Muscle pain perception and sympathetic nerve activity to exercise during opioid modulation

Low-Dose Morphine Reduces Pain Perception and Blood Pressure, but Not Muscle Sympathetic Outflow, Responses During the Cold Pressor Test

Our knowledge about how low-dose (analgesic) morphine affects autonomic cardiovascular regulation is primarily limited to animal experiments. Notably, it is unknown if low-dose morphine affects human autonomic cardiovascular responses during painful stimuli in conscious humans. Therefore, we tested the hypothesis that low-dose morphine reduces perceived pain and subsequent sympathetic and cardiovascular responses in humans during an experimental noxious stimulus. Twenty-nine participants (14F/15M; 29±6 y; 26±4 kg•m^-2, mean ± SD) completed this randomized, crossover, placebo-controlled trial during two laboratory visits. During each visit, participants completed a cold pressor test (CPT; hand in ~0.4 °C ice bath for two minutes) before and ~35 minutes after drug/placebo administration (5 mg IV morphine or saline). We compared pain perception (100 mm visual analog scale), muscle sympathetic nerve activity (MSNA; microneurography; 14 paired recordings), and beat-to-beat blood pressure (BP; photoplethysmography) between trials (at both pre- and post-drug/placebo time points) using paired, two-tailed t-tests. Before drug/placebo infusion, perceived pain (p=0.92), Δ MSNA burst frequency (n=14, p=0.21), and Δ mean BP (p=0.39) during the CPT were not different between trials. After the drug/placebo infusion, morphine versus placebo attenuated perceived pain (morphine: 43±20 vs. placebo: 57±24 mm,p<0.001) and Δ mean BP (morphine: 10±7 vs. placebo: 13±8 mmHg,p=0.003), but not Δ MSNA burst frequency (morphine: 10±11 vs. placebo: 13±11 bursts/minute,p=0.12), during the CPT. Reductions in pain perception and Δ mean BP were only weakly related (r=0.34,p=0.07; post-morphine CPT minus post-placebo CPT). These data provide valuable information regarding how low-dose morphine affects autonomic cardiovascular responses during an experimental painful stimulus.

Immersive virtual reality to relieve exercise-induced pain caused by aerobic cycling

Chronic pain affects 20% of the global population and is incredibly complex to treat. The burden of chronic pain is physical, emotional and financial, and prevalence rates continue to rise. Current treatments are ineffective long-term against pain and common comorbidities, including anxiety and depression, mood and sleep disorders, and social isolation. While a large body of evidence supports regular physical exercise as an effective long-term treatment for chronic pain and its comorbidities, exercise-induced pain and kinesiophobia are significant barriers to participation and adherence. Immersive virtual reality is a powerful short-term pain reliever, that, when combined with exercise, can help overcome these barriers. This perspective argues for the use of combined exercise and virtual reality treatment techniques to mitigate chronic pain.

The Influence of Caffeine Supplementation on Resistance Exercise: A Review

This paper aims to critically evaluate and thoroughly discuss the evidence on the topic of caffeine supplementation when performing resistance exercise, as well as provide practical guidelines for the ingestion of caffeine prior to resistance exercise. Based on the current evidence, it seems that caffeine increases both maximal strength and muscular endurance. Furthermore, power appears to be enhanced with caffeine supplementation, although this effect might, to a certain extent, be caffeine dose- and external load-dependent. A reduction in rating of perceived exertion (RPE) might contribute to the performance-enhancing effects of caffeine supplementation as some studies have observed decreases in RPE coupled with increases in performance following caffeine ingestion. However, the same does not seem to be the case for pain perception as there is evidence showing acute increases in resistance exercise performance without any significant effects of caffeine ingestion on pain perception. Some studies have reported that caffeine ingestion did not affect exercise-induced muscle damage, but that it might reduce perceived resistance exercise-induced delayed-onset muscle soreness; however, this needs to be explored further. There is some evidence that caffeine ingestion, compared with a placebo, may lead to greater increases in the production of testosterone and cortisol following resistance exercise. However, given that the acute changes in hormone levels seem to be weakly correlated with hallmark adaptations to resistance exercise, such as hypertrophy and increased muscular strength, these findings are likely of questionable practical significance. Although not without contrasting findings, the available evidence suggests that caffeine ingestion can lead to acute increases in blood pressure (primarily systolic), and thus caution is needed regarding caffeine supplementation among individuals with high blood pressure. In the vast majority of studies, caffeine was administered in capsule or powder forms, and therefore the effects of alternative forms of caffeine, such as chewing gums or mouth rinses, on resistance exercise performance remain unclear. The emerging evidence suggests that coffee might be at least equally ergogenic as caffeine alone when the caffeine dose is matched. Doses in the range of 3-9 mg·kg^-1 seem to be adequate for eliciting an ergogenic effect when administered 60 min pre-exercise. In general, caffeine seems to be safe when taken in the recommended doses. However, at doses as high as 9 mg·kg^-1 or higher, side effects such as insomnia might be more pronounced. It remains unclear whether habituation reduces the ergogenic benefits of caffeine on resistance exercise as no evidence exists for this type of exercise. Caution is needed when extrapolating these conclusions to females as the vast majority of studies involved only male participants.

Influence of Acetaminophen Consumption on Perceived Exertion at the Lactate Concentration Threshold

The purpose of this investigation was to study effects of acetaminophen consumption on ratings of perceived exertion and estimated time limit responses at the lactate threshold. 98 young regional to national level athletes performed a graded exhausting exercise on an outdoor running track to estimate their maximal aerobic velocity and the velocity associated with their lactate concentration threshold. Urine (30 mL) was collected during this test and analysed for numerous substances. During urinary screening for doping substances, 9 acetaminophen consumers (9.2%) among the 98 included athletes were detected. These acetaminophen consumers have significantly lower perceived exertion at velocity corresponding to the lactate concentration threshold than nonconsumers (11.9 ± 2.1 vs 13.6 ± 2.1, respectively) although they were at the same relative exercise intensity. This result shows that acetaminophen consumption may have mediated the perceived exertion response at the lactate concentration threshold. This may then suggest that the pain induced by training load could be a factor in use of self-prescribed pain relievers. Such consumption must be taken into account by medical staff, trainers, or educators who have to give information on the use and adverse effects of this substance and to propose palliative methods to their athletes.

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.

Muscle pain and blood pressure responses during isometric handgrip exercise in healthy African American and non-Hispanic White adults

It has been shown that African Americans (AAs) are more sensitive to experimental pain stimuli compared to non-Hispanic Whites (NHWs). A single bout of exercise results in naturally-occurring muscle pain and elevation in blood pressure (BP); however, it is currently unclear whether AAs and NHWs differ in muscle pain and BP responses during exercise. Therefore, we examined the differences in muscle pain and blood pressure (BP) during isometric handgrip exercise in African Americans (AAs) and non-Hispanic Whites (NHWs). Fourteen AAs and 14 NHWs completed isometric exercise consisting of squeezing a hand dynamometer at 25% of maximal strength for 3 min. During exercise, muscle pain ratings (MPRs) were assessed every 30s, whereas systolic and diastolic BP (SBP and DBP) were recorded every minute. During exercise, AAs generally reported greater MPR than NHWs (p<0.001), and MPR increased more rapidly during exercise in AAs than NHWs (p<0.05). In contrast, SBP and DBP continued to increase similarly during exercise in both AAs and NHWs (p>0.05). The results suggest that AAs generally experienced a greater intensity of muscle pain than NHWs during isometric handgrip exercise, but both groups exhibited similar elevations in BP during exercise.

Pressure and activity-related allodynia in delayed-onset muscle pain

OBJECTIVES

Muscle pain from different activities was tested with the muscle pain expected to vary in ways that may clarify mechanisms of activity-induced exacerbation of myofascial pain.

METHODS

Participants [N=20; 45% women; 23 y old (SD=2.09)] consented to participate in a 6 session protocol. Bilateral muscle pain ratings and pressure pain thresholds (PPTs) were collected before and for 4 days after lengthening (ie, eccentric) muscle contractions were completed with the nondominant elbow flexors to induce delayed-onset muscle pain. The muscle pain ratings were collected with the arms in several conditions (eg, resting, moving, and contracting in a static position) and PPTs were collected with the arms.

RESULTS

In the ipsilateral arm, muscle pain ratings at rest and during activity significantly increased whereas PPTs significantly decreased after the eccentrics (ηs=0.17 to 0.54). The greatest increases in pain occurred during arm extension without applied load, in which there was more stretching but less force than isometrics. In the contralateral arm, neither muscle pain nor PPTs changed from baseline.

DISCUSSION

These results resemble earlier electrophysiology studies showing differential sensitization across stimuli and support that increased depth of information about aggravating activities from clinical patients is needed.

Effects of naltrexone on pain sensitivity and mood in fibromyalgia: no evidence for endogenous opioid pathophysiology

The pathophysiological mechanisms underlying fibromyalgia are still unknown, although some evidence points to endogenous opioid dysfunction. We examined how endogenous opioid antagonism affects pain and mood for women with and without fibromyalgia. Ten women with fibromyalgia and ten age- and gender-matched, healthy controls each attended two laboratory sessions. Each participant received naltrexone (50mg) at one session, and placebo at the other session, in a randomized and double-blind fashion. Participants were tested for changes in sensitivity to heat, cold, and mechanical pain. Additionally, we collected measures of mood and opioid withdrawal symptoms during the laboratory sessions and at home the night following each session. At baseline, the fibromyalgia group exhibited more somatic complaints, greater sensory sensitivity, more opioid withdrawal somatic symptoms, and lower mechanical and cold pain-tolerance than did the healthy control group. Neither group experienced changes in pain sensitivity due to naltrexone administration. Naltrexone did not differentially affect self-reported withdrawal symptoms, or mood, in the fibromyalgia and control groups. Consistent with prior research, there was no evidence found for abnormal endogenous opioid activity in women with fibromyalgia.

Central modulation of exercise-induced muscle pain in humans

The purpose of the current study was to determine if exercise-induced muscle pain is modulated by central neural mechanisms (i.e. higher brain systems). Ratings of muscle pain perception (MPP) and perceived exertion (RPE), muscle sympathetic nerve activity (MSNA), arterial pressure, and heart rate were measured during fatiguing isometric handgrip (IHG) at 30% maximum voluntary contraction and postexercise muscle ischaemia (PEMI). The exercise trial was performed twice, before and after administration of naloxone (16 mg intravenous; n = 9) and codeine (60 mg oral; n = 7). All measured variables increased with exercise duration. During the control trial in all subjects (n = 16), MPP significantly increased during PEMI above ratings reported during IHG (6.6 +/- 0.8 to 9.5 +/- 1.0; P < 0.01). However, MSNA did not significantly change compared with IHG (7 +/- 1 to 7 +/- 1 bursts (15 s)(-1)), whereas mean arterial blood pressure was slightly reduced (104 +/- 4 to 100 +/- 3 mmHg; P < 0.05) and heart rate returned to baseline values during PEMI (83 +/- 3 to 67 +/- 2 beats min(-1); P < 0.01). These responses were not significantly altered by the administration of naloxone or codeine. There was no significant relation between arterial blood pressure and MSNA with MPP during either IHG or PEMI. A second study (n = 8) compared MPP during ischaemic IHG to MPP during PEMI. MPP was greater during PEMI as compared with ischaemic IHG. These findings suggest that central command modulates the perception of muscle pain during exercise. Furthermore, endogenous opioids, arterial blood pressure and MSNA do not appear to modulate acute exercise-induced muscle pain.

Self-Efficacy Correlates With Leg Muscle Pain During Maximal and Submaximal Cycling Exercise

Quadriceps muscle pain is a naturally occurring consequence of moderate- to high-intensity cycle ergometry. The present study involved an examination of self-efficacy for tolerating muscle pain during exercise as a correlate of muscle pain during maximal and submaximal cycle ergometry. Young adult females (n=16) who were healthy and regularly physically active completed a measure of self-efficacy for tolerating moderate to strong pain in the legs and then undertook a maximal incremental exercise test on a cycle ergometer. Within 1 week, participants completed a 30-minute bout of submaximal exercise (80% VO2peak) on a cycle ergometer. Ratings of quadriceps muscle pain intensity were recorded every minute during the maximal incremental exercise test and every 5 minutes during the submaximal exercise session. Self-efficacy for tolerating pain was moderately inversely correlated with ratings of peak muscle pain during the maximal incremental exercise test (r=-.45) and pain ratings averaged across the submaximal session (r=-.49). Our results are consistent with social-cognitive theory and indicate that self-efficacy for tolerating pain is inversely associated with ratings of muscle pain during maximal and submaximal exercise in healthy and regularly active young adult females. Such findings support a possible examination of strategies for manipulating self-efficacy for tolerating pain and thereby reducing muscle pain during exercise.

PERSPECTIVE

Our data provided evidence that self-efficacy for tolerating moderate to strong muscle pain during exercise correlates with muscle pain intensity ratings during bouts of maximal and submaximal exercise. Future research could identify methods of manipulating self-efficacy as a means of decreasing muscle pain during exercise and ultimately enhancing physical activity participation.

Does Self-Efficacy Influence Leg Muscle Pain During Cycling Exercise?

This experiment examined the effect of a manipulation of self-efficacy beliefs on perceptions of leg muscle pain during moderate-intensity cycling exercise among women. Low to moderately active college-age women (n = 28) were randomly assigned to 1 of 2 conditions that were designed to either increase or decrease efficacy beliefs for engaging in moderate intensity physical activity. Efficacy was manipulated based on bogus feedback after a maximal incremental exercise test. Within 2-3 days after the efficacy manipulation, participants completed 30 min of cycling on an ergometer at 60% peak oxygen consumption. Perceptions of leg muscle pain, as well as work rate and oxygen consumption, were recorded during exercise. There was an initial relationship between baseline self-efficacy and pain ratings during a maximal incremental exercise test. Although the provision of bogus feedback was effective for manipulating self-efficacy, there was no differential effect on leg muscle pain intensity ratings during the 30 min bout of moderate-intensity cycling. The results imply that our manipulation of self-efficacy for prolonged exercise does not influence leg muscle pain during moderate-intensity exercise among low to moderately active young females.

PERSPECTIVE

We examined the influence of self-efficacy on muscle pain during exercise. Although we did not identify a significant influence of efficacy on pain, our research highlights novel research directions. Future research could potentially help identify self-efficacy as a means of decreasing pain during exercise and ultimately enhancing physical activity participation.

Aging, opioid-receptor agonists and antagonists, and the vestibulosympathetic reflex in humans

Animal studies indicate that opioids inhibit the firing rate of vestibular neurons, which are important in mediating the vestibulosympathetic reflex. Furthermore, this inhibition appears to be greater in more mature rats. In the present study, we tested the hypotheses that opioids inhibit the vestibulosympathetic reflex in humans and that endogenous opioids contribute to the age-related impairment of the vestibulosympathetic reflex. These hypotheses were tested by measuring muscle sympathetic nerve activity (MSNA), arterial blood pressure, and heart rate responses to otolith organ engagement during head-down rotation (HDR) in young (24 ± 2 yr old) and older (63 ± 2 yr) subjects before and after administration of either an opioid-receptor antagonist (16 mg naloxone in 9 young and 8 older subjects) or an opioid-receptor agonist (60 mg codeine in 7 young and 7 older subjects). Naloxone did not augment the reflex increase in MSNA during HDR in young (Δ7 ± 2 vs. Δ4 ± 2 bursts/min and Δ81 ± 23 vs. Δ60 ± 24% change in burst frequency and total MSNA before and after naloxone, respectively) or older subjects (Δ2 ± 2 vs. Δ1 ± 2 burst/min and Δ8 ± 7 vs. Δ8 ± 9% before and after naloxone). Similarly, codeine did not attenuate the increase in MSNA during HDR in young (Δ8 ± 1 vs. Δ7 ± 2 bursts/min and Δ53 ± 4 vs. Δ64 ± 16% before and after codeine) or older subjects (Δ6 ± 4 vs. Δ3 ± 3 bursts/min and Δ38 ± 21 vs. Δ33 ± 20%). Mean arterial blood pressure and heart rate responses to HDR were not altered by either naloxone or codeine. These data do not provide experimental support for the concept that opioids modulate the vestibulosympathetic reflex in humans. Moreover, endogenous opioids do not appear to contribute the age-associated impairment of the vestibulosympathetic reflex.

Muscle pain during exercise in normotensive african american women: effect of parental hypertension history

The purpose of the present investigation was to determine the influence of parental hypertension history on leg muscle pain ratings during cycling exercise in African American women. Eighteen women (age, 19 +/- 2 years) with a positive family history (+PH) of hypertension and 16 (age, 19 +/- 1 years) with a negative family history (-PH) underwent maximal exercise and cold pressor testing. Maximal exercise was conducted on a cycle ergometer. Quadriceps muscle pain intensity ratings were obtained each minute during the maximal exercise test by using a category-ratio scale. The hand cold pressor test was used to determine cardiovascular reactivity. Repeated measures analysis of variance showed significantly lower pain ratings during exercise for the +PH group compared to the -PH group. Psychophysical power functions indicated that the +PH participants had significantly lower exponents for pain throughout exercise. Systolic blood pressure reactivity did not significantly predict pain ratings during exercise. Normotensive African American women with +PH of hypertension experienced less muscle pain during exercise compared to normotensive African American women with a -PH of hypertension. The results are consistent with data demonstrating reduced sensitivity to experimental pain stimuli in individuals at risk for developing hypertension and extend them to naturally occurring muscle pain produced by exercise.

PERSPECTIVE

African American women, a sedentary group with an elevated risk for developing hypertension and chronic pain, show the same negative relationship between +PH and pain perception as men, suggesting that central nervous system mechanisms of pain modulation are more related to family history than gender. Acute exercise provides an experimental model for manipulating naturally occurring pain in studies concerned with the association between pain and hypertension.

Effect of caffeine on perceptions of leg muscle pain during moderate intensity cycling exercise

This double-blind, within-subjects experiment examined the effect of ingesting a large dose of caffeine on perceptions of leg muscle pain during moderate intensity cycling exercise. Low-caffeine-consuming college-aged males (n = 16) ingested either caffeine (10 mg x kg(-1) body weight) or placebo and 1 hour later completed 30 minutes of moderate intensity cycling exercise (60% VO(2peak)). The order of drug administration was counter-balanced. Perceptions of leg muscle pain as well as work rate, heart rate, and oxygen uptake (VO(2)) were recorded during exercise. Leg muscle pain ratings were significantly and moderately reduced after a high dose of caffeine. This observation suggests that prior reports showing caffeine improves endurance exercise performance might be partially explained by caffeine's hypoalgesic properties. It also suggests that moderate intensity cycling exercise has promise as a useful experimental model for the study of naturally occurring muscle pain.

RPE, pain, and physiological adjustment to concentric and eccentric contractions

PURPOSE

The purpose of the study was to compare perceptual (RPE and pain), cardiac (heart rate), lactate, and endocrine (cortisol) responses with concentric (CON) and eccentric (ECC) resistance exercise protocols using the same absolute workload.

METHODS

Eight healthy men with resistance-training experience participated in the study. Subjects completed two experimental trials consisting of either CON contractions or ECC contractions at the same absolute workload for each of four exercises: bench press, leg extension, military press, and leg curl. Subjects performed four sets of 12 repetitions at 80% of 10-RM with 90-s rest periods. Blood samples were taken before, immediately after, and 15-min postexercise.

RESULTS

There was a significant trial effect for RPE, with CON exercise eliciting a higher RPE than ECC exercise (6.71 +/- 0.51 and 4.10 +/- 0.27, respectively). A significant trial effect was also demonstrated for pain, with CON exercise producing a higher pain rating than ECC exercise (5.59 +/- 0.41 and 3.23 +/- 0.27, respectively). Significantly higher heart rates and lactates were also demonstrated during the CON trial. For cortisol, a significant interaction was revealed between the pre- and immediate posttrial measures but not an overall trial effect. Correlational analyses revealed a significant relationship between RPE and pain for both trials.

CONCLUSIONS

CON exercise elicits greater perceptual (higher RPE and pain rating), cardiac, lactate and cortisol response than ECC exercise at the same absolute workload. Data demonstrate that relative to absolute load, RPE and pain respond to resistance exercise in a similar fashion. Additionally, physiological cues are consistent with these perceptual data.

Effect of morphine on sympathetic nerve activity in humans

There are conflicting reports for the role of endogenous opioids on sympathetic and cardiovascular responses to exercise in humans. A number of studies have utilized naloxone (an opioid-receptor antagonist) to investigate the effect of opioids during exercise. In the present study, we examined the effect of morphine (an opioid-receptor agonist) on sympathetic and cardiovascular responses at rest and during isometric handgrip (IHG). Eleven subjects performed 2 min of IHG (30% maximum) followed by 2 min of postexercise muscle ischemia (PEMI) before and after systemic infusion of morphine (0.075 mg/kg loading dose + 1 mg/h maintenance) or placebo (saline) in double-blinded experiments on separate days. Morphine increased resting muscle sympathetic nerve activity (MSNA; 17 +/- 2 to 22 +/- 2 bursts/min; P < 0.01) and increased mean arterial pressure (MAP; 87 +/- 2 to 91 +/- 2 mmHg; P < 0.02), but it decreased heart rate (HR; 61 +/- 4 to 59 +/- 3; P < 0.01). However, IHG elicited similar increases for MSNA, MAP, and HR between the control and morphine trial (drug x exercise interaction = not significant). Moreover, responses to PEMI were not different. Placebo had no effect on resting, IHG, and PEMI responses. We conclude that morphine modulates cardiovascular and sympathetic responses at rest but not during isometric exercise.

Interactive effects of mental and physical stress on cardiovascular control

Physiological responses to mental tasks and physical exercise were studied independently and combined. We hypothesized that combined mental and physical stresses produce a synergistic interaction. We studied cardiovascular responses to 5 min of static handgrip, mental arithmetic, and the combined stimuli in random order in 12 healthy subjects. Muscle sympathetic nerve activity (SNA) and mean arterial blood pressure (MAP) responses to handgrip and the combined stimuli exceeded responses to mental arithmetic, yet no significant difference existed between responses to handgrip and the combined stimuli. Peak changes in SNA (in %) were greatest during handgrip (188 +/- 41), followed by the combined stimuli (166 +/- 31) and mental arithmetic (51 +/- 9). Peak changes in MAP (in mmHg) were also greatest during handgrip (26 +/- 4), followed by the combined stimuli (23 +/- 3) and then mental arithmetic (8 +/- 2). Peak changes in heart rate (in beats/min) followed the same trend: handgrip (15 +/- 2), combined (13 +/- 2), and mental arithmetic (10 +/- 2). Mental stimulation did not synergistically interact with or add to the responses elicited by handgrip exercise; in fact, a trend existed for math during handgrip to reduce responses relative to handgrip alone.

Effects of expiratory muscle work on muscle sympathetic nerve activity

We hypothesized that contractions of the expiratory muscles carried out to the point of task failure would cause an increase in muscle sympathetic nerve activity (MSNA). We measured MSNA directly in six healthy men during resisted expiration (60% maximal expiratory pressure) leading to task failure with long [breathing frequency (f(b)) = 15 breaths/min; expiratory time (TE)/total respiratory cycle duration (TT) = 0.7] and short (f(b) = 30 breaths/min; TE/TT = 0.4) TE. Both of these types of expiratory muscle contractions elicited time-dependent increases in MSNA burst frequency that averaged +139 and +239%, respectively, above baseline at end exercise. The increased MSNA coincided with increases in mean arterial pressure (MAP) for both the long-TE (+28 +/- 6 mmHg) and short-TE (+22 +/- 14 mmHg) trials. Neither MSNA nor MAP changed when the breathing patterns and increased tidal volume of the task failure trials were mimicked without resistance or task failure. Furthermore, very high levels of expiratory motor output (95% maximal expiratory pressure; f(b) = 12 breaths/min; TE/TT = 0.35) and high rates of expiratory flow and expiratory muscle shortening without task failure (no resistance; f(b) = 45 breaths/min; TE/TT = 0.4; tidal volume = 1.9 x eupnea) had no effect on MSNA or MAP. Within-breath analysis of the short-expiration trials showed augmented MSNA at the onset of and throughout expiration that was consistent with an influence of high levels of central expiratory motor output. Thus high-intensity contractions of expiratory muscles to the point of task failure caused a time-dependent sympathoexcitation; these effects on MSNA were similar in their time dependency to those caused by high-intensity rhythmic contractions of the diaphragm and forearm muscles taken to the point of task failure. The evidence suggests that these effects are mediated primarily via a muscle metaboreflex with a minor, variable contribution from augmented central expiratory motor output.