Mainly because of the increase of body fat, long-term intake of too many calories can not be completely consumed in the body, and excess calories are converted into fat accumulation in the body, leading to obesity. Another reason is the lack of proper arm movement. Bai Shucheng introduced that in daily life, the arm is the most active part, but most of the stretching direction is only the front or side. Because there is less movement in the back, the inner arm part is easier to relax. Moreover, the unused parts of muscles are very easy to accumulate fat, especially after the age of 25. In any case, to have strong muscles, you must cover everything.