Back swelling behind the left mid-abdominal rib band may be caused by fasciitis, intercostal muscle strain, rhomboid muscle syndrome, etc. 1. Fasciitis: Back swelling is mainly caused by fasciitis. There is also back muscle strain, due to long-term tension, resulting in the muscles can not get enough rest, resulting in back swelling. 2. Intercostal muscle strain: Long-term work and incorrect sitting posture lead to intercostal muscle strain. Long-term twisting can lead to an imbalance in the strength of bilateral intercostal muscles and bilateral spinal vertical muscles. This often causes intercostal muscle strain on one side, the symptoms of which are soreness and pain in the ribcage part. 3. rhomboid syndrome: It is a localized rhomboid exudation caused by acute injury or prolonged strain, which can lead to a sensation of back swelling behind the left mid-abdominal ribcage band. If the symptoms of left mid-abdominal rib band back swelling persist without relief, prompt medical treatment is needed.