Lateral exercises strengthen the lateral deltoids, which are the central muscles on the shoulder that stretch toward the shoulder blade. Exercises that work this muscle include lateral raises, in which you lift dumbbells upward and outward in an arcing motion until your arms are parallel with the ground, and upright rows, in which you lift two dumbbells toward your chin while bending your arms and pointing your elbows outward. If you notice lumps on your shoulder blades after performing these exercises, you might be suffering from one of several conditions.
The acromioclavicular joint -- also referred to as the AC joint -- is formed by the joining of the collar bone, also known as the or clavicle, and the shoulder blade, called the scapula. The ligaments that connect these two bones can become damaged, which can cause these bones to separate. Swelling often occurs with an acromioclavicular joint separation. Lateral exercises might agitate an AC joint separation and cause pain as you raise your arms to shoulder-height.
Tendinitis can occur in your rotator cuff, which attaches your shoulder blade to your upper arm bone. This condition can cause swelling in your shoulder, and is often accompanied by redness and pain. Movements that bring your arms upward could further agitate this area.
A bursa is found in places of the body where muscles and tendons could rub each other. These fluid-filled sacks prevent irritation and serve as a protective barrier at the shoulder joint. However, when a bursa becomes inflamed, painful swelling can occur in your shoulder. Repetitive movements that involve the shoulder such as lateral deltoid exercises can further irritate this condition.
While AC joint separation, tendinitis and bursitis are treatable conditions, a lump on the shoulder could indicate a more serious health complication. See your doctor about your signs and symptoms for a diagnosis and treatment options.