"The results suggest that post-exertional malaise [PEM] is composed of two empirically different experiences, one for generalized fatigue and one for muscle-specific fatigue."
So said the findings reported by Stephanie McManimen and colleagues  looking at one of the most common and debilitating aspects of chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) and how rough-and-ready generalised descriptions often do little to reveal the complexities of this particular symptom.
The research group at the centre of this new paper based at DePaul University under the stewardship of Leonard Jason have some previous interest in this facet of ME/CFS as per other recent publications  talking about the trials and tribulations of defining PEM. Once again, the very multiple and very complicated ways that CFS/ME is currently diagnosed (see here for example) comes into play as the wording used to define PEM seems to count in terms of who is more or less likely to display this symptom. I might also add that even the words 'post-exertional malaise' have been subject to question and redefinition as per the use of the term PENE (Postexertional neuroimmune exhaustion) .
In their latest paper, McManimen et al set out to "discern whether post-exertional malaise is a unified construct or whether it is composed of two smaller constructs, muscle fatigue and generalized fatigue." As per the opening sentence of this post, researchers came down on the side of PEM meaning more than one thing, and with it the possibility of further revisions being required to the criteria to define this concept. Indeed, going back to yet another paper from this group  researchers suggested that several composite items might better define PEM over just one question: "Dead, heavy feeling that occurs quickly after starting to exercise; Next day soreness or fatigue after non-strenuous, everyday activities; Mentally tired after the slightest effort; Physically drained or sick after mild activity; and Minimum exercise makes you physically tired." Words, in a diagnostic sense, need to be used accurately.
And finally, whilst on the topic of CFS/ME, there have been some further developments around the science of CBT and graded exercise...
 McManimen SL. et al. Deconstructing post-exertional malaise: An exploratory factor analysis. J Health Psychol. 2016 Aug 24. pii: 1359105316664139.
 Jason LA. et al. Problems in Defining Post-Exertional Malaise. Journal of prevention & intervention in the community. 2015;43(1):20-31.
 Carruthers BM. et al. Myalgic encephalomyelitis: International Consensus Criteria. Journal of Internal Medicine. 2011;270(4):327-338.
 Jason LA. et al. Fatigue Scales and Chronic Fatigue Syndrome: Issues of Sensitivity and Specificity. Disabil Stud Q. 2011 Winter;31(1). pii: 1375.
McManimen SL, Sunnquist ML, & Jason LA (2016). Deconstructing post-exertional malaise: An exploratory factor analysis. Journal of health psychology PMID: 27557649