Having spoken with doctors and physical therapists who've dealt with CFS patients over the years, there's often a gap in understanding regarding exercise. They're accustomed to seeing deconditioning after a) a severe disease; b) physical trauma; or c) an extended period of sedentary lifestyle. In cases like this, there's always some submaximal level of effort (eg leg lifts with partial assistance by a therapist) that patients can start from to build up to functional again. This, then, is the narrative they are accustomed to, and seek to apply to CFS patients.
My experience with CFS (anecdotal, of course) for those who have milder cases (times when they can work part or full time, do household chores, but flare and might not recover to a normal daily activities level for months) is that clinicians need to be taught a new narrative, similar to the relapsing/remitting model of MS. Just like these MS patients have periods of reduced motor control during a relapse, which can't be forced to improve through willpower or by practicing a motor control activity, CFS patients have times where there is no submaximal, sustainable level of physical activity (or it can only be recognized in hindsight, after that threshold has already been passed). By the same token, a CFS patient's body will tell them which day they feel up to, for example, going for a walk, and it will usually be correct. This is often the start of the period when some stamina gains start to be seen again. The rapid return to a patient's normal levels of activity pre-flare (relapse, crash...) often puts the lie to deconditioning in the case of CFS, as it occurs much more rapidly than true deconditioning usually does. This would also similar to relapsing/remitting MS, where patients who simply could not attain their normal level of motor control regain most or all of their pre-relapse level again.
This is, of course, part of what makes CFS so hard to research and treat. Like any mechanic or tech support worker will tell you, it's almost always the intermittent problems that are the toughest to fix, because you can't take a defined set of steps to define the issue, or to fix it. Studies like this are going to capture the patients in remission mode, akin to taking the car to your the mechanic on the day it's not making that noise.
I really believe that, in a remission phase, there is a regular (even if it is only once a week) level of physical exertion that can improve stamina and exercise capacity over time. However, it won't be in the form of a standard PT exercise plan. Instead, it would need to be more like sessions with a long-term personal trainer, or primary caregiver. This person would have to be able to help a patient set realistic goals and have in their repertoire a series of tools to achieve them, like a PT would. However, this would also need to be someone who has also spent a lot of time with the patient, has developed a rapport with them and a familiarity with their body, and who can both adjust the plan as a patient expresses concerns vocally, and recognize subvocal cues when the patient is at or past their threshold. Personalized treatment like this, while advocated by specialized CFS clinics, is neither easy nor cheap. As a result, until the underlying molecular mechanisms which cause CFS are understood, we will always have studies using these cookie cutter approaches to rehabilitation.
