A muscle strain can also sometimes cause bruising, depending on the location and the extent of the injury. When the muscle tears, it can damage the small blood vessels that surround the muscle and supply the tissue with nutrients. Damaged vessels can trigger local bleeding under the skin. This bleeding shows up as a bruise or multiple bruises around the affected area. Since the blood is not contained by the ruptured vessel walls, it leaks into places it is not supposed to be and pools there.
Bruising is not always serious, though multiple unexplained bruises may be a sign that there is an underlying condition causing clotting problems. There are a variety of other injuries that may mimic a muscle strain and cause bruising as well. One of the most common is an ankle fracture. A strained ankle, sprained ankle, and broken ankle will often present with the same symptoms.