As a result of the initial collision, you break blood vessels, which leak blood into areas under your skin, causing swelling, discoloration and soreness.
Bruises are most common in older people whose skin is thinning with aging.
This is due to the fact that collagen, the connective tissue that cushions skin, breaks down, leaving underlying blood vessels more vulnerable. People who take medications for diseases, including blood thinners like aspirin, also have an increased risk of bruising. A normal bruise should heal on its own within a few days to a couple of weeks. As a general rule, bruises tend to heal more slowly as you go down the body. In other words, a bruise to the arm heals more quickly hen one to the leg.