There are several reasons for the thickening of the arms: First, the epiphysis of the human body gradually ossifies, that is, the normal physiological process of bone growth, the arms will become significantly thicker. Second, due to the usual arm activities more, such as frequent exercise or lifting heavy objects, which leads to a gradual increase in muscle strength, muscle fullness further increase, the arm will become thicker. Third, due to the increase in subcutaneous fat, mainly due to the amount of food usually eaten, especially the calorie-containing ingredients are higher. In addition, the lack of activity will lead to more fat distribution in the human body under the skin, which will also lead to the gradual thickening of the arms. Fourth, due to injury to the arm, resulting in small blood vessels with rupture and bleeding, gradually forming swelling and inflammatory edema, will also lead to thickening of the arm.