I was playing around with a medical dictionary to try and come up with an alternative for 'fatigue' which is more or less the common denominator in all ME/CFS diagnostic criteria. Note: I was not trying to invent a new name for the illness! Anyway, having come up with a couple I thought I'd better do a search and see if they were currently used. Dynapenia came up. From the dictionary I used it means 'energy deficiency'. It is a relatively new term being used in research related to aging. Wikipedia has an entry (most of it is waffle), but the biology bit is interesting: "Studies involving the dissection of cadavers have uncovered a 43% decrease in the cell body size of neurons found in the premotor cortex compared to those of younger adults. New studies have recently verified this finding in living subjects using high resolution magnetic resonance imaging (MRI). Recent evidence suggests that aging is also related to the loss of myelinated nerve fiber length and the mass of white matter, with individuals losing approximately 45% of total nerve fiber length as they age.  These changes that develop through the aging process affect the connectivity of the cortex within itself as well as its connectivity to the rest of the central nervous system. Another potential contributor to dynapenia is a disruption of the process that converts electrical signal given for muscular activation into an actual contraction. Particularly, impairments in the release of calcium (Ca2+) from the sarcoplasmic reticulum have been suggested to explain why decreased muscle quality is so prevalent in older adults. It has also been suggested that nutritional factors may contribute to the onset of dynapenia. It has been proposed that low levels of Vitamin E, carotenoids, and selenium  are associated with lower levels of muscle strength. The activity of Vitamin D receptors on muscle has been found to decrease with aging. These receptors initiate the nuclear response leading to De novo synthesis of proteins. However, the results of multiple studies on the relation between Vitamin D and muscle strength have been highly controversial, making the effect of Vitamin D on muscle strength in need of further investigation." The researchers ( who initially proposed use of dynapenia in 2008) update in 2012; "There is evidence to suggest that dynapenia is, to some extent, attributable to neurologic mechanisms." "Interestingly, over the past couple of years, there have been several reports suggesting a link between muscle weakness and cognitive decline" https://academic.oup.com/biomedgerontology/article/67A/1/28/583360 there is a lot of scientific stuff which maybe someone else would like to look at. The actual condition as described may not be applicable, but the 'effect' sounds much closer to what I for one experience as opposed to 'fatigue'.