How quickly a bruise develops depends on how forceful the trauma is that causes the injury. When this occurs, injury occurs to the muscle fibers as well as the capillaries (which are small blood vessels) and blood leaks out of the injured cells into the surrounding areas beneath the skin. A contusion can result from a blow (or repetitive blows) or a significant strain to a specific part of your body, or can result from you hitting your body against a fixed object or the ground.
