Muscles contain a lot of blood vessels, which are responsible for bringing the oxygen needed by muscles for action. As you exercise, blood flow to the muscles increases significantly to provide needed oxygen and remove waste, such as lactic acid and carbon dioxide.
Often when you pull a muscle, you feel it right away. You may feel the muscle suddenly give, which is usually then accompanied by a strong dose of pain. In severe cases, you may actually hear it. At other times, you don’t feel the pull at the time of injury, but you notice it later on.
When you pull a muscle, blood vessels within the muscle tissue are torn, which damages the circulation to and from the area. Without a proper pathway, fluids “spill” into the muscle tissue, causing swelling and on occasion bruising. Sometimes these fluids may cause the area to visibly swell, but more commonly the result is internal inflammation. This inflammation is the root of pain.
In addition to pain and possible swelling, a muscle pull is usually accompanied by a lack of function.