What is a muscle strain? In short, it is the overstretching or tearing of muscle fibres. The majority of muscle strains occur for one of two reasons: either the muscle has been stretched beyond its capacity or it has beena forced to contract too strongly. The severity of injury can range from only a few muscle fibres that are damaged, and the muscle continues to function, through to complete tears where the muscle may be unable to function.  How do I know if I have strained a muscle? Symptoms include: Pain and tenderness in the muscle, especially after physical activity