Illustration of a hematoma, intramuscular hemorrhage, characterised by a build up of blood in the muscle, shown here in the calf muscle. Hematomas are often as a result of injury but can also appear spontaneously in hemophiliacs. Stage 1, left-hand illustration : healthy leg muscles. Stage 2, middle illustration : blood spreads in the muscle and forms a small hematoma. Stage 3, right-hand illustration : a large quantity of blood has built up in the muscle which has swollen and is affecting the leg's mobility.