When doing flat support or side flat support, the tendon on the side of biceps brachii close to the forearm will have the pain of torsion and stretching, and it is difficult to bear elbow support when the hip touches the ground. But when the body is supported only by elbows and feet, the tendons will gradually become sore until they can't stand it. It is because muscles gradually produce acidic substances in the process of load-bearing and stimulate nerves. It belongs to the normal reaction of the human body.