N Jenkins, R Belnave
Faculty of Health Sciences, The University of Sydney
Effort Perception in CFS
Chronic fatigue syndrome (CFS) is a disorder which presents with the characteristic symptom of an extreme sense of fatigue that is exacerbated by low levels of activity. Studies to date have largely focused on the skeletal muscle in an attempt to find an explanation for the symptom of fatigue. The failure of these studies to demonstrate any consistent abnormalities of the musculature has led to the hypothesis that the fatigue of CFS may represent a perception of increase effort associated with muscular work rather than an inability to perform the required work.
The perception of effort in CFS has been investigated in a small number of studies utilising ratings of perceived exertion (RPE) as the measure of perceived effort. The findings of these studies have been inconclusive. Furthermore, the process of assigning a score to a particular level of effort would involve reference to the subject's normal experience of perceived effort. As the normal experience of effort in individuals with CFS is likely to be very different to that of the healthy population, subjective measures of perceived effort (such as RPEs) may not detect any differences in effort perception that are present in sufferers of CFS.
In the current study, effort perception was investigated in eight subjects fitting the established diagnostic criteria for CFS (Fukuda et al, 1994) an in ten healthy controls by means of a weight matching task which provides an objective measure of perceived effort. All subjects selected weights ("matching weights") lifted by the elbow flexors of their non dominant arm which 'felt the same' as a weight ("reference weight", ~5% max) lifted simultaneously by their dominant elbow flexors. As the sense of effort associated with lifting the reference weight is the primary source of information used by subjects to select the matching weights, the matching weights provided an indicator of the subjects' perceived effort. After three practice matches, all subjects performed twelve matches ("baseline series"). Another ten matches were then completed between each of which the subjects supported a 30% maximum weight for one minute with the reference arm ("fatigue series"). The protocol was completed again 24 hours later.
The CFS group demonstrated a significantly greater upward trend of matching weights across the fatigue series than did the control group. This indicates a greater increase in the perceived effort associated with an equivalent amount of muscular work for the CFS subjects as compared to the controls. It is most likely that this altered effort perception stems from an abnormality of the central nervous system (CNS). As the neural circuits responsible for the perception of effort have not yet been identified, however, it is not possible to implicate a specific region of the CNS in producing this altered effort perception.
In addition, RPEs collected in parallel with the weight matching task failed to demonstrate the differences in perceived effort between the CFS and control subjects. This suggests that subjective reported scores of perceived effort, such as RPEs, are insensitive to the altered effort perception in CFS.
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