Heavy and weak legs are commonly caused by muscle strain, intermittent claudication, low potassium periodic paralysis and so on. 1. Muscle strain: commonly caused by excessive exercise, such as hiking, long-distance running, etc. A large amount of exercise results in excessive production of lactic acid in the muscles, and the lactic acid that is not metabolized within a short period of time accumulates in the muscles, causing muscle soreness, weakness and heaviness. 2.Intermittent claudication (1) Neurogenic intermittent claudication: mostly seen in middle-aged and old people, due to spinal stenosis or lumbar intervertebral disc herniation, compressing the nerves that innervate the lower limbs, causing intermittent claudication, especially seen after long time walking, sitting or standing for a long period of time, which results in soreness and heaviness in the legs. (2) Intermittent claudication of vascular origin: commonly seen in occlusive vasculitis or secondary to atherosclerosis, causing occlusion of the arteries of the lower limbs, with prolonged sitting, prolonged standing or prolonged walking, there is a situation of heaviness and weakness of the affected limbs, which is often accompanied by weakening of dorsalis pedis arterial pulsation, coldness of the lower limbs and other symptoms. 3. Hypokalemic periodic paralysis: characterized by periodic episodes of delayed paralysis, the weakness and heaviness of the limbs often start from the lower limbs and gradually go up and involve the upper limbs. There are many reasons that cause heavy legs and fatigue, it is recommended to consult a doctor in time to clarify the cause of the disease and then carry out targeted treatment.