The heaviness of the body and the feeling of falling downward are mostly caused by physical weakness. At this point, you need to consider that there is a large amount of physical work while eating a light diet, a large amount of mental work, lack of sleep, anxiety, emotional stress caused, and then consider pathological factors. A large amount of sweating and not enough food can lead to low sodium and low blood potassium, causing physical weakness. Diabetic patients suffer from disorders of glucose metabolism and negative nitrogen balance, leading to physical weakness. Long-term anemia leads to a decrease in the ability to carry oxygen within the blood and dizziness and weakness due to lack of oxygen in brain cells. There are also some endocrine diseases, such as hypothyroidism, which can also cause symptoms of mental depression and weakness of the limbs. The main symptom of myasthenia gravis is muscle weakness and continuous and rapid movements, which are more likely to cause limb fatigue, often with morning lightness and evening heaviness. Persistent weakness requires a hospital visit to clarify the cause.