What Happens When We Pull A Muscle?

More than 300 individual muscles enable your limbs to work and give your body shape and definition. If you spend time properly stretching and strengthening them, you're rewarded with strength, power and flexibility. However, if your muscles are injured, you can end up with problems. Many of us have seen the serious consequences of an abrupt muscle tear: A sprinter rounding the turn suddenly drops down clutching a torn hamstring.
A muscle tear may be partial or complete and caused either by a direct blow or by overexertion. There are three degrees of muscle ruptures. A first-degree strain involves less than 5 percent of the muscle. You may notice only mild pain and not lose much strength or range of motion. We sometimes refer to these mild tears as pulled muscles.
A second-degree tear is a greater rupture that stops short of a complete tear. Any contraction of the torn muscle will cause pain. With either a first- or second-degree tear, you may feel a defect of the muscle - a bump or an indentation - at the site of the most pain. You should be able to partially contract the muscle, but you may not be able to walk or stand without pain or a limp.
A third-degree rupture is a complete tear across the width of the muscle. You will be unable to contract the muscle. This is what happens when someone suddenly drops while sprinting. The torn end of the muscle may ball up and form a large lump under the skin, and a great deal of internal bleeding occurs. Severely torn muscles may require surgery to heal properly. Many sportspeople such as footballers get third-degree ruptures which can often end their careers.

Usually athletes get this type of injuries.  You pull a muscle when you stretch too fast, or too hard at once.So the muscle group begins to tear. Some of these muscle pulls could take 1-18 months to heal depending on how bad it is.