The back of the butterfly is two shoulder blades symmetrically distributed on both sides of the shoulder. The scapula of normal people clings to the chest wall, which is completed by the cooperative contraction of serratus anterior and trapezius muscles.

If the serratus anterior and trapezius muscles are paralyzed, the scapula will lose its function of sticking to the chest. When the scapula of the upper arm rotates, it will tilt up due to the loss of traction at the edge of the spine, forming butterfly wings, which are technically called winged scapula.

Butterfly back is very popular in domestic entertainment circles. Many female stars have been practicing butterfly back in order to make themselves look good in dark V clothes. There are also many female stars without butterflies on their backs. Look at the photos, they are all p's. Butterflies are popular in the entertainment circle because they imitate foreign female models. In foreign countries, the back of a butterfly is called the wings of an angel.