Decomposition of Fatigue

One thing I noticed over the years was that lots of low intensity activities spread throughout the day, like running errands at leisurely pace, made me tired like hell and sleep like a baby, yet did not trigger the post-exertional sickness. ('Lots of activities' for CFS patients, of course, a relative term that really means 'a few activities'). On the other hand, a sustained, high intensity activity, even for a short duration, can result in a post-exertional sickness. Taking 2 more steps per minute on a 1 km walk, for example, reliably produces post-exertional sickness for me even though the total calorie expenditure is substantially less than running errands for several hours. These led me to think that there are different types of fatigues.

The first and the most obvious is the fatigue that occurs immediately from the exercise. Muscle activity produces metabolites and myokines. They act on the nerve system to produce the fatigue sensation. You usually recover this fatigue within a few hours. (A short nap after a long slow walk does wonders for me).

Exercise can also produce cellular damage from oxidative stress and micro-tear of the muscle fibers. People get tired at the end of the day and needs sleep to restore their bodies from this damage they accumulated throughout the day. The repair process entails inflammation and that brings about the sensation of fatigue. CFS patients, being sensitive to inflammation, feels this fatigue at much lower level of activities. Mild/moderate form of this fatigue usually resolve within 48 hours.

Finally, there is the extreme fatigue and weakness of post-exertional sickness. Metabolites and pro-inflammatory myokines produced by working muscles manifest as the headwind to the nervous system, and the nervous system must work harder against it to maintain the performance. The CNS of CFS patients, being hyper-sensitive to this headwind, must work even harder. When the effort goes above the threshold, the post-exertional sickness results. This is a neural fatigue that can last 96 hours or more.

In summary, the fatigue is composed of: 1) exercise fatigue; 2) post-exercise fatigue from the tissue /cellular damages 3) CFS neural fatigue in the form of post-exertional sickness.  These fatigues can occur in any combination, producing multiple different (temporal) patterns for recovery from them.  For example, heavy activities can produce an immediate fatigue followed by post-exercise fatigue the next day, typically involving 'happy feeling'. This fatigue then can lead right into post-exertional sickness lasting several days. (This, btw, gives the illusion of 48 hour delay, instead of the usual 24 hour delay, of post-exertional sickness). Or, you could go directly to the post-exertional sickness the next day after a higher intensity activity of shorter duration. With lots of low intensity activities, on the other hand,  you may recover from the post-exercise fatigue after 48 hours without the ensuing post-exertional sickness. Lesser exertion may produce only the immediate exercise fatigue, but not the post-exertional fatigue or sickness.

(Severely ill patients probably don't get the opportunity to get post-exercise fatigue. They are so sensitive to exertion, they go right into the post-exertional sickness at an activity level).

The common denominator of all these fatigues is the inflammation. Muscle activities immediately produce metabolites and pro-inflammatory cytokines. Tissue damages also results in inflammation. These inflammations apply brake and CFS patients end up with post-exertional sickness when they apply more effort than they should against this brake.

(Though the Columbia University study concluded that inflammation levels in CFS patients are not explained by the symptom severity, that could be because the sensitivity to inflammation, rather than inflammation itself, is causing the symptoms.)

So, what's the point of this decomposition of fatigue? For now, it helps me to explain the varying temporal patterns of fatigue that I have observed over the years. But it may also help me to quantitatively predict better how I am going to feel in days following activities. Eventually, I hope.