Contusions are caused by blunt force to the tissues which damage blood vessels beneath the surface, allowing blood to leak into the surrounding tissues. The blood leaks in a diffuse manner spreading under the skin so that the injury usually does not reproduce the pattern of the weapon. But certain contusions take the shape of weapon. Example is a tram line contusion which consists of two parallel lines of bruises with a pale undamaged skin in between. This indicates the impact with a rod or cane like elongated weapon, either cylindrical or square sectioned.

There are specific features of contusions. The bruises may be absent or slight at the moment of infliction, but appear becomes more prominent after some hours, or a day or two. The bruises in dark skinned people may not be visible but there will be a painful swelling over the area.

The colour and the anatomical position of the bruises may change with the time. The collection of the blood moves within the tissues due to the gravity. The collection of blood on the scalp may move down and collect around the eye and the contusion on the thigh may track down and collect around the knees.  As time goes by the colour of the contusion changes due to chemical degradation of the hemoglobin which is a protein in the red blood cells and gives the red colour to the blood. The spectrum of colour change in contusions with the time is red, blue, brown, green yellow and finally fade away to normal skin colour. It has been agreed that red/blue colur indicates a recent contusion of first couple of days whereas green/ yellow transformation is due to late stages of about one week and beyond.