According to researcher Dr. Dane Cook, we still don't know what the scientific relation is between (aerobic) "exercise intolerance" and "PEM/exertion intolerance":
There is currently a theory circulating that tries to link (aerobic) "exercise intolerance"
to "PEM/exertion intolerance"
, by invoking a phenomenon called "ischemia-reperfusion":
- "Ischemia" simply means a restriction of blood to a tissue, such as a muscle.
- "Reperfusion" simply means the restoration of blood flow to that tissue.
- When blood flow is abnormally restricted to a tissue, such as a muscle, and then the blood flow is restored to that tissue, it results in an "ischemia-reperfusion injury".
- An ischemia-reperfusion injury can have different effects on different tissues.
Specifically, the currently popular "ischemia-reperfusion hypothesis
" proposes that PEM is caused by temporarily restricted blood flow to tissues, due to aerobic exercise intolerance (or other problems with blood circulation). Unfortunately, this "ischemia-reperfusion hypothesis" falls flat upon further inspection.
Specifically, the symptoms of PEM are nothing like the symptoms of ischemia-reperfusion injuries, which have been well-studied. Whereas PEM symptoms are generalized and varied, ischemia-reperfusion injuries tend to produce localized, well-defined symptoms.
For example, ischemia-reperfusion injuries in limbs
can produce myalgia and paresthesias, which is something like Delayed-Onset Muscle Soreness (DOMS) with temporary Peripheral Neuropathy (PN). On the other hand, PEM presents with a profound muscle weakness called paresis. Inflammation markers in limb ischemia-reperfusion injuries may be elevated, whereas inflammation markers in PEM are not reliably elevated.
An ischemia-reperfusion injury to the brain
would produce stroke-like symptoms, as it is essentially a Transient Ischemic Attack (TIA). Although cognitive exertion can trigger PEM, it is not at all clear how cognitive exertion could possibly produce a TIA.
Therefore, PEM/exertion intolerance is not at all similar to the effects of ischemia-reperfusion injuries.