Differences in muscle sympathetic nerve response to isometric exercise in different muscle groups

The aim of this study was to examine the effects of muscle fibre composition on muscle sympathetic nerve activity (MSNA) in response to isometric exercise. The MSNA, recorded from the tibial nerve by a microneurographic technique during contraction and following arterial occlusion, was compared in three different muscle groups: the forearm (handgrip), anterior tibialis (foot dorsal contraction), and soleus muscles (foot plantar contraction) contracted separately at intensities of 20%, 33% and 50% of the maximal voluntary force. The increases in MSNA relative to control levels during contraction and occlusion were significant at all contracting forces for handgrip and at 33% and 50% of maximal for dorsal contraction, but there were no significant changes, except during exercise at 50%, for plantar contraction. The size of the MSNA response correlated with the contraction force in all muscle groups. Pooling data for all contraction forces, there were different MSNA responses among muscle groups in contraction forces (P = 0.0001, two-way analysis of variance), and occlusion periods (P = 0.0001). The MSNA increases were in the following order of magnitude: handgrip, dorsal, and plantar contractions. The order of the fibre type composition in these three muscles is from equal numbers of types I and II fibres in the forearm to increasing number of type I fibres in the leg muscles. The different MSNA responses to the contraction of different muscle groups observed may have been due in part to muscle metaboreflex intensity influenced by their metabolic capacity which is related to by their metabolic capacity which is related to the fibre type.     

Inspiratory muscle fatigue increases sympathetic vasomotor outflow and blood pressure during submaximal exercise

The purpose of this study was to elucidate the influence of inspiratory muscle fatigue on muscle sympathetic nerve activity (MSNA) and blood pressure (BP) response during submaximal exercise. We hypothesized that inspiratory muscle fatigue would elicit increases in sympathetic vasoconstrictor outflow and BP during dynamic leg exercise. The subjects carried out four submaximal exercise tests: two were maximal inspiratory pressure (PI(max)) tests and two were MSNA tests. In the PI(max) tests, the subjects performed two 10-min exercises at 40% peak oxygen uptake using a cycle ergometer in a semirecumbent position [spontaneous breathing for 5 min and with or without inspiratory resistive breathing for 5 min (breathing frequency: 60 breaths/min, inspiratory and expiratory times were each set at 0.5 s)]. Before and immediately after exercise, PI(max) was estimated. In MSNA tests, the subjects performed two 15-min exercises (spontaneous breathing for 5 min, with or without inspiratory resistive breathing for 5 min, and spontaneous breathing for 5 min). MSNA was recorded via microneurography of the right median nerve at the elbow. PI(max) decreased following exercise with resistive breathing, whereas no change was found without resistance. The time-dependent increase in MSNA burst frequency (BF) appeared during exercise with inspiratory resistive breathing, accompanied by an augmentation of diastolic BP (DBP) (with resistance: MSNA, BF +83.4%; DBP, +23.8%; without resistance: MSNA BF, +19.2%; DBP, -0.4%, from spontaneous breathing during exercise). These results suggest that inspiratory muscle fatigue induces increases in muscle sympathetic vasomotor outflow and BP during dynamic leg exercise at mild intensity.     

Effect of expiratory muscle fatigue on exercise tolerance and locomotor muscle fatigue in healthy humans

High-intensity exercise (> or =90% of maximal O(2) uptake) sustained to the limit of tolerance elicits expiratory muscle fatigue (EMF). We asked whether prior EMF affects subsequent exercise tolerance. Eight male subjects (means +/- SD; maximal O(2) uptake = 53.5 +/- 5.2 ml.kg(-1).min(-1)) cycled at 90% of peak power output to the limit of tolerance with (EMF-EX) and without (CON-EX) prior induction of EMF and for a time equal to that achieved in EMF-EX but without prior induction of EMF (ISO-EX). To induce EMF, subjects breathed against an expiratory flow resistor until task failure (15 breaths/min, 0.7 expiratory duty cycle, 40% of maximal expiratory gastric pressure). Fatigue of abdominal and quadriceps muscles was assessed by measuring the reduction relative to prior baseline values in magnetically evoked gastric twitch pressure (Pga(tw)) and quadriceps twitch force (Q(tw)), respectively. The reduction in Pga(tw) was not different after resistive breathing vs. after CON-EX (-27 +/- 5 vs. -26 +/- 6%; P = 0.127). Exercise time was reduced by 33 +/- 10% in EMF-EX vs. CON-EX (6.85 +/- 2.88 vs. 9.90 +/- 2.94 min; P < 0.001). Exercise-induced abdominal and quadriceps muscle fatigue was greater after EMF-EX than after ISO-EX (-28 +/- 9 vs. -12 +/- 5% for Pga(tw), P = 0.001; -28 +/- 7 vs. -14 +/- 6% for Q(tw), P = 0.015). Perceptual ratings of dyspnea and leg discomfort (Borg CR10) were higher at 1 and 3 min and at end exercise during EMF-EX vs. during ISO-EX (P < 0.05). Percent changes in limb fatigue and leg discomfort (EMF-EX vs. ISO-EX) correlated significantly with the change in exercise time. We propose that EMF impaired subsequent exercise tolerance primarily through an increased severity of limb locomotor muscle fatigue and a heightened perception of leg discomfort.     

Limb congestion enhances the synchronization of sympathetic outflow with muscle contraction

In this report, we examined if the synchronization of muscle sympathetic nerve activity (MSNA) with muscle contraction is enhanced by limb congestion. To explore this relationship, we applied signal-averaging techniques to the MSNA signal obtained during short bouts of forearm contraction (2-s contraction/3-s rest cycle) at 40% maximal voluntary contraction for 5 min. We performed this analysis before and after forearm venous congestion; an intervention that augments the autonomic response to sustained static muscle contractions via a local effect on muscle afferents. There was an increased percentage of the MSNA noted during second 2 of the 5-s contraction/rest cycles. The percentage of total MSNA seen during this particular second increased from minute 1 to 5 of contraction and was increased further by limb congestion (control minute 1 = 25.6 +/- 2.0%, minute 5 = 32.8 +/- 2.2%; limb congestion minute 1 = 29.3 +/- 2.1%, minute 5 = 37.8 +/- 3.9%; exercise main effect <0.005; limb congestion main effect P = 0.054). These changes in the distribution of signal-averaged MSNA were seen despite the fact that the mean number of sympathetic discharges did not increase over baseline. We conclude that synchronization of contraction and MSNA is seen during short repetitive bouts of handgrip. The sensitizing effect of contraction time and limb congestion are apparently due to feedback from muscle afferents within the exercising muscle.     

Dissociation of muscle sympathetic nerve activity and leg vascular resistance in humans

We examined the hypothesis that the increase in inactive leg vascular resistance during forearm metaboreflex activation is dissociated from muscle sympathetic nerve activity (MSNA). MSNA (microneurography), femoral artery mean blood velocity (FAMBV, Doppler), mean arterial pressure (MAP), and heart rate (HR) were assessed during fatiguing static handgrip exercise (SHG, 2 min) followed by posthandgrip ischemia (PHI, 2 min). Whereas both MAP and MSNA increase during SHG, the transition from SHG to PHI is characterized by a transient reduction in MAP but sustained elevation in MSNA, facilitating separation of these factors in vivo. Femoral artery vascular resistance (FAVR) was calculated (MAP/MBV). MSNA increased by 59 +/- 20% above baseline during SHG (P < 0.05) and was 58 +/- 18 and 78 +/- 18% above baseline at 10 and 20 s of PHI, respectively (P < 0.05 vs. baseline). Compared with baseline, FAVR increased 51 +/- 22% during SHG (P < 0.0001) but returned to baseline levels during the first 30 s of PHI, reflecting the changes in MAP (P < 0.005) and not MSNA. It was concluded that control of leg muscle vascular resistance is sensitive to changes in arterial pressure and can be dissociated from sympathetic factors.     

Thyroarytenoid muscle activity during hypoxia in awake lambs

It is generally accepted that hypoxia in early life results in active laryngeal braking of expiratory airflow via the recruitment of glottic adductor muscles. We examined the electromyogram expiratory activity of the thyroarytenoid muscle in seven 11- to 18-day-old awake nonsedated lambs exposed to an inspired O2 fraction of 0.08 for 18 min. The lambs breathed through a face mask and a pneumotachograph. During baseline prehypoxic breathing, the thyroarytenoid muscle was largely inactive in each awake lamb. Unexpectedly, no recruitment of the thyroarytenoid muscle was recorded during hypoxia in any of the seven lambs; simultaneous examination of the flow-volume curves revealed an absence of expiratory airflow braking. Also unexpectedly, marked expiratory activity of the thyroarytenoid muscle was recorded, with each expiration occurring within less than 10 s after the return to room air. The resulting delay of expiration was apparent in the flow-volume loops. Thus, in awake 11- to 18-day-old lambs, 1) active expiratory glottic adduction is absent during hypoxia and 2) a return from hypoxia to room air results in prolonged expiration as well as active glottic adduction that controls end-expiratory lung volume.     

Sympathetic adaptations to one-legged training.

The purpose of the present study was to determine the effect of leg exercise training on sympathetic nerve responses at rest and during dynamic exercise. Six men were trained by using high-intensity interval and prolonged continuous one-legged cycling 4 day/wk, 40 min/day, for 6 wk. Heart rate, mean arterial pressure (MAP), and muscle sympathetic nerve activity (MSNA; peroneal nerve) were measured during 3 min of upright dynamic one-legged knee extensions at 40 W before and after training. After training, peak oxygen uptake in the trained leg increased 19 +/- 2% (P < 0.01). At rest, heart rate decreased from 77 +/- 3 to 71 +/- 6 beats/min (P < 0.01) with no significant changes in MAP (91 +/- 7 to 91 +/- 11 mmHg) and MSNA (29 +/- 3 to 28 +/- 1 bursts/min). During exercise, both heart rate and MAP were lower after training (108 +/- 5 to 96 +/- 5 beats/min and 132 +/- 8 to 119 +/- 4 mmHg, respectively, during the third minute of exercise; P < 0.01). MSNA decreased similarly from rest during the first 2 min of exercise both before and after training. However, MSNA was significantly less during the third minute of exercise after training (32 +/- 2 to 22 +/- 3 bursts/min; P < 0.01). This training effect on MSNA remained when MSNA was expressed as bursts per 100 heartbeats. Responses to exercise in five untrained control subjects were not different at 0 and 6 wk. These results demonstrate that exercise training prolongs the decrease in MSNA during upright leg exercise and indicates that attenuation of MSNA to exercise reported with forearm training also occurs with leg training.     

Distinct profiles of neuromuscular fatigue during muscle contractions below and above the critical torque in humans

Whether the transition in fatigue processes between "low-intensity" and "high-intensity" contractions occurs gradually, as the torque requirements are increased, or whether this transition occurs more suddenly at some identifiable "threshold", is not known. We hypothesized that the critical torque (CT; the asymptote of the torque-duration relationship) would demarcate distinct profiles of central and peripheral fatigue during intermittent isometric quadriceps contractions (3-s contraction, 2-s rest). Nine healthy men performed seven experimental trials to task failure or for up to 60 min, with maximal voluntary contractions (MVCs) performed at the end of each minute. The first five trials were performed to determine CT [~35-55% MVC, denoted severe 1 (S1) to severe 5 (S5) in ascending order], while the remaining two trials were performed 10 and 20% below the CT (denoted CT-10% and CT-20%). Dynamometer torque and the electromyogram of the right vastus lateralis were sampled continuously. Peripheral and central fatigue was determined from the fall in potentiated doublet torque and voluntary activation, respectively. Above CT, contractions progressed to task failure in ~3-18 min, at which point the MVC did not differ from the target torque (S1 target, 88.7 ± 4.3 N·m vs. MVC, 89.3 ± 8.8 N·m, P = 0.94). The potentiated doublet fell significantly in all trials, and voluntary activation was reduced in trials S1-S3, but not trials S4 and S5. Below CT, contractions could be sustained for 60 min on 17 of 18 occasions. Both central and peripheral fatigue developed, but there was a substantial reserve in MVC torque at the end of the task. The rate of global and peripheral fatigue development was four to five times greater during S1 than during CT-10% (change in MVC/change in time S1 vs. CT-10%: -7.2 ± 1.4 vs. -1.5 ± 0.4 N·m·min(-1)). These results demonstrate that CT represents a critical threshold for neuromuscular fatigue development.     

Ventilation and respiratory pattern and timing in resting awake cats

The purpose of this study was to characterize the variability and patterns of spontaneous respiratory behaviour in awake cats. Respiration was measured in six cats over 80 or 90 min by the plethysmographic technique. In three cats, arterial blood gases were measured. Breath frequency (f) and tidal volume (VT) varied considerably breath-to-breath, although on average, these measurements as well as average ventilation remained relatively constant. The incidence of breath ventilation (VT X 60/TTOT) and VT were distributed unimodally but the incidence of breath f had a bimodal distribution. In the low f range, average f was 22.5 breaths/min, and in the high f range, average f was 41.6 breaths/min. The latter range appeared to be associated with purring. Inspiratory duration (TI) was less than expiratory duration (TE) at low f but exceeded TE at high f. For a given breath ventilation there was a predictable f and VT. At shorter TI (higher f) mean inspiratory flow, an index of central respiratory drive, increased but VT decreased. This study indicates that "normal" control respiratory behaviour in awake cats is better described by the range and pattern of breathing than by average values.     

Neural and cardiovascular responses to emotional stress in humans

Sympathetic neural responses to mental stress are well documented but controversial, whereas sympathetic neural responses to emotional stress are unknown. The purpose of this study was to investigate neural and cardiovascular responses to emotional stress evoked by negative pictures and reexamine the relationship between muscle sympathetic nerve activity (MSNA) and perceived stress. Mean arterial pressure (MAP), heart rate (HR), MSNA, and perceived stress levels were recorded in 18 men during three randomized trials: 1) neutral pictures, 2) negative pictures, and 3) mental stress. MAP and HR increased during mental stress (Delta14 +/- 2 mmHg and Delta15 +/- 2 beats/min, P < 0.001) but did not change during viewing of negative or neutral pictures. MSNA did not change during viewing of neutral (Delta1 +/- 1 burst/min, n = 16) or negative (Delta0 +/- 1 burst/min, n = 16) pictures or during mental stress (Delta1 +/- 2 burst/min, n = 13). Perceived stress levels were higher during mental stress (3 +/- 0 arbitrary units) than during viewing negative pictures (2 +/- 0 arbitrary units, P < 0.001). Perceived stress levels were not correlated to changes in MSNA during negative pictures (r = 0.10, P = 0.84) or mental stress (r = 0.36, P = 0.23). In conclusion, our results demonstrate robust increases in MAP and HR during mental stress, but not during emotional stress evoked by negative pictures. Although the influence of mental stress on MSNA remains unresolved, our findings challenge the concept that perceived stress levels modulate MSNA during mental stress.     

Accessory muscle activity contributes to the variation in time to task failure for different arm postures and loads.

Time to failure and electromyogram activity were measured during two types of sustained submaximal contractions with the elbow flexors that required each subject to exert the same net muscle torque with the forearm in two different postures. Twenty men performed the tasks, either by maintaining a constant force while pushing against a force transducer (force task), or by supporting an equivalent load while maintaining a constant elbow angle (position task). The time to failure for the position task with the elbow flexed at 1.57 rad and the forearm horizontal was less than that for the force task (5.2 +/- 2.6 and 8.8 +/- 3.6 min, P = 0.003), whereas it was similar when the forearm was vertical (7.9 +/- 4.1 and 7.8 +/- 4.5 min, P = 0.995). The activity of the rotator cuff muscles was greater during the position tasks (25.1 +/- 10.1% maximal voluntary contraction) compared with the force tasks (15.2 +/- 5.4% maximal voluntary contraction, P < 0.001) in both forearm postures. However, the rates of increase in electromyogram of the accessory muscles and mean arterial pressure were greater for the position task only when the forearm was horizontal (P < 0.05), whereas it was similar for the elbow flexors. These findings indicate that forearm posture influences the difference in the time to failure for the two fatiguing contractions. When there was a difference between the two tasks, the task with the briefer time to failure involved greater rates of increase in accessory muscle activity and mean arterial pressure.     

Effect of frequency on perceived magnitude of added loads to breathing

Using open-magnitude scaling, six normal subjects estimated the perceived magnitude of a range of added elastic loads (20-76 cmH2O/l), applied for a sequence of five breaths, at frequencies varying from 5 to 26.4 breaths/min. Two experiments were performed. In the first, frequency was increased by a reduction in expiratory duration (TE), and the duty cycle (ratio of inspiratory duration to total breath duration, TI/TT) ranged between 0.10 and 0.52. The perceived magnitude psi increased significantly with the peak airway pressure (Pm) (P less than 0.0001) but did not reach conventional significance with frequency (fb) (P = 0.15): psi = K0Pm1.23fb0.07 (r = 0.911). However, the sensory magnitude increased significantly as the duty cycle increased (P less than 0.01), but when it was included, the magnitude decreased minimally with frequency (P less than 0.01): psi = K0Pm1.3fb-0.97 TI/TT1.14 (r = 0.92). In the second experiment the duty cycle (TI/TT) was kept constant [(0.43 +/- 0.008 (SE)] and frequency (5-26.4 breaths/min) increased at the expense of shortening both TI and TE. The perceived magnitude of the added elastances decreased with the increase in frequency. However, when the perceived magnitude was corrected for the duration of inspiration, which is known to increase the sensory magnitude, psi = K0Pm1.3TI0.56, the sensory magnitude increased significantly with frequency (P less than 0.001): psi/TI0.56 = K0Pm1.21fb0.28 (r = 0.773). The decrease in inspiratory duration had a greater quantitative effect decreasing sensory magnitude than frequency had on increasing the magnitude. The effect of increasing frequency is complex and depends on the simultaneous intensity, duration of inspiratory pressure, and the duty cycle.     

Characteristics of the muscle mechanoreflex during quadriceps contractions in humans

We examined muscle sympathetic nerve activity (MSNA) in thenonexercising lower limb during repetitive static quadricepscontraction paradigm at 25% maximal voluntary contraction in eightmen. Subjects performed 20-s contractions with 5-s rest periods for upto 12 contractions. Although the workload was constant, we found that MSNA amplitude rose as a function of contraction number [0.6 ln (amplitude/min)/contraction]; this suggests chemicalsensitization of the muscle reflex response. We employedsignal-averaging techniques and then integrated the data to examine theonset latency of the MSNA response as a function of the 25-scontraction-rest period. We observed an onset latency of ~4-6 s.Moreover, although the onset latency did not appear to vary as afunction of contraction number, the rate of MSNA increase tookapproximately four contractions to reach a steady-state rate of rise;this suggests contraction-induced sensitization. The onset latencyreported here is similar to findings in recent animal studies, but itis at odds with latencies determined in prior human handgripcontraction studies. We believe our data suggest that1) mechanically sensitive afferentscontribute importantly to the MSNA response to the paradigm employedand 2) these afferents may besensitized by the chemical products of muscle contraction.     

Effects of spontaneous swallows on breathing in awake goats.

The effects of spontaneous swallows on breathing before, during, and after solitary swallows were investigated in 13 awake goats. Inspiratory (TI) and expiratory (TE) time and respiratory output were determined from inspiratory airflow [tidal volume (VT)] and peak diaphragmatic activity (Dia(peak)). The onset time for 1,128 swallows was determined from pharyngeal muscle electrical activity. During inspiration, the later the swallowing onset, the greater increase in TI and VT, whereas there was no significant effect on TE and Dia(peak). Swallows in early expiration increased the preceding TI and reduced TE, whereas later in expiration swallows increased TE. After expiratory swallows, TI and VT were reduced whereas minimal changes in Dia(peak) were observed. Phase response analysis revealed a within-breath, phase-dependent effect of swallowing on breathing, resulting in a resetting of the respiratory oscillator. However, the shift in timing in the breaths after a swallow was not parallel, further demonstrating a respiratory phase-dependent effect on breathing. We conclude that, in the awake state, within- and multiple-breath effects on respiratory timing and output are induced and/or required in the coordination of breathing and swallowing.     

Inspiratory and expiratory patterns of the pectoralis major muscle during pulmonary defensive reflexes

Experiments wereconducted to determine the discharge pattern of the pectoralis majormuscle during pulmonary defensive reflexes in anesthetized cats(n = 15). Coughs andexpiration reflexes were elicited by mechanical stimulation of theintrathoracic trachea or larynx. Augmented breaths occurredspontaneously or were evoked by the same mechanical stimuli.Electromyograms (EMGs) were recorded from the diaphragm, rectusabdominis, and pectoralis major muscles. During augmented breaths, thepectoralis major had inspiratory EMG activity similar to that of thediaphragm, but during expiration reflexes the pectoralis major also hadpurely expiratory EMG activity similar to the rectus abdominis. Duringtracheobronchial cough, the pectoralis major had an inspiratory patternsimilar to that of the diaphragm in 10 animals, an expiratory patternsimilar to that of the rectus abdominis in 3 animals, and a biphasicpattern in 2 animals. The pectoralis major was active during both the inspiratory and expiratory phases during laryngeal cough. We conclude that, in contrast to the diaphragm or rectus abdominis muscles, thepectoralis major is active during both inspiratory and expiratory pulmonary defensive reflexes.

     

Transient changes in expiratory time during hypercapnia in premature infants

The transient ventilatory responses to hypercapnia were studied in nine healthy preterm infants. We administered 4% CO2 in air for at least 7 min during quiet sleep and measured frequency (f), inspiratory time (TI), expiratory time (TE), tidal volume (VT), and minute ventilation (VI). Frequency increased over the first 2 min of CO2 inhalation (P less than 0.05) and then decreased to control values (P less than 0.05). This response was secondary to changes in TE, which decreased over the first 2 min (P less than 0.05) and then returned to control values, whereas TI did not change. The late increase in TE was associated with an increased percent of breaths exhibiting retardation of expiratory flow (braking) (P less than 0.05). These breaths had longer TE than the breaths without braking (P less than 0.05). Exponential curves made to fit the increases in VI and VT revealed that only 67% of the infants reached 90% of steady state for both VI and VT over the 7-min study period. The time to 90% of steady state was always shorter for VI than VT (P less than 0.05) due to the transient changes in f. The results indicate that the transient changes of f in response to hypercapnia are secondary to changes in TE, which appear unique to human infants. We speculate that the expiratory braking that develops during the course of CO2 inhalation increases lung volume, resulting in prolongation of TE via mechanoreceptor-mediated reflexes.     

Sternomastoid, rib cage, and expiratory muscle activity during weaning failure.

We hypothesized that patients who fail weaning from mechanical ventilation recruit their inspiratory rib cage muscles sooner than they recruit their expiratory muscles, and that rib cage muscle recruitment is accompanied by recruitment of sternomastoid muscles. Accordingly, we measured sternomastoid electrical activity and changes in esophageal (DeltaPes) and gastric pressure (DeltaPga) in 11 weaning-failure and 8 weaning-success patients. At the start of trial, failure patients exhibited a higher DeltaPga-to-DeltaPes ratio than did success patients (P = 0.05), whereas expiratory rise in Pga was equivalent in the two groups. Between the start and end of the trial, failure patients developed additional increases in DeltaPga-to-DeltaPes ratio (P < 0.0014) and the expiratory rise in Pga also increased (P < 0.004). At the start of trial, sternomastoid activity was present in 8 of 11 failure patients contrasted with 1 of 8 success patients. Over the course of the trial, sternomastoid activity increased by 53.0 +/- 9.3% in the failure patients (P = 0.0005), whereas it did not change in the success patients. Failure patients recruited their respiratory muscles in a sequential manner. The sequence began with activity of diaphragm and greater-than-normal activity of inspiratory rib cage muscles; recruitment of sternomastoids and rib cage muscles approached near maximum within 4 min of trial commencement; expiratory muscles were recruited slowest of all. In conclusion, not only is activity of the inspiratory rib cage muscles increased during a failed weaning trial, but respiratory centers also recruit sternomastoid and expiratory muscles. Extradiaphragmatic muscle recruitment may be a mechanism for offsetting the effects of increased load on a weak diaphragm.     

Effect of resistive loads on pattern of respiratory muscle recruitment during exercise.

In healthy subjects, we compared the effects of an expiratory (ERL) and an inspiratory (IRL) resistive load (6 cmH2O.l-1.s) with no added resistive load on the pattern of respiratory muscle recruitment during exercise. Fifteen male subjects performed three exercise tests at 40% of maximum O2 uptake: 1) with no-added-resistive load (control), 2) with ERL, and 3) with IRL. In all subjects, we measured breathing pattern and mouth occlusion pressure (P0.1) from the 3rd min of exercise, in 10 subjects O2 uptake (VO2), CO2 output (VCO2), and respiratory exchange ratio (R), and in 5 subjects we measured gastric (Pga), pleural (Ppl), and transdiaphragmatic (Pdi) pressures. Both ERL and IRL induced a high increase of P0.1 and a decrease of minute ventilation. ERL induced a prolongation of expiratory time with a reduction of inspiratory time (TI), mean expiratory flow, and ratio of inspiratory to total time of the respiratory cycle (TI/TT). IRL induced a prolongation of TI with a decrease of mean inspiratory flow and an increase of tidal volume and TI/TT. With ERL, in two subjects, Pga increased and Ppl decreased more during inspiration than during control suggesting that the diaphragm was the most active muscle. In one subject, the increases of Ppl and Pga were weak; thus Pdi increased very little. In the two other subjects, Ppl decreased more during inspiration but Pga also decreased, leading to a decrease of Pdi. This suggests a recruitment of abdominal muscles during expiration and of accessory and intercostal muscles during inspiration. With IRL, in all subjects, Ppl again decreased more, Pga began to decrease until 40% of TI and then increased.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of concentric and eccentric muscle actions on muscle sympathetic nerve activity

The purpose ofthis study was to determine the effects of concentric (Con) andeccentric (Ecc) muscle actions on leg muscle sympathetic nerve activity(MSNA). Two protocols were utilized. In protocol1, eight subjects performed Con and Ecc arm curls for 2 min, with a resistance representing 50% of one-repetition maximum forCon curls. Heart rate (HR) and mean arterial pressure (MAP) weregreater (P < 0.05) during Con thanduring Ecc curls. Similarly, the MSNA was greater(P < 0.05) during Con than during Ecc curls. In protocol 2, eightdifferent subjects performed Con and Ecc arm curls to fatigue, followedby postexercise muscle ischemia, by using the same resistanceas in protocol 1. Endurance time wassignificantly greater for Ecc than for Con curls. The increase in HR,MAP, and MSNA was greater (P < 0.05)during Con than during Ecc curls. However, when the data werenormalized as a function of endurance time, the differences in HR, MAP,and MSNA between Con and Ecc curls were no longer present. HR, MAP, andMSNA responses during postexercise muscle ischemia were similar for Con and Ecc curls. Con curls elicited greater increase(P < 0.05) in blood lactateconcentration than did Ecc curls. In summary, Con actions contributesignificantly more to the increase in cardiovascular and MSNA responsesduring brief, submaximal exercise than do Ecc actions. However, whenperformed to a similar level of effort (i.e., fatigue), Con and Eccmuscle actions elicit similar cardiovascular and MSNA responses. Theseresults indicate that the increase in MSNA during a typical bout ofsubmaximal dynamic exercise is primarily mediated by the musclemetaboreflex, which is stimulated by metabolites produced predominantlyduring Con muscle action.     

Thixotropy of rib cage respiratory muscles in normal subjects.

In this study, we searched for signs of thixotropic behavior in human rib cage respiratory muscles. If rib cage respiratory muscles possess thixotropic properties similar to those seen in other skeletal muscles in animals and humans, we expect resting rib cage circumference would be temporarily changed after deep rib cage inflations or deflations and that these aftereffects would be particularly pronounced in trials that combine conditioning deep inflations or deflations with forceful isometric contractions of the respiratory muscles. We used induction plethysmography to obtain a continuous relative measure of rib cage circumference changes during quiet breathing in 12 healthy subjects. Rib cage position at the end of the expiratory phase (EEP) was used as an index of resting rib cage circumference. Comparisons were made between EEP values of five spontaneous breaths immediately before and after six types of conditioning maneuvers: deep inspiration (DI); deep expiration (DE); DI combined with forceful effort to inspire (FII) or expire (FEI); and DE combined with forceful effort to inspire (FIE) or expire (FEE), both with temporary airway occlusion. The aftereffects of the conditioning maneuvers on EEP values were consistent with the supposition that human respiratory muscles possess thixotropic properties. EEP values were significantly enhanced after all conditioning maneuvers involving DI, and the aftereffects were particularly pronounced in the FII and FEI trials. In contrast, EEP values were reduced after DE maneuvers. The aftereffects were statistically significant for the FEE and FIE, but not DE, trials. It is suggested that respiratory muscle thixotropy may contribute to the pulmonary hyperinflation seen in patients with chronic obstructive pulmonary disease.