There are two reasons for shoulder pain after fitness: First, lactic acid buildup; the first case is because the upper arm muscles, especially the deltoid muscle, move too much to cause the muscle fibers to produce lactic acid, when too much lactic acid buildup and no way to eliminate it in time will lead to shoulder pain. Secondly, bursitis of the subacromial or deltoid muscles. The second condition is caused by excessive shoulder joint activity and repeated friction, which stimulates the bursa of the subacromial and deltoid muscles, causing edema and exudation of the bursa, resulting in a sterile inflammation and causing shoulder pain. How to eliminate two kinds of shoulder pain: First, proper rest. Second, apply local heat and do not overdo it when exercising.