended for treatment of chronic pain and its effectiveness has been established in clinical trials for people with a variety of pain conditions. However, exercise can also increase pain making participation in rehabilitation challenging for the person with pain. Animal models of exercise-induced pain have been developed and point to central mechanisms underlying this phenomena, such as increased activation of NMDA receptors in pain-modulating areas. Meanwhile, a variety of basic science studies testing different exercise protocols, show exercise-induced analgesia involves activation of central inhibitory pathways. Opioid, serotonin and NMDA mechanisms acting in rostral ventromedial medulla promote analgesia associated with exercise. This review explores and discusses current evidence on central mechanisms underlying exercised-induced pain and analgesia.

Nociceptive Threshold and Physical Activity

Previous studies using subjective tools to measure pain have shown that muscle exercise can have analgesic effects in man. The nociceptive leg flexion reflex (or RIII reflex) is a useful objective tool for assessing human pain. In this study, the pain threshold was assessed using the nociceptive flexion reflex in six high-level athletes 1) at rest in comparison with 8 control subjects and 2) after exercise requiring the production of a 200-Watt force over a period of 20 minutes. The nociceptive flexion reflex threshold at rest was found to be spontaneously higher in the athletes than in the controls. Physical activity resulted in a significant increase (+53%) in the threshold of the nociceptive reflex in the athletes. The role of stress-induced analgesia, the reduction in perceived intensity of stimuli during movement, and the release of opioids are discussed.

Endogenous opioid antagonism in physiological experimental pain models: a systematic review

Opioid antagonists are pharmacological tools applied as an indirect measure to detect activation of the endogenous opioid system (EOS) in experimental pain models. The objective of this systematic review was to examine the effect of mu-opioid-receptor (MOR) antagonists in placebo-controlled, double-blind studies using ʻinhibitoryʼ or ʻsensitizingʼ, physiological test paradigms in healthy human subjects. The databases PubMed and Embase were searched according to predefined criteria. Out of a total of 2,142 records, 63 studies (1,477 subjects [male/female ratio = 1.5]) were considered relevant. Twenty-five studies utilized ʻinhibitoryʼ test paradigms (ITP) and 38 studies utilized ʻsensitizingʼ test paradigms (STP). The ITP-studies were characterized as conditioning modulation models (22 studies) and repetitive transcranial magnetic stimulation models (rTMS; 3 studies), and, the STP-studies as secondary hyperalgesia models (6 studies), ʻpainʼ models (25 studies), summation models (2 studies), nociceptive reflex models (3 studies) and miscellaneous models (2 studies). A consistent reversal of analgesia by a MOR-antagonist was demonstrated in 10 of the 25 ITP-studies, including stress-induced analgesia and rTMS. In the remaining 14 conditioning modulation studies either absence of effects or ambiguous effects by MOR-antagonists, were observed. In the STP-studies, no effect of the opioid-blockade could be demonstrated in 5 out of 6 secondary hyperalgesia studies. The direction of MOR-antagonist dependent effects upon pain ratings, threshold assessments and somatosensory evoked potentials (SSEP), did not appear consistent in 28 out of 32 ʻpainʼ model studies. In conclusion, only in 2 experimental human pain models, i.e., stress-induced analgesia and rTMS, administration of MOR-antagonist demonstrated a consistent effect, presumably mediated by an EOS-dependent mechanisms of analgesia and hyperalgesia.

Cerebral responses to innocuous somatic pressure stimulation following aerobic exercise rehabilitation in chronic pain patients: a functional magnetic resonance imaging study

OBJECTIVE

The purpose of this research was to assess the functional brain activity and perceptual rating of innocuous somatic pressure stimulation before and after exercise rehabilitation in patients with chronic pain.

MATERIALS AND METHODS

Eleven chronic pain patients and eight healthy pain-free controls completed 12 weeks of supervised aerobic exercise intervention. Perceptual rating of standardized somatic pressure stimulation (2 kg) on the right anterior mid-thigh and brain responses during functional magnetic resonance imaging (fMRI) were assessed at pre- and postexercise rehabilitation.

RESULTS

There was a significant difference in the perceptual rating of innocuous somatic pressure stimulation between the chronic pain and control groups (P=0.02) but no difference following exercise rehabilitation. Whole brain voxel-wise analysis with correction for multiple comparisons revealed trends for differences in fMRI responses between the chronic pain and control groups in the superior temporal gyrus (chronic pain > control, corrected P=0.30), thalamus, and caudate (control > chronic, corrected P=0.23). Repeated measures of the regions of interest (5 mm radius) for blood oxygen level-dependent signal response revealed trend differences for superior temporal gyrus (P=0.06), thalamus (P=0.04), and caudate (P=0.21). Group-by-time interactions revealed trend differences in the caudate (P=0.10) and superior temporal gyrus (P=0.29).

CONCLUSION

Augmented perceptual and brain responses to innocuous somatic pressure stimulation were shown in the chronic pain group compared to the control group; however, 12-weeks of exercise rehabilitation did not significantly attenuate these responses.

A meta-analytic review of the hypoalgesic effects of exercise

The purpose of this article was to examine the effects of acute exercise on pain perception in healthy adults and adults with chronic pain using meta-analytic techniques. Specifically, studies using a repeated measures design to examine the effect of acute isometric, aerobic, or dynamic resistance exercise on pain threshold and pain intensity measures were included in this meta-analysis. The results suggest that all 3 types of exercise reduce perception of experimentally induced pain in healthy participants, with effects ranging from small to large depending on pain induction method and exercise protocol. In healthy participants, the mean effect size for aerobic exercise was moderate (d(thr) = .41, d(int) = .59), while the mean effect sizes for isometric exercise (d(thr) = 1.02, d(int) = .72) and dynamic resistance exercise (d(thr) = .83, d(int) = .75) were large. In chronic pain populations, the magnitude and direction of the effect sizes were highly variable for aerobic and isometric exercise and appeared to depend on the chronic pain condition being studied as well as the intensity of the exercise. While trends could be identified, the optimal dose of exercise that is needed to produce hypoalgesia could not be systematically determined with the amount of data available.

PERSPECTIVE

This article presents a quantitative review of the exercise-induced hypoalgesia literature. This review raises several important questions that need to be addressed while also demonstrating that acute exercise has a hypoalgesic effect on experimentally induced pain in healthy adults, and both a hypoalgesic and hyperalgesic effect in adults with chronic pain.

High-intensity extended swimming exercise reduces pain-related behavior in mice: involvement of endogenous opioids and the serotonergic system

The present study examined the hyponociceptive effect of swimming exercise in a chemical behavioral model of nociception and the mechanisms involved in this effect. Male mice were submitted to swimming sessions (30 min/d for 5 days). Twenty-four hours after the last session, we noticed that swimming exercise decreased the number of abdominal constriction responses caused by acetic acid compared with the nonexercised group. The hyponociception caused by exercise in the acetic acid test was significantly attenuated by intraperitoneal (i.p.) pretreatment of mice with naloxone (a nonselective opioid receptor antagonist, 1 mg/kg), ρ-chlorophenylalanine methyl ester (PCPA, an inhibitor of serotonin synthesis, 100 mg/kg once a day for 4 consecutive days), and by bilateral adrenalectomy. Collectively, the present results provide experimental evidences indicating for the first time that high-intensity extended swimming exercise reduces pain-related behavior in mice. The mechanisms involve an interaction with opioid and serotonin systems. Furthermore, endogenous opioids released by adrenal glands probably are involved in this effect.

PERSPECTIVE

Our results indicate that high-intensity extended exercise endogenously controls acute pain by activation of opioidergic and serotonergic pathways. Furthermore, these results support the use of exercise as a nonpharmacological approach for the management of acute pain.

The Influence of Education and Exercise on Neck Pain

Neck pain is a significant contributor to worldwide disability and poses a considerable financial burden to its stakeholders. The prognosis for chronic neck pain is generally poor, and the associated disability seems to be more persistent than low back pain. It has been suggested that the goals of a rehabilitation program are to maximize return to function, limit progression of degenerative changes, and prevent further injury. The variety of treatment options can make it difficult for clinicians to agree on the most effective treatment intervention. This article reviews noninvasive treatment considerations for patients with neck pain. Exercise-based interventions, including aerobic conditioning, stretching, and strengthening, are addressed. Moreover, concepts related to education are covered, including the effects of posture and ergonomic counseling.

Implications for Physical Activity in the Population With Low Back Pain

Low back pain is considered the most prevalent pain complaint affecting the general population, with a reported lifetime prevalence of up to 75%. It is a recognized societal problem from both a disablement and economic perspective, with costs exceeding that of coronary artery disease, respiratory infections, and diabetes. The traditional concept of low back pain having a favorable natural history paints an overly optimistic prognosis when considering recent epidemiological data. From a prognostic perspective, more than three quarters of individuals will have a recurrence of their symptoms within a year from the initial onset, and up to 40% report a permanent reduction in activity participation. Fear of movement and avoidance of physical activity participation have been associated with a less than desirable prognosis, often times leading to chronicity. Although numerous interventions with often contradictory findings have been described for the population with low back pain, a consensus of research has implicated the resumption and pursuit of appropriate physical activity as an efficacious intervention. This article reviews the current evidence pertaining to physical activity participation in the population with low back pain and provides recommendations for identifying individuals who may be at risk for chronicity resulting from fear and avoidance of activity.

Nociception before and after exercise in rats bred for high and low aerobic capacity

Exercise and stress are known to influence pain perception. However, little is known about how level of fitness influences pain perception and the experience of pain. In the present study, pain perception before and after exercise to exhaustion was examined in 6 rats systematically bred to have a high aerobic capacity (HCR animals) and 6 rats systematically bred to have a low aerobic capacity (LCR animals). HCR animals had significantly higher pain thresholds compared to LCR animals before and after exercise (7.66 s compared to 6.01 s, t=-3.07, p<0.05; and 6.89 s versus 4.73 s, t=-3.73, p<0.01, respectively). In addition, both groups of animals displayed evidence of hyperalgesia following exercise compared to baseline. However, the pain thresholds of HCR animals returned to baseline levels faster than LCR animals following exercise. The findings support the hypothesis that level of fitness plays a role in the perception of pain. In addition, a higher level of fitness may serve as buffer against the effects of stress and help to reduce or prevent the experience of clinical pain. Further research is needed to examine the mechanisms that underlie this phenomenon.

Does aerobic exercise improve pain perception and mood? A review of the evidence related to healthy and chronic pain subjects

Aerobic exercise can cause an acute improvement in mood as well as a reduction in the perception of pain from a painful stimulus. Regular exercise training also may offer some protection from depression, is clinically useful in treating certain psychiatric and chronic pain conditions, and may allow for an enhancement of the acute improvements in mood from a single exercise session. The utility of aerobic exercise training for improving mood disturbances and pain perception among patients with chronic pain requires further investigation.

Effects of steady-state exercise on perceived pain: comparison of sedentary students and cyclists

In this study, the effect of fitness level on perceived pain before and after a steady state exercise was investigated. Ten trained cyclists (M age=25.2 yr., SD=4.9) and 10 sedentary men (Mage=24.5 yr., SD=2.2) performed a maximal graded test on a cycle ergometer. At least 48 hr. later the participants of both groups performed a 30-min. steady-state cycling test at 75% of VO2 max. Before the steady-state exercise and 5- and 30-min. postexercise, a pressure pain stimulation test was applied on the finger of each participant. Perceived pain was measured with Borg's CR10 scale at the end of each pain stimulation. The results indicated no significant changes in perceived pain between the pre-exercise and 5- and 30-min. postexercise values (effect sizes=.07 and .19, respectively). Moreover, no significant difference in perceived pain was found between the groups for pre-exercise (ES=.02) and 5- and 30-min. postexercise (ES=.16 and .21, respectively) values. These results do not confirm the analgesic effects usually observed after steady state exercise. Therefore, it is not possible to compare the analgesic effect of this exercise mode between participants characterized by different fitness levels.

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

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

OBJECTIVE

To determine the effects of low-intensity exercise on chronic muscle pain and potential activation of the endogenous opioid system.

DESIGN

Randomized placebo-controlled trial.

SETTING

Animal laboratory.

ANIMALS

Sixty-three male Sprague-Dawley rats.

INTERVENTIONS

Rats performed a low-intensity exercise protocol for 5 consecutive days after the induction of chronic muscle pain. In a separate experiment, naloxone or saline was administered systemically before 5 low-intensity exercise sessions.

MAIN OUTCOME MEASURE

Mechanical hyperalgesia was measured using von Frey filaments to determine the mechanical withdrawal threshold.

RESULTS

Low-intensity exercise increased mechanical withdrawal threshold in the chronic muscle pain model. Naloxone attenuated the antihyperalgesic effects of low-intensity exercise.

CONCLUSIONS

Low-intensity exercise reversed mechanical hyperalgesia in the chronic muscle pain model through activation of the endogenous opioid system.

Thermal Pain Perception After Aerobic Exercise

OBJECTIVE

To examine thermal pain perception before, 5 minutes after, and 30 minutes after 30 minutes of treadmill exercise at 75% of maximal oxygen uptake (V o 2 max).

DESIGN

Repeated-measures.

SETTING

Sports science laboratory.

PARTICIPANTS

Convenience sample of 14 healthy male and female volunteers (mean age +/- standard deviation, 32+/-3y).

INTERVENTIONS

Sensory thresholds, pain thresholds, and pain ratings to hot and cold stimuli were measured before and after 30 minutes of treadmill exercise at 75% of V o 2 max. The hot and cold stimuli were delivered by using a thermode placed on the thenar eminence of the nondominant hand. Thermal sensory and pain thresholds were determined during continuous ramps in temperature of the thermode.

MAIN OUTCOME MEASURES

Pain ratings were measured on a visual analog scale at 10-second intervals over 2 minutes of thermal pain stimulation.

RESULTS

There were no significant changes in thermal sensitivity, pain thresholds, or pain ratings for either heat or cold after 30 minutes of exercise at 75% of V o 2 max.

CONCLUSIONS

Pain perception to thermal stimuli was unaltered after 30 minutes of exercise at 75% of V o 2 max, an intensity and duration of exercise previously shown to alter pain perception to electric and mechanical stimuli.

Intensity and duration threshold for aerobic exercise-induced analgesia to pressure pain

OBJECTIVE

To examine how exercise-induced analgesia is affected by the duration and intensity of aerobic exercise.

DESIGN

Repeated-measures design.

SETTING

Exercise science laboratory.

PARTICIPANTS

Convenience sample of 12 healthy male and female volunteers (mean age +/- standard deviation, 32+/-9 y).

INTERVENTIONS

Pain ratings were assessed before and at 5 and 30 minutes after treadmill exercise of 10 minutes at 75% maximal oxygen uptake (Vo(2)max), 30 minutes at 50% Vo(2)max, and 30 minutes at 75% Vo(2)max (randomized order and no less than 48 h between each bout).

MAIN OUTCOME MEASURES

Pain ratings were measured on a visual analog scale at 10-second intervals during a 2-minute pressure-pain stimulus to the nondominant index finger.

RESULTS

Pain ratings were significantly decreased (P<.05) from pre-exercise values 5 minutes after 30 minutes of exercise at 75% Vo(2)max but returned toward baseline by 30 minutes after exercise. There were no significant changes in pain ratings after 10 minutes of exercise or after exercise at 50% Vo(2)max.

CONCLUSIONS

There are thresholds for both the intensity (>50% Vo(2)max) and duration (>10 min) of exercise required to elicit exercise analgesia.

The unique and transient impact of acute exercise on pain perception in older, overweight, or obese adults with knee osteoarthritis

This study examined the unique contribution of acute exercise to perceptions of pain in 32 older, overweight, or obese adults with knee osteoarthritis (OA), statistically controlling for the effect of diurnal variation, supplemental medication intake, and stress. Using an ecological momentary assessment method, 964 pain appraisals were recorded and coded into experience samplings that occurred either on a nonexercise day or before or following scheduled activity on an exercise day. Univariate and multivariate multilevel modeling analyses controlling for supplemental medication intake and stress revealed a quadratic trend in diurnal pain variations with the peak occurring mid-afternoon. Although pain was significantly elevated following exercise in comparison with the predicted diurnal pattern, pain reports later in the day following exercise were significantly lower than immediately following exercise. We conclude that the pain associated with acute exercise by older, overweight, or obese adults who have knee OA is transient. Findings are discussed in terms of the implications of exercise therapy for patients with knee OA.

Exercise-induced hypoalgesia and intensity of exercise

Pain sensitivity has been found to be altered following exercise. A number of investigators have found diminished sensitivity to pain (hypoalgesia) during and following exercise. However, currently it is unknown whether there is a specific intensity of exercise that is required to produce this hypoalgesia response. Aerobic exercise, such as cycling and running, have been studied most often, and a number of different exercise protocols have been used in this research including: (i) increasing exercise intensity by progressively increasing the workloads; (ii) prescribing a particular exercise intensity based on a percentage of maximum; and (iii) having participants self-select the exercise intensity. Results indicate that hypoalgesia occurred consistently following high-intensity exercise. In the studies in which exercise intensity was increased by increasing workloads, hypoalgesia was found most consistently with a workload of 200 W and above. Hypoalgesia was also found following exercise prescribed at a percentage of maximal oxygen uptake (e.g. 60 to 75%). Results are less consistent for studies that prescribed exercise based on percentage of heart rate maximum, as well as for studies that let participants self-select the exercise intensity. However, there has not been a systematic manipulation of exercise intensity in most of the studies conducted in this area. In addition, the interaction between exercise intensity and exercise duration, more than likely influences whether hypoalgesia occurs following exercise. There is a need for research to be conducted in which both intensity and duration of exercise are manipulated in a systematic manner to determine the 'optimal dose' of exercise that is required to produce hypoalgesia. In addition, there is a need for more research with other modes of exercise (e.g. resistance exercise, isometric exercise) to determine the optimal dose of exercise required to produce hypoalgesia.

Pain threshold changes following acupuncture, measured with cutaneous argon laser and electrical tooth pulp stimulation, a comparative study

1. The present study compared the argon laser technique for pain threshold measurements with the previously often used threshold measurements with electrical tooth pulp (TP) stimulation. 2. Conditioning stimuli with acupuncture was given since it previously has been shown to induce pain threshold changes. Acupuncture needles were inserted in the hands and arms and electrically stimulated. 3. The experiments were performed in a single blind fashion comparing pain threshold effects after either acupuncture or placebo. Pain threshold measurements with a method of limits were used with both electrical TP and the argon laser method. 4. In comparison to placebo the pain thresholds increased after acupuncture, both on the face and on the hand, with the argon laser method, while the TP thresholds were not influenced. No significant pain threshold change was detected with either the argon laser or the TP method following placebo treatment. 5. The results indicate that the argon laser used for sensory testing with a method of limits is a useful method for pain threshold measurements. The difference in pain threshold effects induced by acupuncture measured with the argon laser and the TP stimulation, are discussed and related to mechanisms that may differ between the two types of pain threshold measurements.

Analgesia following exercise: a review

Over the past 20 years a number of studies have examined whether analgesia occurs following exercise. Exercise involving running and cycling have been examined most often in human research, with swimming examined most often in animal research. Pain thresholds and pain tolerances have been found to increase following exercise. In addition, the intensity of a given pain stimulus has been rated lower following exercise. There have been a number of different noxious stimuli used in the laboratory to produce pain, and it appears that analgesia following exercise is found more consistently for studies that used electrical or pressure stimuli to produce pain, and less consistently in studies that used temperature to produce pain. There is also limited research indicating that analgesia can occur following resistance exercise and isometric exercise. Currently, the mechanism(s) responsible for exercise-induced analgesia are poorly understood. Although involvement of the endogenous opioid system has received mixed support in human research, results from animal research seem to indicate that there are multiple analgesia systems, including opioid and non-opioid systems. It appears from animal research that properties of the exercise stressor are important in determining which analgesic system is activated during exercise.

Somatic nerve stimulation and cholera-induced net fluid secretion in the small intestine of the rat: evidence for an opioid effect

The effects of somatic nerve stimulation on cholera toxin induced secretion was investigated in vivo in anaesthetised rats. Small intestinal secretion was induced with cholera toxin and measured by a gravimetric technique. Afferent stimulation (pulse frequency within train; 100 Hz; train duration: 50 ms; train frequency: 3 Hz) of the sciatic nerve over 30 min significantly reduced the net fluid secretion both during (P < 0.05) and after cessation of the stimulation (P < 0.01). The greatest effect was obtained immediately after the termination of the nerve stimulation when the secretion was reversed to net fluid absorption. The opioid receptor antagonist naloxone (10 mg kg(-1) i.v.) administrated during the stimulation, significantly inhibited the antisecretory effect seen after the stimulation, thus no significant difference was seen between the control period and the periods after cessation of the stimulation. The opioid receptor antagonist naloxone methiodide (10 mg kg(-1) i.v.), which does not cross the blood-brain barrier, partly inhibited the antisecretory effects but not with the same magnitude as naloxone, thus the net fluid secretion was still significantly inhibited after the stimulation (P < 0.05). We conclude that afferent stimulation of the sciatic nerve strongly inhibits the cholera toxin induced secretion in the small intestine. This inhibition involves primarily a central opioid mechanism and to a lesser extent peripheral opioid mechanism.

Different effects of physical exercise on cold pain sensitivity in fighter pilots with and without the history of acute in-flight neck pain attacks

PURPOSE

The cold pain sensitivity in fighter pilots was studied by using a cold pressor test.

METHODS

The pilots were divided into two groups: one group consisting of eight pilots (N = 8) who had experienced several acute in-flight neck pain attacks, and the control group (N = 8) who had not experienced these pain conditions under similar work and environment conditions. In each pilot cold pain thresholds and pain and unpleasantness responses to suprathreshold cold stimulations were recorded during repeated tests. The ratings of pain and unpleasantness responses to cold stimulations were evaluated by visual analog scales (VAS). The effect of exercise on cold pain sensitivity was tested in a separate experiment. Exercise was performed on a cycle ergometer at different workload levels (50-200 W).

RESULTS

In the control conditions (resting measures) of this study during repeated cold pressor tests, the average pain thresholds and pain or unpleasantness responses to suprathreshold cold stimulation were not different between groups. Physical exercise increased pain thresholds (P < 0.001) in pilots with a history of neck pain attacks but not in control group. Exercise induced a significant decrease in pain responses and unpleasantness responses to suprathreshold stimulation in both groups. This exercise effect was more marked both in pain intensity (P < 0.05) and unpleasantness responses (P < 0.01) in pilots with a history of neck pain attacks. Moreover, exercise more markedly (P < 0.05) decreased unpleasantness than pain intensity responses in both groups of pilots.

CONCLUSIONS

The results suggest that exercise stress-related analgesia mechanisms may be enhanced in pilots with a history of acute in-flight neck pain attacks. Moreover, sensory and nonsensory aspects of pain experience may be differentially influenced by exercise stress.

Perception of pain after resistance exercise

OBJECTIVES

The main objective was to assess the influence of resistance exercise on pain threshold and pain ratings. Secondary objectives included measuring state anxiety, body awareness, systolic blood pressure, diastolic blood pressure, and heart rate responses.

METHODS

Pressure (3 kg force) was applied to the middle digit of the left hand for two minutes using a Forgione-Barber pain stimulator before and after (five minute and 15 minutes) resistance exercise and quiet rest. Resistance exercise consisted of 45 minutes of lifting three sets of 10 repetitions at 75% of an individual's one repetition maximum. Quiet rest consisted of sitting quietly in a room free from distractions.

RESULTS

Data were analysed by repeated measures analysis of variance for multifactor experiments. Pain threshold was significantly higher (p<0.05) and pain ratings were significantly lower (p<0.05) five minutes after resistance exercise. Changes in pain perception were accompanied by changes in heart rate, systolic blood pressure, and body awareness after exercise.

CONCLUSION

A single bout of resistance exercise is capable of modifying the sensation of experimentally induced pain.

Effect of Interferential Current on Perceived Pain and Serum Cortisol Associated with Delayed Onset Muscle Soreness

The purpose of this study was to assess the effect of interferential current (IFC) on perceived pain and serum Cortisol levels in subjects with delayed onset muscle soreness (DOMS). DOMS was induced in 10 subjects through repeated eccentric contractions of the elbow flexors. Forty-eight hours later subjects were evaluated. Starting at t = 0:00, blood samples were withdrawn from a superficial vein every 5 min for 65 min. At t = 0:05, subjects received IFC of 10 bps or IFC of 100 bps. Perceived pain levels were evaluated prior to catheter insertion and at t = 0:35, 0:50, and 0:65. Two mixed-model analyses of variance revealed a significant decrease in perceived pain scores across time for both treatment groups but no significant difference in serum Cortisol for the two groups. It was concluded that IFC of high and low beat frequency is effective in controlling the pain of DOMS but does not elicit a generalized stress response as indexed by increasing serum Cortisol levels.

Perception of pain following aerobic exercise

The literature regarding the effect of exercise on pain is equivocal. The purpose of this investigation was to quantify the influence of an acute bout of exercise on pain threshold (PT) and pain ratings (PR). A secondary purpose was to measure heart rate (HR), systolic blood pressure (SBP), diastolic blood pressure (DBP), and state anxiety (SA) responses. Pressure (3000-g force) was applied to the middle digit of the right forefinger for 2 min with the Forgione-Barber pain stimulator before and following (5 min of cycling at 75% VO2max. Quiet rest consisted of sitting quietly for 30 min in a sound-dampened chamber. Order was counterbalanced, randomly assigned, and performed on different days. Data was analyzed with a repeated measures ANOVA for multifactor experiments. PT was significantly higher (P < 0.05) and PR were significantly lower (P < 0.05) in the exercise condition 5 min post-exercise in comparison with quiet rest. Changes in pain perception were accompanied by lower SBP (P < 0.03) and higher HR (P < 0.0001). in the exercise condition. It is concluded that an acute bout of exercise is associated with changes in pain perception.

Mechanical and physical responses to stretching with and without preisometric contraction in human skeletal muscle

OBJECTIVE

To examine electromyography (EMG) activity, passive torque, and stretch perception during static stretch and contract-relax stretch.

DESIGN

Two separate randomized crossover protocols: (1) a constant angle protocol on the right side, and (2) a variable angle protocol on the left side.

SUBJECTS

10 male volunteers.

INTERVENTION

Stretch-induced mechanical response in the hamstring muscles during passive knee extension was measured as knee flexion torque (Nm) while hamstring surface EMG was measured. Final position was determined by extending the knee to an angle that provoked a sensation similar to a stretch maneuver. Constant angle stretch: The knee was extended to 10 degree below final position, held 10sec, then extended to the final position and held for 80 sec. Variable angle stretch: The knee was extended from the starting position to 10 degrees below the final position, held 10sec, then extended to the onset of pain. Subjects produced a 6-sec isometric contraction with the hamstring muscles 10 degrees below the final position in the contract-relax stretch, but not in the static stretch.

MAIN OUTCOME MEASURES

Passive torque, joint range of motion, velocity, and hamstring EMG were continuously recorded.

RESULTS

Constant angle contract-relax and static stretch did not differ in passive torque or EMG response. In the final position, passive torque declined 18% to 21% in both contract-relax and static stretch (p<.001), while EMG activity was unchanged. In the variable angle protocol, maximal joint angle and corresponding passive torque were significantly greater in contract-relax compared with static stretch(p<.01), while EMG did not differ.

CONCLUSION

At a constant angle the viscoelastic and EMG response was unaffected by the isometric contraction. The variable angle protocol demonstrated that PNF stretching altered stretch perception.

Are runners stoical? An examination of pain sensitivity in habitual runners and normally active controls

Anecdotal and clinical reports suggest that athletes are stoical. However, there are few studies comparing persons who exercise regularly with those who do not. This study compared two independent samples of regular runners and normally active controls, both without recent exercise, on cold pressor, cutaneous heat, and tourniquet ischemic pain tests. Results demonstrated that the runners' threshold for noxious cold was significantly higher than that of controls. The heart rate and blood pressure responses to cold were similar in the 2 groups, suggesting that differences in cold pain report did not result from differences in autonomic reactivity to cold. Signal detection theory measures demonstrated that runners discriminated among noxious thermal stimuli significantly better than controls, but neither noxious nor innocuous thermal report criteria differed between groups. The cohorts also did not differ in their report of ischemic pain sensations. Thus, these data do not generally support the hypothesis of pain insensitivity or stoicism in habitual runners. Rather, insensitivity occurs only in their response to noxious cold, which is suggested to be an adaptation to regular training.

A test of exercise analgesia using signal detection theory and a within-subjects design

Evidence for the existence of exercise-induced analgesia has been weak, inconsistent, or anecdotal. The present study addresses criticisms of previous research by using a signal-detection analysis of pain perception in a within-subjects, repeated-measures research design with 22 endurance athletes who exercised in a naturalistic setting. Analysis showed discriminability measures from the lower intensities of a radiant-heat pain-perception task were significantly lower after exercise than the subjects' discriminability after a no-exercise period. This is consistent with discriminability decreases seen in studies of subjects who are under the influence of analgesic chemicals such as morphine and nitrous oxide. Discriminability differences were not seen in this study at the higher intensities of the stimulus, suggesting that exercise generates perceptual changes that may not reach pain-mediating levels. Visual analogue data suggest cognitive and motivational factors also play a role in report of pain after exercise.

Cerebrospinal fluid dynorphin-converting enzyme activity is increased by voluntary exercise in the spontaneously hypertensive rat

The activity in cerebrospinal fluid (CSF) of dynorphin-converting enzyme (DCE) has been studied after voluntary exercise in the spontaneously hypertensive (SH) rat. The exercise consisted of spontaneous running in wheels for 4-5 weeks and the average running distance during the last two weeks was 4.0 km/24h. CSF samples were obtained under anaesthesia from the cisterna magna after penetration of the atlanto-occipital membrane. DCE transforms the members of the dynorphin family of opioid peptides into Leu-enkephalin-Arg6. In the present investigation a radioimmunoassay was used for quantitation of Leu-enkephalin-Arg6 release from dynorphin A1-17 and dynorphin B1-13. The rats that were running had a DCE activity (vs. both substrates) in CSF that was approximately 6-12 times higher than in animals not given the running opportunity. A statistically significant correlation between the two prodynorphin-derived substrate peptides was found in terms of DCE activity. We therefore propose that a single enzyme activity may be responsible for the hydrolysis of dynorphin B1-13 and dynorphin A1-17. Furthermore, a significant correlation was also found between running activity and DCE activity 12-14 h before the CSF was withdrawn. Besides measurement of DCE activity by radioimmunoassay, the formation of Leu-enkephalin-Arg6 was identified by reversed-phase micro-column liquid chromatography and plasma desorption mass spectrometry. The experiment shows that voluntary exercise affects opioid peptidergic mechanisms.

Relations between experimentally induced tooth pain threshold changes, psychometrics and clinical pain relief following TENS. A retrospective study in patients with long-lasting pain

The present study investigates the relationships between clinical pain relief, physiological and psychological parameters. Out of 50 patients with long-lasting musculoskeletal neck- and shoulder-pain treated with transcutaneous electrical nerve stimulation (TENS), 21 were selected and classified as responders (n = 13) or non-responders (n = 8). Tooth pain thresholds (PT) were measured before and after an experimental TENS treatment and the relative change in PT following the stimulation was calculated. Three psychometric self-inventories were administered: Zung Depression Scale, Spielberger's Trait Anxiety Scale and the Multidimensional Health Locus of Control Scale. Responders (R) and non-responders (NR) differed significantly from each other in the PT measurements as well as on the psychometric scales. NR exhibited higher levels of anxiety and depression, a more pronounced powerful other orientation and no change or a decrease in PT following TENS compared to R. These findings indicate relationships and interactions between physiological and psychological factors in patients with long-lasting pain.

Modulation of skin sensitivity by dynamic and isometric exercise in man

The effect of dynamic cycle ergometer exercise and isometric leg exercise on skin sensitivity was studied in man. Exercise was performed at different loads. Cutaneous sensitivity to innocuous and noxious thermal stimuli was tested using a contact thermostimulator and sensitivity to tactile stimuli was tested using electrical stimuli. During isometric exercise a segmental (the exercising limb), but not a multisegmental, phasic decrease of cutaneous thermal sensitivity to innocuous stimuli was found. At the isometric forces used the effect on tactile and heat pain sensitivity was not significant. During dynamic exercise a multisegmental, load-dependent decrease of sensitivity in all tested sensory modalities was found and this attenuation disappeared gradually after the end of exercise. In contrast to isometric exercise, the decrease of sensitivity produced by dynamic exercise was most evident in tactile sensitivity. The size of the stimulus area (7.9 vs 11.8 cm2) did not have a significant effect on the magnitude of the exercise-induced decrease of cutaneous thermal sensitivity to innocuous stimuli. It was concluded that underlying the modulation of skin sensitivity by dynamic and isometric exercise were mechanisms that were different, at least to a small extent. Isometric exercise produced a segmental modulation of skin sensitivity due to central neuronal mechanisms, independent of exercise-induced stress. Exercise-induced stress could have caused the modulation of skin sensitivity by dynamic exercise.

Corticotropin-releasing hormone and opioid peptides in reproduction and stress

Increased knowledge on the mechanisms whereby corticotropin releasing hormone (CRH) and opioid peptides mediate the effects of stress has helped us to understand the relationship between stress and disturbed reproductive function. Increases of CRH and beta-endorphin in the hypothalamus in stressful situations inhibits the secretion of gonadotropins, oxytocin and vasopressin. This may lead to amenorrhea, which often is a consequence of intensive training or psychological stress, or it may disrupt parturition and lactation. There is a relationship between ovarian function and opioid peptides in the hypothalamus. Opioid peptides increase during puberty and fall at the menopause. Oestradiol and progesterone increase beta-endorphin concentrations in the luteal phase of the menstrual cycle, and this is followed by a rapid fall at menstruation. These changes may mediate symptoms typical of the premenstrual syndrome. Rather intensive exercise is required to increase plasma concentrations of beta-endorphin and corticotropin. During labour the amounts of beta-endorphin and corticotropin reach the values found in athletes during maximal exercise. The placenta produces increasing amounts of CRH towards the end of pregnancy which may help the mother and fetus to withstand the increased demands of labour. The placenta may thus be involved in the adaptation of the stress mechanism during pregnancy. CRH has also a paracrine function in different biological processes of the placenta and fetal membranes. It is possible to counteract the deleterious effects of stress on reproductive function by the administration of opiate antagonists. Induction of ovulation with naltrexone has been shown in patients with hypothalamic amenorrhea but the effect on fertility is not known.

The effects of mu, delta- and kappa-opioid receptor antagonists on the pain threshold increase following muscle stimulation in the rat

In a previous study, prolonged low-frequency muscle stimulation, inducing dynamic contractions in the hind leg of unanaesthetized rats, was shown to give rise to a hypoalgesia. The increase in pain threshold, measured as squeak threshold to noxious electric pulses, lasted 3 h. In the present study, the involvement of the endogenous opioid system in the post-stimulatory analgesia was investigated using selective opioid receptor antagonists. The post-stimulatory analgesia was completely reversed back to prestimulatory control levels by naloxone, 1 mg kg-1. ICI 154,129 and MR 2266 BS, selective delta- and kappa-receptor antagonists respectively, did not significantly influence the post-stimulatory analgesia, although ICI 154,129 had a minor pain threshold-lowering effect. Rats pretreated with beta-funaltrexamine, a mu-receptor antagonist, did not exhibit any post-stimulatory analgesia. These results suggest that opioid systems are involved in the increase in pain threshold after muscle stimulation and that the analgesic response is both elicited and maintained by the mu-receptor.

Role of opioid receptors in the long-lasting blood pressure depression after electric muscle stimulation in the hind leg of the rat

In a previous study, electrically induced contractions of the gastrocnemius muscle in conscious spontaneously hypertensive rats were shown to induce a blood pressure reduction of 15-20 mmHg lasting several hours. We showed in that study that endogenous opioid systems were involved. In this study, drugs with selective affinity for different opioid receptors were used to analyse further the involvement of endogenous opioid systems in the post-stimulatory drop in blood pressure in spontaneously hypertensive rats. Prestimulatory intracerebroventricular administration of beta-FNA (a mu-receptor antagonist) did not significantly influence the response at all, nor did a lower intravenous dose of naloxone reverse the post-stimulatory drop in blood pressure. High-dose naloxone (15 mg kg-1) increased post-stimulatory blood pressure by around 10 mmHg. About 50% of the drop thus remained after this treatment. A similar, partial reversal of the decreased blood pressure was seen after intravenous administration of a delta-receptor antagonist, ICI 154,129. However, the depressor response was completely reversed by a low intravenous dose of MR 2266 BS (a kappa-receptor antagonist). These results suggest that the reduction in blood pressure after muscle stimulation is mainly mediated by the opioid kappa-receptor. A certain involvement of the delta-receptor is also indicated.

Dexamethasone attenuates exercise-induced dental analgesia in man

The effect of dexamethasone on exercise-induced adrenocorticotropin (ACTH) secretion and dental analgesia was studied in healthy human subjects. Different levels of exercise (100-200 W) were produced by a cycle ergometer. Dental pain thresholds were tested with a constant current stimulator. Dental pain thresholds were elevated with increasing work loads, and the elevation was still significant 30 min after the end of the exercise. Dexamethasone produced a significant reversal of exercise-induced pain threshold elevations concomitantly with the suppression of exercise-induced ACTH release. The results suggest that the corticotropin releasing factor-ACTH axis is involved in the exercise-induced analgesia.

Cerebrospinal fluid immunoreactive beta-endorphin concentration is increased by voluntary exercise in the spontaneously hypertensive rat

The effect of voluntary exercise on cerebrospinal fluid (CSF) levels of immunoreactive beta-endorphin has been studied in the spontaneously hypertensive rat (SHR). The exercise consisted of 5-6 weeks of spontaneous running in wheels and the average running distance was 3.5 +/- 0.4 km/24 h. CSF samples were obtained under anaesthesia from the cisterna magna. Five experimental groups were examined, four groups of runners and one group of sedentary controls. The runners were sampled either (a) shortly (0-3 h) after termination of exercise, or after the wheel had been locked for (b) 24, (c) 48 or (d) 96 h. The runners in group a had significantly higher immunoreactive beta-endorphin levels than the controls. The levels remained increased as compared with controls after 24 and 48 h of enforced abstinence but had returned to control after 96 h. The data indicate that voluntary exercise induces adaptive changes in central beta-endorphin systems.

Electrical stimulation of the gastrocnemius muscle in the spontaneously hypertensive rat increases the pain threshold: role of different serotonergic receptors

In a previous study, prolonged low-frequency muscle stimulation in the hind leg of the fully conscious spontaneously hypertensive rat (SHR) was shown to induce a long-lasting reduction of blood pressure. It was also shown that opioid and serotonergic (5-HT) systems were involved. More recently, we have shown that the 5-HT1 receptors are involved in the post-stimulatory decrease in blood pressure. In the present study, the influence of this type of muscle stimulation on the pain threshold was investigated. Pain perception was measured as the squeak threshold to noxious electric pulses. After cessation of the stimulation, an analgesic response was elicited within 60 min and peak analgesia developed after 120 min, being 139 +/- 10% (P less than 0.01) of the prestimulatory control value. The increased pain threshold lasted for another 2 h. One group of SHR was pretreated with PCPA, a serotonin synthesis blocker, which completely abolished the post-stimulatory analgesia. To analyse further the involvement of different serotonin systems, drugs with selective affinity for 5-HT receptors were used. In one group a prestimulatory dose of metitepine maleate (a 5-HT1&2 receptor antagonist) abolished the post-stimulatory elevation of the pain threshold. The prolonged analgesic response was still present after prestimulatory treatment with ritanserin or ICS 205-930 (5-HT2 and 5-HT3 blocking agents respectively). In another group of experiments, the serotonin receptor antagonists were administered post-stimulation to animals with fully elicited analgesia. None of the antagonists used could reverse the elevation of pain threshold towards prestimulatory levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscular fatigue during repeated isokinetic shoulder forward flexions in young females

Peak torque, work, mean power and electromyographic (EMG) activity were recorded for each of 150 repeated isokinetic maximal shoulder flexions (45 degrees-90 degrees) in 23 healthy females. From the EMG signals of trapezius, deltoid, infraspinatus and biceps brachii the mean power frequency and the signal amplitude were determined in real time. The mechanical output showed a steep decrease during the first 40 contractions, followed by a plateau maintained until the end. In all muscles, except the biceps brachii, significant decreases in mean power frequency occurred during the first 40 contractions, showing a tendency to stabilize around the same absolute frequency value. Signal amplitude increased in the trapezius, the deltoid and the infraspinatus, but was constant in the biceps brachii. For some individuals rather high EMG activity was recorded in the muscles during the time the arm was supposed to be passively extended to the starting position, and this was found to be associated with lower strength and endurance levels. Longitudinal analyses showed that the mean power frequencies correlated better than the signal amplitudes with the three mechanical variables. The results suggest that the initial steep decrease in mechanical performance and mean power frequency is caused by fatiguing of type 2 motor units.

Analgesia by low-frequency nerve stimulation mediated by low-threshold afferents in rats

Several authors claim that analgesia by low-frequency peripheral nerve stimulation (transcutaneously or through inserted needles) depends on the activation of small-diameter A delta afferents. In the present study, a marked increase in pain threshold assessed by the squeak threshold test (vocalization to electric shocks delivered to the tail) was obtained by 2 Hz sciatic or radial nerve low-intensity stimulation, using single pulses. Conduction velocities of the activated nerve fibers ranged between 33.6 and 46.4 m/sec, which is consistent with activation of large afferent A fibers. This analgesic effect was reversed by naloxone (3 mg/kg i.v.). Thus, low-threshold afferents may contribute to the analgesic effect in addition to the previously reported high-threshold induced effect. High-intensity stimulation (5-6 times the threshold for visible muscular twitches), activating small-diameter afferents (A delta fibers) resulted in a decrease in pain threshold. Hyperalgesia also resulted from low-intensity stimulation using trains of pulses. The analgesic or hyperalgesic effects were correlated with the animal's behavior during stimulation. Analgesia was obtained in calm rats, who went to sleep regularly. When the rats showed signs of being distressed, hyperalgesia resulted. These findings may explain the occasional failure to obtain reduction in pain in anxious human patients by peripheral stimulation.