The amount of bruising after a muscle strain occurs is dependent on the amount of tissue injured and the severity of the muscle strain. A muscle strain can be categorized in degrees from first to third degree, with a first degree strain being least severe and third degree strain being most severe.
In first degree muscle strain, 20% of the muscle fibers or fewer are damaged. There is little damage to the blood vessels, and there is also very little cellular fluid released.
In a second degree strain there can be up to 70% of the muscle fibers torn. This amount of tissue damage can rupture capillaries and a large number of cells causing an increase in fluids outside of the muscle cells. This fluid buildup can lead to what is known as ecchymosis, a visible change in color of the skin, or more commonly, a bruise. With hamstring injuries this bruising often appears in the back of the knee, even if the site of the muscle tear is higher, due to gravity pulling the fluid downward and causing it to pool away from the injury.
This answer provided for NATA by Loras College.
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