Because the chronic fatigue syndrome (CFS) manifests itself primarily as an inability to carry out physically active tasks due to debilitating fatigue, skeletal muscle has been a focus of much CFS research attention. The majority of studies, investigating many areas of muscle function including morphometric analysis (15), central activation (8), muscle membrane function, excitation-contraction coupling (6), and muscle contractile strength (4), have failed to locate a consistent abnormality at the muscular level. Studies investigating potential dysfunctions in muscle metabolism have produced conflicting results (6,22) but with a degree of consensus that the mitochondria are the likely loci, if any such dysfunction exists. Indeed, recent observations of low plasma glutamine concentrations in athletes suffering from the closely related overtraining syndrome (17) and serum acylcarnitine deficiency in CFS sufferers (7) have both been associated with a possible mitochondrial dysfunction in the muscle. Such a dysfunction would logically give rise to a reduced aerobic capacity after development of CFS. However, previous reports of reduced aerobic capacity in CFS sufferers (16) could be entirely due to reduced muscle use, inactivity, and deconditioning (2). There has been speculation that many cases of CFS are misattributed and are in fact a result of hyperventilation. Nixon (13) proposed that hyperventilation, causing a depletion of the body's alkaline buffering system, and consequently a reduction of the anaerobic threshold, could lead to "effort syndrome"-a discrete subdivision of CFS. In this present study, the development of CFS in an elite ultra-endurance athlete who had previously undergone extensive physiological and performance testing, and his subsequent recovery, offered a unique opportunity for research into this area of CFS etiology.
Subject history. An elite ultra-endurance cyclist (age: 37 yr, height: 180 cm, weight: 81.4 kg), having given written informed consent, as required by the Human Ethics Committee of the University of Western Australia (UWA), underwent an incremental exercise test, described below, at the Human Physical Performance Laboratory, UWA 12 months before setting a world ultra-endurance cycling record-the "Pre-CFS" trial. Two years later, the same athlete was diagnosed by a qualified medical practitioner as suffering from CFS, as defined by criteria previously set out (18). The athlete reported unusual tiredness and fatigue, exacerbated by physical activity, muscular aches and pains, poor memory and concentration difficulties, headaches, influenza symptoms, insomnia, depression, and increased irritability. A series of blood tests failed to detect any coexistent conditions, and a muscle biopsy sample was obtained from the vastus lateralis muscle for examination by electron microscopy. The athlete had been unable to compete or complete any training for 7 months, was only able to work part time, and attributed the onset of the CFS symptoms to overtraining, overwork, a viral infection, or a combination of all three. However, according to diagnostic criteria, the subject could not be subtyped as postinfectious fatigue syndrome, because there was no laboratory evidence of the self-reported infection. The athlete completed an 18-point symptomatic rating questionnaire (Table 1), based on symptoms reported in a previous study investigating overload training in elite army soldiers (3). The incremental exercise test was repeated-referred to as the "CFS" trial-using the same experimental protocol as 2 yr previously. On returning to the laboratory a further 6 months later, the subject reported considerable improvement in his condition, and this was also assessed using the symptomatic rating questionnaire described above. At this time he did not fulfill the CFS criteria, and it was concluded that the athlete had begun to recover from the condition. He had begun to resume light training and was able to return to work full time. He was retested again using the same experimental protocol-the "post-CFS" trial.
Exercise test protocol. On all three occasions, the subject completed a discontinuous incremental exercise test (4-min work intervals separated by 1-min rest periods) carried out on a cycle ergometer (Monark). Pedal rate was set at 90 rpm, and the workload was increased in 45-W increments until volitional exhaustion was reached. After completion of each workload, a 100-μL blood sample was taken from the earlobe into a capillary tube for blood lactate (BLa) analysis (Accusport, Boehringer Mannheim). Throughout the test, heart rate (HR) was recorded using a PE3000 HR monitor (Polar Electro, Finland), and expired gas analysis was conducted using a metabolic trolley consisting of an oxygen analyzer (Applied Electrochemistry, Sunnyvale, CA), carbon dioxide analyzer(Applied Electrochemistry), and ventilometer (Morgan, Gillingham, U.K.). Anaerobic threshold (AT) was the point at which BLa began to increase nonlinearly during the incremental cycling test. AT was identified from a plot, on linear coordinates, of BLa versus workload for each test, by two independent judges who were naive to the source of the data.
Clinical assessment. The athlete's self-reported symptomatic ratings, at the CFS assessment, and at the post-CFS assessment are reported in Table 2. No data were available for the pre-CFS assessment.
Muscle biopsy. Electron microscopic examination of the muscle biopsy sample obtained at the time of the CFS trial revealed retention of the myofibrillar architecture within the majority of the muscle fibers, but with focal necrosis and myofibrillar disorganization in a few fibers. Glycogen and lipid content were reported to be within normal limits, and there was no evidence of any structural abnormalities within the mitochondria.
Exercise tests. The performance data obtained during all three exercise tests is presented in Table 3. Between the pre-CFS and CFS trials, there was a decrease in the maximum workload (Wmax) achieved (−11.3%), the maximum oxygen uptake (V˙O2max; −12.5%), and the anaerobic threshold (AT; −14.3%). However, there was no difference between tests in either maximum heart rate (HRmax) or peak BLa concentration(Table 3). When the CFS trial was compared with the post-CFS trial, there were further decreases in the athlete's Wmax (−7.9%), V˙O2max(−10.2%), and AT (−8.3%), whereas HRmax and peak BLa were again unchanged.
The athlete's heart rate (HR) at submaximal workloads increased by between 10 and 16 beats·min−1 at the CFS trial compared to the pre-CFS trial, whereas HR during the post-CFS trial was virtually unchanged from the CFS trial (Table 4). At submaximal workloads below AT, there were no differences in oxygen uptake (V˙O2) or minute ventilation(V˙E) between all three exercise trials. At the only submaximal workload that was above anaerobic threshold in all three trials (315 W), there was a large increase in V˙E (+13.9% and +12.4%) and a small decrease in V˙O2(−2.4% and −4.3%) in both the CFS and the post-CFS trials, compared with the pre-CFS assessment. These changes were consistent with the decreases in AT, and the leftward shifts in the BLa/workload curves during the CFS and post-CFS trials(Fig. 1).
Although rating of perceived exertion scores was not recorded, the athlete reported that the CFS trial was subjectively more strenuous than either of the other two tests. Furthermore, after the CFS exercise test, the athlete's fatigue symptoms were exacerbated, such that he required up to 18 hr of bed rest each day for the following 6 days. Conversely, after the pre-CFS and post-CFS trials, the athlete was capable of completing light training on the following day.
In this case study, we observed substantial decreases in an elite ultra-endurance athlete's performance and aerobic capacity between a pre-CFS exercise test and the same test repeated once he had developed CFS, evidenced by a reduced V˙O2max, Wmax, and AT. However, a third test conducted after the same athlete had shown indications of substantial, almost complete, recovery from clinical symptoms showed a further decrease in all three physiological measures. If the development of CFS was the cause of an impairment in aerobic metabolism, and hence the reduction in performance capacity, as has been suggested (22), an improvement at the time of the post-CFS trial may have been expected. However, the further decrease in performance capacity, even after the athlete showed clinical improvement, suggests an alternative mechanism. Because none of the observed changes in performance measures were reversed once the athlete showed indications of symptomatic recovery, it is suggested that these are not a direct effect of CFS per se, but a result of the detraining process in an elite athlete forced, by CFS, to live a relatively sedentary lifestyle.
Despite prolonged aerobic training over many years, previous studies have reported reductions in both maximal and submaximal exercise performance within only a few weeks of cessation of training (12). Studies into the effects of detraining in endurance athletes have reported up to 17% decreases in V˙O2max(20), 21% decreases in endurance capacity (9), and submaximal HR increases of 11 beats·min−1(5) after only 12 wk, 4 wk, and 2 wk of detraining, respectively. The reduction in aerobic capacity documented for the athlete in this study could be fully accounted for by the effects of 7 months of detraining after the athlete's onset of CFS, and a further 6 months of detraining during his recovery. Because there was little change in HRmax between the three exercise trials, the data would suggest that the decreases in V˙O2max may be mediated by a reduced stroke volume, and hence a decreased maximal cardiac output, as has been previously observed with detraining (10,12).
Decreases in skeletal muscle oxidative enzyme capacity (11,12,19), and consequently the mitochondrial adenosine triphosphate (ATP) production rate (21), have also been observed after detraining in endurance athletes. These changes are thought to be functionally related to accelerated lactate production during submaximal exercise (12) and would account for the observed decreases in AT, although a peak BLa of 5.1 mmol·L−1 may be considered quite low, even for an ultra-endurance athlete. Submaximal ventilation rates at all workloads below AT were unchanged between the three exercise trials (Table 4). This would tend to exclude hyperventilation as a contributing factor to the CFS symptoms in this athlete as has been proposed (13). Elevated ventilation at the 270-W and 315-W workloads during the CFS and post-CFS trials could be attributed to the normal response of increased ventilation at AT, because AT had been reduced through detraining. The shortfall in aerobic ATP production at these workloads in the CFS and post-CFS trials, evident from the slight reductions in V˙O2, would be made up by increased anaerobic metabolism and lactic acid production (Fig. 1), bringing about an increased ventilation response.
The electron microscopic examination of the muscle biopsy, obtained at the same time as the CFS trial, did not reveal any structural abnormalities in the mitochondria. This observation is in agreement with previous studies (14), whereas other researchers have highlighted mitochondrial abnormalities in CFS sufferers(1). Although nonstructural abnormalities cannot be excluded, these data would tend to suggest that a mitochondrial dysfunction, and hence impaired aerobic metabolism due to CFS, was unlikely to contribute to the debilitating fatigue experienced by this athlete. In agreement with previous studies (4,6), these findings suggested that following the development of CFS there was no apparent physiological basis, as defined by the measurements made in the present study, for the fatigue experienced by the athlete during either the CFS exercise test or the days following. Because the athlete's higher perception of exertion and fatigue during the CFS trial than both the pre-CFS and post-CFS trials does not seem to be attributable to any of the measured physiological mechanisms, this may be indicative of central, possibly neurological, factors influencing fatigue perception in CFS sufferers.
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