Why is it difficult to exercise for someone having myasthenia gravis?

Muscle fatigue: Mg is a disorder of neuromuscular transmission. As you continue to stimulate a nerve for muscular contraction, the degree of contraction gradually diminishes as the muscle fatigues. This is due to the breakdown in neuromuscular transmission. If the muscle(patient) rests, it(they) can return to exercise. Patients with mg usually tell you they are stronger in the am but fatigue as the day progresses.

Myasthenia Gravis: Mg is a disorder characterized by weakness of skeletal muscles and rapid fatigue with exercise. Typically muscle weakness worsens throughout the day and improves with rest. Myasthenia gravis is a neuromuscular disorder. Neuromuscular disorders involve the muscles and the nerves that control them. http://www.ncbi.nlm.nih.gov/pubmedhealth/pmh0001731/.