I started looking at the paper closely and I've got to say that it's a mixed bag. The discussion is a lot more nuanced than the abstract. Here are some notes I started. I'll park them here in case I don't get back to the job.
Selection criteria
The paper uses the Fukuda criteria to select the CFS cohort. In this criteria, post-exertional malaise (PEM), the defining feature of ME/CFS, is not required. Therefore it is impossible to know how many of the participants did in fact have ME/CFS. Although this paper was written in 2011, and the centrality of PEM in ME/CFS symptomology has been better described since then, researchers are still using the Fukuda criteria. The AHRQ in 2015 (from memory) noted that papers with Fukuda selected cohorts did not provide useful information about ME/CFS.
Short time period for activity monitoring
While wearable activity monitors are much better than a recall survey, this study only had participants record activity for 7 days. Monitoring, even if only a step count, over a much longer time period, perhaps 3 months, is required to ensure that any Hawthorne effect is minimised. It is likely that people who are monitored make an effort to be more active than normal initially, but this will not be sustained. People with ME/CFS are particularly unlikely to be able to sustain a high level of activity.
Fatigue Impact Scale (FIS)
This scale is likely to have resulted in people with a wide range of fatigue severities being assessed as having the same amount of fatigue. This scale gives options from 0 ‘no problem’, to 4 ‘extreme problem’ for 40 questions. Most people with ME/CFS who are able to participate in a study are unlikely to respond to many questions with a ‘4’, as there is still some capacity. However, to receive a diagnosis of CFS, it is necessary to have a substantial reduction in capacity (50% in some criteria), and so they are unlikely to respond to many questions with ‘0’ either.
Furthermore, someone only moderately incapacitated is likely to feel the incapacity is a significant problem compared to their previous level of function and may score the incapacity the same as someone more profoundly disabled. For example, with the question ‘I have to reduce my workload or responsibilities’, someone who can now only go to work three days a week might choose 3 ‘a big problem’. Another person who has given up working entirely, but is still able to help in the care of their child might also choose ‘3’, comparing it to a period when they were completely bedbound.
The scale therefore lacks sensitivity and does not accurately measure the severity or frequency of fatigue. It is likely that the participants’ scores were clustered, and not indicative of the level of fatigue.
It is good to see that the authors also realised that the FIS was not a good measure of fatigue
We were surprised to find that there was no relationship between increased fatigue and reduced habitual physical activity, which is in contrast to other studies.20 We would suggest that this is explained by a ceiling affect in the symptom assessment tools i.e. a relatively low symptom burden leads to significant impact upon physical activity levels.
In addition, there are issues with some questions not, or not necessarily, relating to fatigue. For example, a question about physical discomfort increasing says very little about fatigue and much more about pain. A question about clumsiness might be rated highly if there are balance issues.
Relationship between fatigue and activity level
The issues with the activity monitoring period potentially being too short and the fact that the FIS doesn't provide a good measure of fatigue are enough to explain why the study found no relationship between fatigue and activity level.
But, there is another problem. There is an implicit assumption that CFS is, as the name implies, all about fatigue and that people who have more fatigue will have lower levels of activity . Actually, the illness involves fatigability and PEM.
If a person reduces their activity level below a certain level, they may be able to avoid 'fatigue'. Therefore, a person with a low level of activity may have a low level of fatigue, while a person with a high level of activity may have a high level of fatigue. It is also possible that someone experiencing PEM or a period of a flare may have a high level of fatigue and a very low activity level.
It is therefore unrealistic to expect a close relationship between fatigue and activity across a cohort of people with ME/CFS.