A “strain” refers to a muscle injury while a “sprain” refers to ligament injury. Common true strains in the shoulder region include pec muscle strains. An overworked muscle such as the deltoid or biceps can cause pain as well. Furthermore there is an entity called “DOMS” or delayed onset muscle soreness that can result from heavy, typically unaccustomed, exercise.
A muscle strain occurs when the structural integrity of the muscle fibers is overwhelmed by the demand you are asking of your body. Sudden eccentric contractions can often cause strains. Strains are more common when muscles are in positions with poor leverage. Biomechanical and other factors, some as simple as fatigue, may increase the risk of muscle strain.
Muscle strain is often felt as sharp, throbbing pain, worse especially when the strained muscle is recruited to fire or contract.
Muscle strains often resolve on their own over the course of weeks. Hematoma (a collection of blood that collects from more significant strains) can take months to resolve fully. Some patients and physicians are experimenting with regenerative-type injections including platelet-rich plasma (PRP) to help heal a muscle strain and return a patient to sport or activity faster, but this is a relatively novel treatment and there are only case reports suggesting mixed efficacy.