Perceived exertion is defined as “the act of detecting and interpreting sensations arising from the body during physical exercise” (20). Ratings of perceived exertion (RPE) represent a complex psychophysiological process that integrates several exertional symptoms and their underlying psychological and physiological mediators, resulting ultimately in a verbal report. For example, a person engaged in physical activity such as hiking receives input from the heart and lungs, active skeletal muscles, and other nonspecific signals (e.g., catecholamines and beta-endorphins), as well as information concerning previous experience with hiking and intended duration and intensity. The hiker uses the integration of this information to determine the amount of effort being put forth. Thus, the perception of effort during exercise represents an integration of information, where the self-reported changes in effort reflect the physiological and psychological processes that under some conditions induce fatigue (20).
The measurement of perceived exertion has proven useful in many clinical conditions including heart disease, diabetes, hypertension, and chronic obstructive pulmonary disease (7,21,25) where pharmacological manipulations used in treatment of the disease, or the nature of the disease itself, alters the normal physiological responses to exercise. Additionally, ratings of perceived exertion have provided insight into physiological processes that underlie clinical conditions and have been integral in exercise prescription (20). Little is known about the clinical usefulness of RPE in people whose primary complaint is severe fatigue.
Chronic fatigue syndrome (CFS) is a medically unexplained illness characterized by the primary symptom of severe fatigue. It is defined as a minimum of 6 months of continuous fatigue, which is severe enough to produce a substantial decrease in activity and which is accompanied by four or more of the following symptoms: impaired memory or concentration, sore throat, tender lymph nodes, muscle and/or multi-joint pain, headaches, unrefreshing sleep, and postexertional malaise (11). A diagnosis of CFS is based on the patient’s self-report of symptoms, after ruling out several potential medical explanations for their fatigue such as untreated hypothyroidism, anemia, HIV, or side effects associated with medications.
Previous researchers have reported that RPE are elevated in CFS (10,13,14,22) and that central dysregulation of the perception of effort contributes to increased fatigue and a decreased exercise capacity (10). In fact, some have argued that CFS is a disease of increased effort sense (10,17). However, methodological limitations have rendered this conclusion suspect. In particular, the majority of studies examining perceived exertion have done so in a nonstandardized fashion often using a single measure of exertion postexercise. Moreover, aerobic exercise studies of RPE in CFS have failed to account for differences in exercise capacity (13,14,22). Therefore, differences in submaximal exertion would likely be a function of differences in relative exercise intensity. Often, it is not clear what RPE scale was used or whether standard instructional sets were employed. These methodological shortcomings limit the strength of the conclusions that central nervous system dysregulation leads to altered RPE in CFS. Thus, there is a need for research that more thoroughly examines RPE in fatiguing illness.
The present investigation reports on RPE during exercise in female civilians with and without CFS. Based on the desire to control for differences in exercise capacity and the known linear relationship between RPE and %V̇O2max (20), we examined RPE in CFS as a function of percent peak V̇O2. We hypothesized that there would be no effect of CFS on RPE when the data were expressed relative to peak oxygen consumption. A companion paper follows, where we use this same approach to examine perceived exertion in Gulf War veterans with CFS to further determine the effect of illness related fatigue on RPE.
METHOD
Subjects.
The sample included 39 female participants (N = 19 CFS;N = 20 healthy). Participants with CFS were recruited from a large patient pool available through the New Jersey CFS Cooperative Research Center. Twenty subjects per group provides statistical power of 0.80 to detect a moderate to large effect (Cohen’s SD = 0.7) between groups for four repeated measures given a two-tailed alpha of 0.05 and a high correlation (r = 0.7 to 0.8) between repeated measures (27). All CFS patients met the current Centers for Disease Control case definition of CFS (11,15) and had no known medical causes for their symptoms. Only patients with an illness duration of < 6 yr and who had no major psychiatric diagnosis in the 5 yr before illness onset as assessed by the Quick Diagnostic Interview Schedule (Q DIS) (WSU School of Medicine, St. Louis, MO) were included. CFS patients were also excluded if they reported current use of steroids, antihypertensives, antibiotics, daily inhalers, or benzodiazepines. Sedentary healthy control subjects were also recruited from a control subject pool of the CFS Center. These subjects were matched for gender and age with the CFS group. “Sedentary” was defined as working in an occupation that did not require moderate-to-intense physical labor and not participating in physical exercise for more than one session per week. Control subjects were also excluded if they had a history of medical illness or a major psychiatric diagnosis in the 5 yr before the study as determined by the Q DIS, or were taking any medications other than oral contraceptives. Before participation, all civilian participants signed a consent form approved by the University of Medicine and Dentistry of New Jersey–New Jersey Medical School, Newark, NJ.
Experimental procedures.
Subjects reported to the human performance laboratory having abstained from nicotine and food for at least 3 h and from caffeine for 24 h. After informed consent, subjects completed a modified questionnaire that requested ratings of self-perceived fatigue, confusion, weakness, and achiness during the past 24 h on a Likert scale ranging from 0 “not at all” to 3 “much more than usual” (6). With this scale, a total fatigue score is obtained by summing the four Likert scores. CFS symptom severity was measured via a 16-item questionnaire containing each of the symptoms listed in the 1988 case definition for CFS (15). Each item of this scale is scored along a Likert scale ranging from 0 “not at all” to 4 “very severe.” Item scores were summed to give a total CFS symptom severity score.
Before testing, subjects were instrumented for monitoring of heart rate (HR) and metabolic responses to exercise. During exercise HR was monitored by ECG using the Q4000 electrocardiographic monitor, and metabolic variables were analyzed using a Q-PLEX I metabolic system (Quinton Instruments, Seattle, WA). Expired air was collected from the mouth via a two-way nonrebreathing valve (Hans-Rudolph, Kansas City, MO). Before each test, the Q-PLEX I system was calibrated for ambient conditions.
Ratings of perceived exertion.
RPE were obtained during exercise using a standardized 6–20 category scale (4). Before exercise, subjects were given standard instructions as to the proper use of the 6–20 scale. Briefly, subjects were instructed that the 6–20 scale would be used to determine: 1) “the intensity of effort, stress or discomfort felt during exercise”; 2) that all feelings and physiological cues should be integrated into an overall feeling of effort; 3) that each number represents a category of sensation that is ordered according to its intensity; and 4) that the verbal anchors should be used to help determine the level of effort at that particular moment. Subjects were also provided cognitive anchors at the high and low ends of the perceptual continuum. Specifically, consistent with Borg’s range model, participants were instructed the number 6 represents the lowest exertion imaginable and that the number 20 should be reserved for maximal effort or the greatest effort they could imagine (3). During the exercise test RPE were obtained during the last 15 s of each stage of exercise.
Maximal exercise testing.
Maximal exercise was performed on a motorized treadmill. Before exercise, baseline measurements were obtained during 4 min of seated rest. Each exercise stage was 3 min, and the exercise test began at 67 m·min−1 and no incline. For stage 2 of exercise, the treadmill speed was increased to 94 m·min−1. For the remaining stages, speed was kept constant and intensity was increased by raising the incline of the treadmill by 2% at each stage until the end of the test. Subjects were encouraged to give a maximal effort and to complete as many stages as possible. Subjects were instructed not to use the sidebars for support and were fitted with a safety harness that was suspended from the ceiling in case of a loss of balance. When the subjects indicated that they could no longer continue, the treadmill was stopped and lowered.
During the exercise test, cardiorespiratory and metabolic variables including oxygen consumption (V̇O2), carbon dioxide production (V̇CO2), ventilation (V̇E), and respiratory rate (RR) were measured breath by breath. Respiratory exchange ratio (RER) was determined by the ratio of V̇CO2 produced to V̇O2 consumed. Maximum effort was determined based on meeting at least two of the following criteria: 1) respiratory exchange ratio ≥ 1.1, 2) attainment of 90% of age-predicted maximum HR (220 − age), and 3) plateau or decline of V̇O2 despite increasing workload.
The gas exchange threshold (GET), a noninvasive index of the onset of exercise-induced metabolic acidosis, was determined using the V-slope method as described by Sue et al. (26). V̇O2 and V̇CO2 data were averaged for each 20-s period of exercise. Using plots of V̇O2 versus V̇CO2, two independent blinded researchers determined the first point of V̇CO2 to depart from linearity. The average corresponding V̇O2 was designated the GET. In the event of a >200 mL discrepancy, the average V̇O2 was calculated including a third researcher’s evaluation.
Statistical analysis.
Subject characteristics and peak exercise variables between experimental and control groups were analyzed using independent samples t-tests. Ratings of perceived exertion during exercise were analyzed between groups using ANOVA with repeated measures for exercise intensity. Self-reported fatigue was entered as a covariate to determine the influence of preexercise fatigue on RPE. For absolute exercise intensities (i.e., exercise stage), the data sets for both groups were complete for stages 2 through 5. Therefore, repeated measures analysis was conducted on these four stages. For relative exercise intensities, RPE were available for both groups at 60%, 80%, 90%, and 100% of peak exercise intensity. Therefore, the repeated measures analysis was conducted on these four intensities. Independent samples t-tests with Bonferroni corrections were used to determine differences in the event of a significant interaction term. Power functions for each subjects’ complete exercise test were determined by linear regression of log transformed (log10) perceived exertion versus log transformed exercise intensity. According to Borg’s modification of Steven’s power law [R = a + c(S − b)n], a resting value of 6, to account for the exertion required for standing, was used as the constant “a” in determining the power function [R − a = c(S)n] (2,3,5). One sample t-tests were performed to examine whether the group-averaged exponents were significantly different than a linear value of 1. Independent samples t-tests were used to examine group differences in the regression derived exponents and intercepts.
RESULTS
Subject characteristics.
Civilian females with CFS were characterized by a mean age of 34.0 ± 7 yr, height of 165.3 ± 6 cm, and weight of 67.9 ± 14 kg. Healthy civilian females were characterized by a mean age of 33.0 ± 7 yr, height of 161.9 ± 7 cm, and weight of 62.1 ± 12 kg. There were no significant differences in age, height or weight between the two groups.
Gas exchange threshold.
Select variables measured at the GET can be seen in Table 1. Subjects with CFS rated the exercise as more effortful than their controls (P = 0.003). There were no significant differences in oxygen consumption at the GET.
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TABLE 1: Variables at the gas exchange threshold.
Perceived exertion during exercise.
Ratings of perceived exertion at selected absolute exercise stages is presented in Figure 1a. The data are limited to stages 2 through 5 of treadmill exercise, intensities that all subjects in both groups were able to complete. ANOVA revealed significant main effects for group [F (1,37) = 10.8, P = 0.002] and exercise intensity [F (3,111) = 169.0, P < 0.001], as well as the group × exercise intensity interaction [F (3,111) = 3.6, P = 0.017]. Figure 1a illustrates that RPE increased as a function of each stage of exercise completed, and subjects with CFS reported higher RPE. Post hoc analysis revealed that CFS patient’s reported higher RPE at stages 3 through 5 (P < 0.02). Covariate analysis with preexercise fatigue entered into the model did not eliminate the group × exercise intensity interaction [F (3,108) = 4.3, P = 0.006]. Log transformed plots of RPE and exercise stage can be seen in Figure 2a. The average exponent for the CFS and healthy control groups were 1.3 ± 0.3 and 1.4 ± 0.6, respectively. There were no significant group differences and both exponents were significantly greater than a linear value of one (P < 0.02).
FIGURE 1: Mean RPE (± SE) as a function of absolute (a) and relative (a) exercise intensities for civilian CFS (N = 19) and healthy control (N = 20) subjects.
FIGURE 2: Log transformed plots of RPE as a function of absolute (a) and relative (b) exercise intensities for civilian CFS (N = 19) and healthy control (N = 20) subjects.
Ratings of perceived exertion at relative exercise intensities are presented in Figure 1b. RPE data for both groups were complete for 60%, 80%, 90%, and 100% of peak oxygen consumption. Repeated measures ANOVA revealed a significant main effect for exercise intensity [F (3,111) = 225.3, P < 0.001] but no significant group effect [F (1,37) = 1.9, P = 0.17]. When the data were expressed relative to peak oxygen consumption, civilians with CFS did not rate the exercise as being more effortful than healthy civilian controls (Fig. 1b). Log transformed plots of RPE and relative oxygen consumption can be seen in Figure 2b. The average exponent for the CFS and healthy control groups were 1.9 ± 0.8 and 1.9 ± 0.8, respectively. There were no significant group differences and both exponents were significantly greater than a linear value of one (P < 0.02).
Peak data.
Peak exercise variables can be seen in Table 2. In our sample, healthy controls exercised for a significantly longer time (P = 0.01) and had higher peak HR (P = 0.03) than civilians with CFS.
TABLE 2: Peak exercise variables.
DISCUSSION
The purpose of the present investigation was to examine RPE during exercise in civilians with CFS. Our aim was to derive some insight into whether perceived exertion is altered in fatiguing illness when clear instructions were provided and when the data were adjusted for potential differences in exercise capacity. Based on earlier work, we expected that RPE would be elevated in CFS when comparisons were made at the same absolute exercise intensities but that these differences would not remain when comparisons were made at the same relative exercise intensities. Consistent with these hypotheses, civilians with CFS reported greater RPE when compared by stage. However, when the data were expressed relative to peak V̇O2 the differences were no longer apparent.
Previous researchers have concluded that perceived exertion in CFS is dysregulated and contributes to the CFS patients’ symptoms of fatigue (10,13,14,22). Our results do not support this view. When RPE were expressed relative to a common maximum, the CFS group did not report greater RPE than controls. This finding challenges the assumption that central nervous system alterations in effort sense result in exacerbated feelings of fatigue during exercise in civilians with CFS. Our results also highlight the importance of making appropriate comparisons through the use of absolute and relative physiological reference criteria and employing standard procedures when examining RPE in CFS.
Studies that have examined RPE in civilians with CFS have generally failed to account for exercise capacity differences between CFS and control groups, or have relied on a single measure of exertion obtained postexercise (13,14,19,22). In the present investigation, the control group exercised an average of 4 min longer than the CFS group. Therefore, any comparison at an absolute exercise intensity would represent a greater relative metabolic strain for the CFS group (i.e., they would be exercising at a greater percentage of their capacity). Differences in exertion would be mistakenly attributed to having CFS when in fact they were most likely a function of differences in exercise duration. This argument is supported by the failure of preexercise fatigue ratings to eliminate the group differences at stages 3 through 5 of exercise and the near identical power functions obtained for the two groups. Thus, differences in exercise capacity and not symptoms of fatigue associated with CFS are the most likely explanation for the higher exertion ratings.
An alternative explanation might be that the CFS subjects were less economic exercisers, using greater oxygen resources at submaximal intensities and reaching volitional exhaustion at an earlier stage than the controls, while achieving similar peak oxygen consumption values. Our exercise protocol employing 3-min stages and modest increases in grade was intended to take our participants to exhaustion. Therefore, peak oxygen consumption values did not necessarily correspond with the exact end-point of exercise. Indeed, several control subjects were able to maintain exercising into part of the next stage without a concomitant increase in V̇O2. Examination of our exercise tests indicated that controls subjects were able to continue exercising for an average of 105 s post their peak oxygen consumption value whereas the CFS subjects averaged 57 s. The reason for the CFS subjects’ inability to maintain exercising cannot be directly assessed in the current project; however, CFS subjects did have a significantly lower heart rate reserve (peak HR − rest HR: CFS = 97 bpm vs controls = 108 bpm, P = 0.01). Limitations in heart rate reserve could contribute to poor exercise tolerance. Moreover, we have previously reported gait abnormalities during walking and running in CFS patients and sedentary healthy controls (1), and gait abnormalities could result in increased oxygen demand during submaximal exercise (9). Even so, poor exercise tolerance does not mean that perceived exertion is dysregulated in CFS. In fact, it further emphasizes the need for relative comparisons when examining RPE in CFS patients and not mistake limitations in exercise capacity as evidence for altered ratings of exertion.
For many of the studies in CFS, it is unclear what scale was used to assess RPE and whether standard instructional sets defining the construct of exertion had been administered (13,14,19,22). According to Borg’s perceptual range model (3,5), perceptual intensities are approximately equal when assessed relative to each individual’s subjective maximum, and interindividual comparisons can be made when the perceptual range is set equal for all individuals (for complete review on this topic see ref. 20). It is unclear whether this theoretical basis (3,5) was applied in the prior investigations. That lack of a standardized approach to the measurement of exertion could account for the differences that have been reported.
Our results are in agreement with previous work done at the New Jersey CFS Cooperative Research Center and one other study. Sisto et al. (24), using an incremental treadmill test to exhaustion, reported no difference in RPE between CFS and control groups when the data were compared at the same relative workload. Lloyd et al. (18), using intermittent, submaximal isometric exercise (30% of peak isometric torque) of the elbow flexors, reported no difference in RPE or relative torque between CFS patients and controls. They concluded that perceived effort could not account for subjective complaints of fatigue during exercise in CFS patients. Thus, for both large muscle aerobic exercise and small muscle dynamic resistance exercise, perceived exertion does not appear to be elevated in civilians with CFS when relative reference criteria are used. However, Sacco et al. (23) reported greater RPE in their CFS participants during sustained, submaximal isometric exercise (20% of peak isometric torque) of the elbow flexors. The differences between the results obtained by Sacco et al. and those by Lloyd et al. may be due to the type of exercise employed. Sacco and colleagues had their subjects sustain an isometric contraction until volitional fatigue while Lloyd and colleagues had subjects perform 6 s of isometric contraction followed by 4 s of rest. This suggests that CFS patients may have difficulty sustaining isometric efforts, resulting in elevated RPE relative to controls. Additionally, Lloyd and colleagues tested an all male sample whereas Sacco and colleagues had a mixed sex sample. Sex differences in RPE, which have been documented previously in healthy individuals (8), could account for the discrepant results.
It is unclear why we saw differences in RPE at the GET; however, these results are in agreement with previous work (24). Although the GET is considered a valid index of relative strain, we feel that the nonsignificant differences from 60% to 100% (five time points) of peak oxygen consumption are more compelling than a single measure obtained at one point during the test. Future studies that examine RPE across the entire range of exercise intensities are necessary to determine whether perceived exertion is altered at the lower ends of the exercise intensity continuum.
The results of the present investigation have implications for using RPE to design and monitor exercise programs to treat CFS. There are reports that exercise therapy is beneficial in terms of both psychological well-being and physical function in patients with CFS (12,28). However, data regarding how exercise is perceived over the course of treatment are lacking. If exercise is to be used as a treatment for CFS then understanding how effortful these patients perceive exercise is important. This has implications for adherence to an exercise program as well as ensuring CFS patients’ appropriately self-regulate exercise intensities and do not overexert themselves. This is underscored by data showing lower heart rate responses at equivalent relative exercise intensities in CFS (16). In accordance, our CFS subjects had significantly lower peak heart rates. Thus, heart rate may not be a good indicator of exertion in the CFS patient. To this end, it appears that for civilians with CFS, relative metabolic strain (%V̇O2max) is a good indicator of exertion.
In summary, consistent with previous reports CFS patients reported greater perceived exertion at absolute exercise intensities during treadmill exercise. Consistent with our hypothesis, when RPE were expressed relative to peak exercise capacity, civilians with CFS did not differ from controls.
Future research examining RPE in CFS using different exercise modes (e.g., treadmill, arm ergometry, or resistance exercise), intensities (e.g., maximal or submaximal exercise), and durations, as well as examining potential mediators (physiological and psychological) of perceived exertion is warranted. It is important for future studies to use validated RPE scales and standardized instructions. Appropriate anchors and distinctions between exertion and fatigue are crucial if potential biases are to be minimized. When possible, the data should be expressed relative to a common maximum to establish more valid comparisons between groups.
The authors wish to thank Theresa Policastro and Pamela Flippin for their assistance in subject recruitment, data collection, and the preparation of this manuscript.
This work was supported by NIH Center grant no. U01 AI-32246 and the DVA NJ Center for Environmental Hazards Research no. 561–003.
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