Sense Of Effort Research Articles

Expiratory muscle training and sensation of respiratory effort during exercise in normal subjects.

The sensation of respiratory effort may increase as expiratory muscles become fatigued during expiratory loading. A study was performed to determine whether expiratory muscle training (EMT) affects the sensation of respiratory effort during exercise in healthy subjects. Six subjects performed EMT for 15 minutes twice daily for four weeks using a pressure threshold device; another six subjects served as a control group. The expiratory threshold was set at 30% of the individual's maximum expiratory mouth pressure (PEmax). The sensation of respiratory effort was evaluated during a progressive exercise test using the Borg scale. After EMT PEmax increased by 25% in the training group. The Borg score increased as exercise grade increased before and after EMT, but scores for each grade were lower after EMT. Minute ventilation during exercise decreased after EMT, as did the breathing frequency during exercise, while the expiratory time increased. Although there was no difference in the relationship between Borg score and minute ventilation before or after EMT, the curve shifted to a lower Borg score after EMT. There were no changes in PEmax, Borg score, minute ventilation, or breathing pattern after the four week study period in the control group. These findings suggest that EMT increases expiratory muscle strength and reduces the sensation of respiratory effort during exercise, presumably by reducing minute ventilation.

Modelling the Additivity of Perceived Exertion in Symmetric, Mid-Sagittal Lifting

Psychophysical approaches to quantifying perceived effort have been used to evaluate the physical demand of many industrial work activities. An experiment was conducted to examine the relationship between ratings of whole-body perceived exertion and differentiated, regional ratings of exertion. The Borg, CR-10 scale was used by 16 subjects performing a simulated repetitive lifting task. Ratings of perceived exertion were obtained for the arms, legs, torso, and central (cardiorespiratory) effort sensations as well as a rating of overall, whole-body exertion. A multiple linear regression analysis was used to predict the whole-body rating of exertion from the differentiated ratings in lifting tasks using both a squat and stoop posture. In the stoop posture condition the coefficient of determination between whole-body perceived exertion and the model including arm, torso, and central ratings was R2=0.81. In the squat posture condition, the final regression model predicting whole-body exertion contained only the rating from the legs (R2 = 0.62). Differentiated ratings explained the majority of the variance in whole-body perceived exertion for squat and stoop lifting tasks.

Respiratory effort perception at rest and during carbon dioxide rebreathing in patients with dystrophia myotonica.

Breathlessness appears to be closely related to the perception of the outgoing motor command to breathe and should be increased in the presence of muscle weakness. However, breathlessness is not a common symptom in patients with chronic muscle disease who have weak respiratory muscles. The factors that determine the perception of respiratory effort in such patients have not been examined. The inspiratory effort sensation during resting breathing and progressive hypercapnia was investigated in 12 patients with dystrophia myotonica with weak respiratory muscles (nine men and three women of mean (SD) age 41.1 (10.5) years; maximum inspiratory pressure 43.1 (17.2) cm H2O) and an age and sex matched control group of normal subjects of mean age 39.6 (10.6) years and a maximum inspiratory pressure of 123 (15.2) cm H2O. During resting breathing with a mouthpiece no differences were seen in inspiratory effort sensation, mouth occlusion pressure, or tidal volume, but inspiratory time and cycle duration were significantly shorter in the patients with dystrophia. Minute ventilation (VE) was significantly higher in the patients (15.8 (4.0) l/min v 12.5 (2.6) l/min), while resting breathing was no more variable in the patients than in controls. The ventilatory response to carbon dioxide (VE/PCO2) was not significantly lower in the patients (14.9 (6.9) l/min/kPa) than in the controls (17.4 (4.3) l/min/kPa). Effort sensation responses to carbon dioxide driven breathing were similar in the control subjects and the patients. With regression analysis of pooled data neither maximum inspiratory pressure nor disease state contributed to perceived inspiratory effort during hypercapnia. Moderately severe global respiratory muscle weakness does not appear to influence the ventilatory response to rising carbon dioxide tension or the perception of inspiratory effort in patients with dystrophia myotonica.

What do fully paralyzed awake humans feel when they attempt to move?

Subjects are able to judge the strength of muscle contraction. In theory, the force of muscular exertion could be perceived either from mechanoreceptor afferents or from knowledge of central motor command (corollary discharge). Sensations of great effort or exerted force have been described by subjects when their limbs were weakened by fatigue or partial paralysis. This has been taken as evidence that effort sensations arise from central motor commands rather than from mechanoreceptor afferent signals produced by muscle contraction. To differentiate between these possibilities, we used neuromuscular block to completely paralyze four waking subjects and required them to attempt maximal contraction of inspiratory muscles and of hand muscles. They were questioned after recovery about what their sensations were when attempting these contractions. None described the sensations of exerted force, great effort, or heaviness, which would have been expected if motor commands alone were the source of these sensations. The contradiction between our findings and those previously reported suggests that the specific neural mechanisms for effort sensations must be reexamined.

Physiological Determinants of Inspiratory Effort Sensation during CO2 Rebreathing in Normal Subjects

1. The physiological basis of inspiratory effort sensation remains uncertain. Previous studies have suggested that pleural pressure, rather than inspiratory muscle fatigue, is the principal determinant of inspiratory effort sensation. However, only a limited range of inspiratory flows and breathing patterns have been examined. We suspected that inspiratory effort sensation was related to the inspiratory muscle tension-time index developed whatever the breathing pattern or load, and that this might explain the additional rise in sensation seen with hypercapnia. 2. To investigate this we measured hypercapnic rebreathing responses in seven normal subjects (six males, age range 21-38 years) with and without an inspiratory resistive load of 10 cm H2O. Pleural and transdiaphragmatic pressures, mouth occlusion pressure and breathing pattern were measured. Diaphragmatic and ribcage tension-time indices were calculated from these data. Inspiratory effort sensation was recorded using a Borg scale at 30s intervals during each rebreathing run. 3. Breathing pattern and inspiratory pressure partitioning were unrelated to changes in inspiratory effort sensation during hypercapnia. Tension-time indices reached pre-fatiguing levels during both free breathing and inspiratory resistive loading. 4. Stepwise multiple regression analysis using pooled mechanical, chemical and breathing pattern variables showed that pleural pressure was more closely related to inspiratory effort sensation than was transdiaphragmatic pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous Opioid System during Inspiratory Loading in Patients with Type I Diabetes

To investigate the activity of the endogenous opioid system in patients with insulin-dependent diabetes mellitus during ventilatory stress situations, we measured plasma beta-endorphin levels in six male and five female diabetic patients breathing against fatiguing inspiratory resistive loads. The patients had to generate with each inspiration an esophageal pressure (Pes) 80% of maximum until they were exhausted and could no longer develop target Pes. The loaded breathing run was repeated three times with a 1-min interval between each run. Duty cycle, tidal volume, and breathing frequency were constant in all tasks. For each run plasma beta-endorphin levels were measured, inspiratory effort sensation assessed using a modified Borg scale, and inspiratory muscle endurance evaluated by the length of time the task could be maintained (Tlim). A group of 11 sex-, age-, height-, and weight-matched healthy individuals served as control subjects. Tlim was significantly lower in the diabetic patients. Evaluating respiratory effort during the three test runs in control subjects at a time corresponding to the break point of loaded breathing in patients showed significantly lower Borg ratings in the control group than in the patient group. Baseline plasma beta-endorphin was significantly lower in the diabetic patients (10.6 +/- 2.1 versus 27.0 +/- 4.2 pg/ml, p < 0.01). Additionally, whereas resistive loaded breathing caused a further increase in plasma beta-endorphin concentration in the control group (p < 0.005), absolutely no increase was found in the diabetic patients. We conclude that the endogenous opioid system in insulin-dependent diabetic patients does not respond to stress caused by breathing against fatiguing inspiratory resistive loads.

Exercise performance and fatiguability in patients with chronic fatigue syndrome.

To examine the role of delay in recovery of peripheral muscle function following exercise in the fatigue experienced by patients with the chronic fatigue syndrome (CFS) and to examine the influence of effort perception in limiting exercise performance in these patients, a study was carried out on a group of twelve patients with chronic fatigue syndrome and 12 sex and age-matched sedentary control subjects. Symptom limited incremental cycle exercise tests including measurements of perceived exertion were performed followed by examination of the contractile properties of the quadriceps muscle group for up to 48 hours. Muscle function was assessed by percutaneous electrical stimulation and maximum voluntary contractions. Muscle function at rest and during recovery was normal in CFS patients as assessed by maximum isometric voluntary contraction, 20:50 Hz tetanic force ratio and maximum relaxation rate. Exercise duration and the relationship between heart rate and work rate during exercise were similar in both groups. CFS patients had higher perceived exertion scores in relation to heart rate during exercise representing a reduced effort sensation threshold of 3.2 units on an unmodified Borg scale in CFS patients. Patients with chronic fatigue syndrome show normal muscle physiology before and after exercise. Raised perceived exertion scores during exercise suggest that central factors are limiting exercise capacity in these patients.

The effect of opioids on inspiratory muscle fatigue during inspiratory resistive loading.

The effect of opioids on inspiratory muscle function under high mechanical load is still unknown. Even less clear is the extent to which opioids influence the shift of the electromyographic power spectrum of the inspiratory muscles to lower frequencies during ventilatory stress. We studied seven healthy subjects breathing against high inspiratory threshold loads until exhaustion while keeping the minute ventilation constantly high. We compared runs with and without administration of 0.2 mg kg-1 of morphine sulphate intramuscularly; two subjects were given 30 mg morphine sulphate so that we could study the effect of higher opioid concentration. The endurance time (Tlim), the diaphragmatic electromyogram (EMG), the transdiaphragmatic pressures (Pdi) and the ventilatory effort sensation were analysed. Morphine did not have any effect on Tlim or on the effort sensation elicited by the inspiratory resistance in both concentrations. Analysing the spectral shifts of the diaphragmatic EMG, we did not find any significant difference in the decrease of the centroid frequency between drug and control runs. Furthermore, the activation pattern of the diaphragm and the intercostal muscles, evaluated from the percentage contribution of oesophageal and gastric pressures on the transdiaphragmatic pressure swings, did not change following the administration of morphine. Our study shows that morphine does not change the function of the inspiratory muscles during high-resistive breathing. Morphine does not affect the electromyographic power spectrum of the diaphragm during those resistive breathing runs, either. This points out that during stressful ventilatory situations, the shift of the electromyographic power spectrum is attributed to a peripheral (muscular) event consequent to muscle fatigue and not to the elaboration of endogenous opioids.

The Assessment of Respiratory Effort Sensation: A Comparison of two Scaling Methods

The Assessment of Respiratory Effort Sensation: A Comparison of two Scaling Methods

Dissociation between dyspnea and respiratory effort.

Breathlessness induced by hypercapnia may be related to the sensation of respiratory effort or to the central or peripheral effects of CO2. To examine the relationship among breathlessness, respiratory effort, and hypercapnia, we studied eight normal naive subjects. By using a visual feedback system, subjects maintained a constant ventilation of 50-60 L/min. PETCO2 was held at 40 mm Hg during the first 2 min of each trial (control period), then for 4 min (test period) was either kept at 40 mm Hg or elevated to 50 mm Hg. At the end of each control and test period, subjects were asked to give separate ratings for dyspnea (an unpleasant urge to breathe) and for the sense of respiratory effort (analogous to lifting a weight) on a 50-cm visual analog scale. Hypercapnia was associated with a significant reduction in effort ratings (-7.3 +/- 6.4, mean +/- SD, p < 0.05) and a concomitant increase in dyspnea (+6.6 +/- 6.0, p < 0.05). We conclude that dyspnea associated with hypercapnia is dissociated from changes in respiratory effort, and that CO2 has a direct central effect that leads to breathlessness. Our data also suggest that the sense of effort at a given level of ventilation is less when the ventilation is the result of "reflex" stimuli to breathe rather than "voluntary" signals to the respiratory muscles.

Effort sensation, chemoresponsiveness, and breathing pattern during inspiratory resistive loading.

Although inspiratory resistive loading (IRL) reduces the ventilatory response to CO2 (VE/PCO2) and increases the sensation of inspiratory effort (IES), there are few data about the converse situation: whether CO2 responsiveness influences sustained load compensation and whether awareness of respiratory effort modifies this behavior. We studied 12 normal men during CO2 rebreathing while free breathing and with a 10-cmH2O.l-1.s IRL and compared these data with 5 min of resting breathing with and without the IRL. Breathing pattern, end-tidal PCO2, IES, and mouth occlusion pressure (P0.1) were recorded. Free-breathing VE/PCO2 was inversely related to an index of effort perception (IES/VE; r = -0.63, P less than 0.05), and the reduction in VE/PCO2 produced by IRL was related to the initial free-breathing VE/PCO2 (r = 0.87, P less than 0.01). IRL produced variable increases in inspiratory duration (TI), IES, and P0.1 at rest, and the change in tidal volume correlated with both VE/PCO2 (r = 0.63, P less than 0.05) and IES/VE (r = -0.69, P less than 0.05), this latter index also predicting the changes in TI with loading (r = -0.83, P less than 0.01). These data suggest that in normal subjects perception of inspiratory effort can modify free-breathing CO2 responsiveness and is as important as CO2 sensitivity in determining the response to short-term resistive loading. Individuals with good perception choose a small-tidal volume and short-TI breathing pattern during loading, possibly to minimize the discomfort of breathing.

Mechanical load on the inspiratory muscles during exercise hyperpnea in patients with type 1 (insulin-dependent) diabetes mellitus.

The aim of this study was to evaluate the difference between Type 1 (insulin-dependent) diabetic patients and healthy control subjects regarding inspiratory muscle load during exercise hypernea. For this purpose an incremental progressive exercise test on a cycle ergometer was performed by 36 Type 1 diabetic patients and 40 healthy subjects. In order to determine the mechanical load on the inspiratory muscles breath by breath, we selected the following two parameters, which represent the pressure generated by the inspiratory muscles as well as the duration and velocity of their contraction: (1) the oesophageal tension time index, which is the product of the duty cycle (ratio of inspiratory time to total breath cycle duration) and the mean oesophageal pressure expressed as a percentage of the maximal oesophageal pressure and (2) the mean oesophageal pressure change per time unit during the inspiratory phase of each breathing manoeuver, which is expressed as a fraction of the subject's maximal oesophageal pressure. Comparison of the two groups revealed that at similar levels of ventilation the mechanical load on the inspiratory muscles was significantly higher in the Type 1 diabetic patients than in the control subjects. When the loading was stopped the maximal ventilation was lower in the patients. Nevertheless, they reported a degree of respiratory effort sensation comparable to the control group, which seems to have been caused by an increase of the mechanical load on the ventilatory muscles.

Relationship of Respiratory Effort Sensation to Expiratory Muscle Fatigue during Expiratory Threshold Loading

We investigated whether fatigue of the expiratory muscle, that is, the abdominal muscle, may account for a change in the respiratory effort sensation in normal subjects during expiratory threshold loading. The respiratory effort sensation was scored using a modified Borg scale. Expiratory muscle fatigue was assessed both from changes in the maximal static expiratory pressure and in the centroid frequency (fc) of the abdominal muscle electromyogram (EMG). Expiratory threshold loading (magnitude of threshold; 40 to 60% of the maximal expiratory pressure at FRC, breathing frequency = 15/min, and duty cycle = 0.5) was continued until exhaustion or for 30 min. Loading was repeated following a 15-min recovery period after the end of the first expiratory loading. The maximal static expiratory pressure during loading (Pmmax) decreased initially and then remained decreased. Decreases were smaller with the 40% load (22 +/- 6%, SEM) than with the 60% load (37 +/- 3%) (p less than 0.05). The decrease during the second run of the 60% load was greater than during the first (p less than 0.01 by ANOVA). The maximal expiratory pressure at TLC before the second run of the 60% load was decreased by 9 +/- 3% compared with the control (p less than 0.02) but that with the 40% load was not. The fc with the 60% load decreased initially by 8 +/- 1% and then remained constant, although no change was observed with the 40% load.(ABSTRACT TRUNCATED AT 250 WORDS)

Learned industriousness.

Extensive research with animals and humans indicates that rewarded effort contributes to durable individual differences in industriousness. It is proposed that reinforcement for increased physical or cognitive performance, or for the toleration of aversive stimulation, conditions rewards value to the sensation of high effort and thereby reduces effort's aversiveness. The conditioning of secondary reward value to the sensation of effort provides a dynamic mechanism by which reinforced high performance generalizes across behaviors. Applications to self-control, moral development, and education are described.

Sensation of effort and duration of mentally executed actions

The aim of this study was to examine whether a sensation of effort would arise in subjects requested, by verbal instructions, to mentally perform motor tasks with an internal imagery strategy. Sixteen subjects had to imagine themselves, from a first person perspective, writing a sentence and drawing a Necker's cube either with their right dominant hand or with their left hand, as well as hopping around a square either on their right or left foot. The time needed to mentally execute these actions was measured. The sensation of effort following the mental performance and the difficulty of imaging the tasks were assessed by means of two analog rating scales. The results indicate that the sensation of effort increased across the trials. Furthermore, a negative correlation coefficient (mean r = -0.99) was found between the difficulty to imagining a given motor task and the subjective sensation of effort across the trials. Moreover, the sensation of effort was more pronounced when the tasks involved the non-dominant limb.

Effects of changes in level and pattern of breathing on the sensation of dyspnea

Breathing during hypercapnia is determined by reflex mechanisms but may also be influenced by respiratory sensations. The present study examined the effects of voluntary changes in level and pattern of breathing on the sensation of dyspnea at a constant level of chemical drive. Studies were carried out in 15 normal male subjects during steady-state hypercapnia at an end-tidal PCO2 of 50 Torr. The intensity of dyspnea was rated on a Borg category scale. In one experiment (n = 8), the level of ventilation was increased or decreased from the spontaneously adopted level (Vspont). In another experiment (n = 9), the minute ventilation was maintained at the level spontaneously adopted at PCO2 of 50 Torr and breathing frequency was increased or decreased from the spontaneously adopted level (fspont) with reciprocal changes in tidal volume. The intensity of dyspnea (expressed as percentage of the spontaneous breathing level) correlated with ventilation (% Vspont) negatively at levels below Vspont (r = -0.70, P less than 0.001) and positively above Vspont (r = 0.80, P less than 0.001). At a constant level of ventilation, the intensity of dyspnea correlated with breathing frequency (% fspont) negatively at levels below fspont (r = -0.69, P less than 0.001) and positively at levels above fspont (r = 0.75, P less than 0.001). These results indicate that dyspnea intensifies when the level or pattern of breathing is voluntarily changed from the spontaneously adopted level. This is consistent with the possibility that ventilatory responses to changes in chemical drive may be regulated in part to minimize the sensations of respiratory effort and discomfort.

Effect of brain and spinal cord injuries on motor imagery

The timing of mentally executed movements was measured in ten patients with hemiplegia, tetraplegia and paraplegia. In hemiplegic patients a significant difference in mental duration times was found between the paralysed and the normal "represented limb". The paralysed limb was mentally much slower than the healthy one. In contrast, movement times in tetraplegic and paraplegic patients did not differ from those in normal subjects. All patients reported a sensation of subjective effort accompanying the execution of the mental tasks. These observations are compatible with an outflow processing underlying motor imagery.

Effect of Codeine on the Sensations Elicited by Loaded Breathing

The present study examined the effect of codeine, a centrally acting opiate, on the respiratory sensations elicited in normal subjects by breathing to exhaustion against externally applied inspiratory threshold loads. Subjects were tested on two separate days following the double-blind, randomized administration of either placebo or codeine (90 mg). The intensity of the sensations of effort and discomfort experienced during two loaded breathing trials (a "high" load that was 73% of the maximum inspiratory pressure (MIP) and a "low" load that was 63% of the MIP) was evaluated using category (Borg) scores on each day of study. To verify that the dosage of codeine administered was sufficient to produce analgesia, we also determined the effect of this dosage on the time that subjects could tolerate immersion of one hand in ice water. Codeine altered neither the perceived effort nor the sense of discomfort associated with breathing against external loads and had no appreciable effect on the time to exhaustion during loaded breathing trials. This dose of codeine did, however, increase the time that ice water immersion could be tolerated and reduced the rate at which the sense of discomfort increased over time during ice water trials. These results indicate that, provided the pressure-time index of respiratory muscle contraction remains constant, analgesic doses of codeine alter neither the sensations elicited by loaded breathing nor the total time that breathing against a fatiguing inspiratory load can be tolerated.

Relationship between inspiratory drive and perceived inspiratory effort in normal man

1. To examine the relationship between the inspiratory effort sensation (IES) and respiratory drive as reflected by mouth occlusion pressure (P0.1) we have studied loaded and unloaded ventilatory responses to CO2 in 12 normal subjects. 2. The individual coefficient of variation of the effort sensation response to CO2 (IES/PCO2) between replicate studies was 21% and was similar to the variability of the ventilatory response (VE/PCO2) (18%) and the occlusion pressure response (P0.1/PCO2) (22%). 3. IES was well correlated with P0.1 (r greater than 0.9) for both free-breathing and loaded runs. 4. Resistive loading reduced the ventilatory response to hypercapnia from 19.3 l min-1 kPa-1 (SD 7.5) to 12.6 l min-1 kPa-1 (SD 3.9) (P less than 0.01). IES and P0.1 responses increased with resistive loading from 2.28 (SD 0.9) to 3.15 (SD 1.1) units/kPa and 2.8 (SD 1.2) to 3.73 (SD 1.5) cmH2O/kPa, respectively (P less than 0.01). 5. Experimentally induced changes in PCO2 and respiratory impedance were accompanied by increases in IES and P0.1. We found no evidence that CO2 increased IES independently of its effect on respiratory drive.

Respiratory sensation and pattern of respiratory muscle activation during diaphragm fatigue.

We have examined the relationship between respiratory effort sensation (modified Borg scale) and amplitude of the integrated surface electromyogram of the diaphragm (Edi, esophageal electrode), rib cage muscles (Erc), and sternomastoid muscle (Esm) during the development of diaphragm fatigue in five normal subjects. Three conditions were studied: run A: transdiaphragmatic pressure (Pdi), 65% Pdimax; esophageal pressure (Pes), 60% Pesmax; run B: Pdi, 50% Pdimax; Pes, 60% Pesmax; and run C: Pdi, 50% Pdimax; Pes, 20% Pesmax. During all runs there was a progressive rise in sensation, which was greater in runs A and B than in run C (P less than 0.05, analysis of variance). There was no difference between runs A and B. At the end of run C subjects did not report a maximal Borg score despite their inability to generate the target Pdi. The increase in sensory score with fatigue correlated highly with Esm/Esmmax and with Erc/Ercmax. There was no correlation between sensory score and Edi/Edimax. We conclude that the increase in respiratory effort sensation that accompanies diaphragm fatigue is not due to perception of increased diaphragmatic activation. It may reflect increased overall respiratory motor output not directed to the diaphragm.