Journal: Journal of Chronic Fatigue Syndrome, Vol. 11(4) 2003, pp. 19-32 Authors: Karen Wallman, BSc (Hons); Carmel Goodman, MBBCH, FACSP; Alan Morton, EdD, FACSM; Robert Grove, PhD; Brian Dawson, PhD Affiliation: Karen Wallman, Carmel Goodman, Alan Morton, Robert Grove, and Brian Dawson are affiliated with Department of Human Movement, University of Western Australia, Nedlands, 6008, Western Australia, Australia.
ABSTRACT. Use of maximal aerobic exercise testing in a chronically ill population may not only deter potential subjects from participating in trials, or returning for repeat trials, but may also result in the exacerbation of symptoms related to CFS. The Aerobic Power Index represents a submaximal exercise test that forms the aerobic component of the TriLevel Fitness Profile. This incremental bike test has a predetermined termination point based on a target heart rate (THR) of 75% of age predicted heart rate maximum, making successful completion of the test more likely in chronically ill subjects. The aim of this study was to determine reliability of the Aerobic Power Index in 20 CFS subjects. Results for the17 subjects who reached THR for both trials, demonstrate high reliability for watts per kilogram and oxygen uptake (mI*kg^-1*mm^ -1 ), as demonstrated by an intraclass correlation coefficient (ICC) of .97 and .91, respectively, while RPE resulted in moderate reliability (ICC = .87). The results of this study indicate that the Aerobic Power Index is a reliable submaximal test for use in a CFS population.
KEYWORDS. Chronic fatigue syndrome, maximal, submaximal, reliability
INTRODUCTION Chronic fatigue syndrome (CFS) is a debilitating disorder characterised by persistent and relapsing fatigue that has not been satisfactorily explained by any known chronic medical or psychological condition (1). Symptoms are generally not alleviated by rest and are often exacerbated by physical or mental activity (1). The cyclical nature of CFS is a further perplexing aspect of the disorder, with patients often reporting symptoms that fluctuate regularly over periods of days or even weeks (2-4).
Research investigating CFS has intensified in recent years with many studies employing exercise tests designed to assess physical function in subjects suffering from this disorder. Typically, assessment of cardiorespiratory function has involved the use of maximal oxygen consumption (VO2 max) testing, which is generally recognised as the gold standard method for this measurement (5,6). While a plateau in oxygen uptake during the last two stages of an incremental maximal exercise test represents the classic criteria in achieving VO2 max (7), this is rarely achieved in healthy, motivated subjects (5), let alone in sedentary subjects (8). Therefore, researchers often apply secondary criteria in order to determine whether a maximal effort has been made. These secondary criteria typically involve the attainment of specified physiological responses generally related to blood lactic acid, respiratory exchange ratio (RER), and heart rate. Studies in CFS that employed secondary criteria have often reported that many subjects had difficulty in reaching required endpoints. For example, Bazelmans et al. (9) found that only 1 of 20 CFS subjects was able to meet a physical criterion relating to the attainment of a target heart rate (THR) based on age-predicted HRmax. Another study by La Manca et al. (10) reported that only 16 of 20 CFS female subjects reached a THR of 90% of age predicted HRmax, while a later study by Inbar et al. (7) reported that none of the enrolled 15 CFS subjects were able to reach an identical THR. Further studies by De Becker et al. (11) and Fischler et al. (12) used a lower THR consisting of 85% of age predicted HRmax, and reported that many CFS subjects (37% and 50%, respectively) were unable to achieve this target. Another study by Montague et al. (13) that did not measure oxygen uptake but employed an incremental cycle test that terminated when a THR of 85% of age predicted HRmax was achieved, reported that only 4 of the 31 CFS subjects achieved this target.
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Questions arise regarding the suitability of employing maximal exercise testing in a population characterised by debilitating physical fatigue, particularly when this fatigue is reported to be exacerbated by physical activity. This concern is reinforced by Lapp (14), who reported that 74% (23/31) of subjects diagnosed with CFS experienced worsening fatigue after VO2 max testing that resulted in an average relapse period of 8.82 days. Other researchers also reported increased sensations of fatigue in CFS subjects ranging from 24 hours up to 2 weeks after maximal exercise (10,15-18). Another important aspect to consider is that employment of an exercise test that pushes subjects to their physiological limits can be intimidating to some potential subjects and may deter them from participation. Consequently, research studies in CFS that employ VO2 max testing may attract only the more robust/healthier subjects, resulting in the exclusion of a vast and perhaps more representative section of the population under study from being assessed. This notion is supported by De Becker et al. (11), who note that failure to assess more severely affected patients may result in a disparity in study conclusions regarding the exercise capacity of CFS patients.
A further problem with the employment of VO2 max testing in a CFS population relates to research that requires repeat testing, such as studies that evaluate therapeutic interventions. If some CFS subjects suffer a relapse or even discomfort after their initial exercise test, then this may discourage them from participating in further trials, resulting in high attrition rates. One study that required subjects to return for repeat VO2 max testing reported 21 dropouts from an original cohort of 135 subjects after initial testing, followed by a further 18 dropouts prior to the third testing session (19).
An alternative to maximal testing is sub-maximal exercise testing. While sub-maximal exercise tests have been reported to produce a 10-15% error in predicting VO2 max in healthy subjects (20), they are considered sufficiently accurate for classifying fitness ability and for monitoring changes to fitness over time, as long as limitations associated with this form of testing are minimized (6). Limitations associated with sub-maximal testing generally relate to diurnal and daily heart rate fluctuations, learning effects associated with repeat testing, and the use of generic maximal heart rate formulas that do not cater for individual differences (6,20,21). If efforts are made to minimize limitations, submaximal exercise testing may represent a suitable and safe alternative to the use of maximal testing in a CFS population.
A sub-maximal exercise test that may be suitable for use in a CFS population is the Aerobic Power Index. The Aerobic Power Index represents the aerobic component of a series of three tests that comprise the Tri-Level Fitness Profile (22) and is a modification of the PWC-170 (physical work capacity at a heart rate of 170 bpm) sub-maximal test. The Aerobic Power Index test differs to the PWC170 in that it consists of gradual increments in power output that terminate when a predetermined THR based on 75% of individual age predicted HRmax is achieved. Use of a THR based on 75% of age predicted HRmax may encourage more CFS subjects with varying exercise capabilities to participate in research trials, as well as increase the chances of subjects reaching a THR criterion. The average duration of the test is 7 minutes, but can range from 2 to 12 minutes depending on the subject's level of fitness (21).
While generally not used to predict VO2 max, the Aerobic Power Index has established normative values for both males and females of varying ages and fitness levels. Reliability studies of the Aerobic Power Index have reported highly reliable results in a small group of trained subjects (22), while a recent study that assessed 20 healthy sedentary subjects also reported high reliability with an intra-class correlation of 0.98 for test scores expressed in W*kg^-1 (23). Associated physiological and psychophysical variables were also assessed in this test-retest of healthy sedentary subjects, with results demonstrating high reliability for both oxygen uptake (mI*kg^-1*min^-1) (ICC = 0.974) and rate of perceived exertion (RPE) (ICC = 0.915).
To date, reliability of the Aerobic Power Index has not been demonstrated in a chronically ill population. Therefore, the purpose of this study was to assess the reliability of the Aerobic Power Index component of the Tri-Level Fitness Profile, and associated measures of RPE and oxygen uptake (ml*kg^-1*min^-1) in a CFS population, as well as to determine if subjects could successfully achieve a THR criterion.
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