There are many reasons for pectoral muscle asymmetry: first, due to lifestyle habits, most are right-handed situation, life is used to the right limb, so that the right arm shoulder muscles are more developed than the left side, as resistance to complete the action, resulting in training the right side of the pectoral muscle is easier to complete the action, resulting in left and right asymmetry. Second, the training force imbalance, also related to the side of life, when training a little inattention, body tilt, especially when the large weight training, meet the huge pressure, all the way to complete the action, naturally difficult to maintain balance, more likely to have problems. Third, the bones are not symmetrical due to this situation sternum and the corresponding two ribs lead to the pectoral and abdominal muscles are not symmetrical, for this phenomenon caused by the situation can not pursue perfection.