(A) Myasthenia gravis # Fatigue: Any disorder that causes muscle weakness can also bring on fatigue, which in this context means an inability to maintain a force.> With weak muscles, individuals cannot sustain physical activities for very long; they become excessively tired.> After all, they are attempting to perform the same movements with fewer or damaged muscle fibers.> Such fatigue occurs in a variety of conditions: Myasthenia gravis, disorders altering the energy production of the muscles, such as mitochondrial myopathies and many of the myopathies that cause a significant loss of muscle (such as muscular dystrophy). Fatigue from myopathies is different from the excess tiredness or lack of energy that many individuals experience, although people may confuse these conditions. Tiredness usually also involves feeling sleepy and having difficulty concentrating.