Muscle fatigability

Muscle fatigability in ME is a symptom in which muscles become weaker after minor exertion and a long period (3-5 days or longer) may elapse before full muscle power is restored. According to Melvin Ramsay, it is the defining feature of myalgic encephalomyelitis, without which a diagnosis of ME should not be made, though this symptom is noted to improve during remission. Similar muscle effects are known to occur in other neurological diseases such as multiple sclerosis and post-polio syndrome.

Possible causes of muscle fatigability in ME/CFS
Muscle biopsies have shown evidence of mitochondrial degeneration, deletions of mitochondrial DNA , and the reduction of mitochondrial activity.

In addition, evidence of oxidative damage to muscles has been found in CFS.

Studies have found reduced levels of serum carnitine which return to normal after recovery and correlate with symptom severity.

Exercise has also been found to induce both early and excessive lactic acid formation in the muscles with a reduced intraceullar concentrations of ATP and acceleration of glycolysis.