Muscles can get strained (pulled muscle), which effectively means that muscle fibres are torn. Strains can be graded as mild, moderate and severe, and may or may not be associated with bleeding inside the muscle. Sometimes there is visible skin bruising after a muscle strain. Muscle strains are acute and usually happen when the muscle is stretched whilst running.
Treatment: Initial treatment is rest (stop the activity), and the use of ice to reduce the swelling and inflammation. It can take between two and eight weeks for the muscle to heal, and rehabilitation of the surrounding joints and muscles is essential to avoid a recurrence. In particular an early return to sports when rehabilitation is not complete is a recipe for repeated muscle strains. Muscle power usually fully recovers.