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Chronic fatigue syndrome is a complex condition characterised by fatigue on relatively trivial exertion, accompanied by various other complaints including myalgia, impaired memory and concentration, depressed mood, and anxiety. 1 The aetiology and pathophysiology of the disorder remain uncertain, although there is some evidence for the importance of both infectious and psychosocial factors in causation. 2-4 Despite the subjective experience of muscle fatigue and pain, neurophysiological investigations have shown normal muscle strength and function in chronic fatigue syndrome; however, patients have reduced exercise tolerance and consistently report increased perception of effort at maximal and submaximal levels of exertion compared with healthy controls. 5-8 This suggests CNS dysfunction, as do several reports of disturbed cognitive performance in chronic fatigue syndrome. 9-13 The nature and extent of cognitive impairment is uncertain as memory, information processing, and attention may all be affected, but there is a gradually emerging consensus that patients with chronic fatigue syndrome may show particular deficits in effortful cognition. 11-13 Further research is therefore necessary to examine the extent to which cognitive performance and effort may be linked to physical exertion in this syndrome.
We have recently proposed that chronic fatigue syndrome is best understood as a primary disturbance in the sense of effort accompanying physical and cognitive exertion. 14 This hypothesis is in keeping with patient reports of symptom exacerbation after motor or mental activity and predicts that greater deficits will be apparent during the performance of controlled than during automatic tasks. However, as many patients with chronic fatigue syndrome also satisfy criteria for depressive disorders, 4 and abnormalities on effortful cognition may also be found in depression, 14 any test of this hypothesis must also include depressed patients. We therefore examined the muscular, cognitive, symptomatic, and effort perception responses to physical exertion in patents with chronic fatigue syndrome, patients with major depressive disorder, and healthy subjects. Our specific prediction was that patients with chronic fatigue syndrome would perform effortful cognitive tasks less well after exercise than the control groups, and that these deficits would be associated with increases in fatigue and effort perception.
Methods
We recruited 10 ambulant patients satisfying criteria for chronic fatigue syndrome 1 from a local infectious diseases clinic, 10 patients with unipolar major depressive disorder 15 from a local...