A side strain is a muscle strain caused by tearing of the transversalis fascia or the internal oblique muscle.^{[1][unreliable medical source?]} When this occurs, stretching the muscle will be severely painful, making it difficult to move the arm, and the strain may later be marked by swelling or bruising.^[2]

The strain usually occurs due to the internal oblique muscle contracting suddenly from a stretched position. It may happen suddenly, or over time from regular repetitive activity.^[1] It is common in cricket players, and occurs on the opposite side of the arm that is used for bowling.^[2] The injury has also been reported in rowing, baseball and ice hockey and pool.^[3] The preferred treatment for a side strain is regular rest and inactivity on the affected muscle. With appropriate rest, regular activity on the muscle can occur between 4 and 6 weeks, though it may take longer.^[1]