When muscles are overstretched, or if their strength is overwhelmed by an excessive force when attempting to contract and shorten, a muscle strain results. Muscles consist of a bundle of fibers (like a rope) that attaches one bone to another at a joint.
When they contract (in order to move the bones in the joint), each of the fibers in the muscle shortens. What happens with a strain is that some of the fibers tear, causing inflammation, pain and weakness.
Depending on what percentage of fibers in the muscle tears determines how bad the strain is (Graded from 1-4). Grades 1-3 are treated successfully by chiropractors using various therapies like ice/heat, ultrasound, electrical muscle stimulation, manipulation, massage and stretching. These modalities and manual techniques reduce inflammation and pain allowing the muscle to heal quickly.