Many layers of anatomy behind the left breast can cause pain during exercise. The breast sits on top of the pectoralis major and minor muscles. Than there is the rib cage with bones, cartilage, intercostal muscles and nerves. The heart and lungs lay within the thoracic cavity and can contribute to pain, as well as other internal organs that may refer pain to the area under the left breast.
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There are two pectoralis muscles, pectoralis major and minor. These muscles attach to the clavicle, sternum, costal cartilages, and ribs on the broadest parts of the muscles. The muscles then stretch across the rib cage and attach to the upper humerus or the scapula. Movement of the shoulder joint and the scapula are accomplished with the help of these two muscles. Like every other muscle in the body, these muscles can be injured or strained. This injury usually occurs when the muscle is stretched beyond its elastic capabilities during lifting or exercising. The pain of a strained muscle commonly occurs when the muscle is in use and may not initially be felt until after the exercising has been completed and the body has had time to calm.
The rib cage can also cause pain during exercise. The ribs are connected to the posterior spine by joints and the anterior sternum via costal cartilages. The ribs are also connected to each other via intercostal muscles and nerves that run between the ribs. If a rib has somehow moved out of place, there may be a strain on the costal cartilage, muscles or nerves in the area. This can be caused by twisting movements, coughing or sneezing. This pain is usually a sharp pain and hurts worse with breathing. It may be exacerbated by exercise because of the change in the muscles that are used to breathe heavily during a workout. Icing, stretching and seeing a chiropractor can help to relieve a rib that is out of place.
Heart and Lungs
The heart and lungs can also cause pain that is felt behind the left breast. The heart's most common pain is called angina and occurs when the heart is not getting enough blood or oxygen. It can be felt as an intermittent tightness, or pressure, a squeeze or a crush. The lungs can cause pain when part of a lung spontaneously collapses, the lung is inflamed, or irritation is caused by asthma. If the heart or lung is thought to be the cause of the pain, seek a doctor's opinion before you exert yourself.
Other organs throughout the abdomen can refer pain to this area. The gallbladder, when inflamed, can refer intermittent pain. Problems that occur in the esophagus and stomach can also refer to the left chest area. This can be from acid reflux, esophageal spasm, or gastritis. These problems are best treated with the assistance of a medical professional.