A:

Bruises that develop after running could be from a number of reasons. But poor circulation is unlikely to be the cause.

Bruises, which are bleeding under the skin, are normal in areas that are bumped or injured in some way.

For example:

• Bruises under the toenails of runners are common because of the pounding the feet take over the course of the run.
• If one side of your foot or leg strikes the other side, this could cause bruises.

Some people bruise easily because they take a medicine that “thins” the blood (for example, aspirin or ibuprofen). Or, they have a condition that makes the tiniest blood vessels, the capillaries, break easily (for example, the corticosteroid medication prednisone). The trauma may be so minor in these situations that they don’t remember it.

The arterial system brings blood full of oxygen to your arms and legs. Poor circulation in the arterial system would cause muscle pain (also called vascular claudication), and likely stop you from running. But this is an unlikely condition in an athletic person your age.

The venous system returns blood to the lungs for more oxygen. Poor circulation in the venous system generally causes swelling in the feet or painful, inflamed blood vessels near the surface of the leg (called superficial thrombophlebitis). Neither of these circulation problems commonly causes bruising.

Rarer blood disorders could cause easy bruising. One example is a low platelet count (thrombocytopenia). Because platelets prevent excessive bleeding, a low count can lead to easy bruising.

But in most situations, no abnormality is found to explain the easy bruising. Minor trauma is probably to blame.

See your doctor if this is a new problem, or if there is still noticeable bruising without any major trauma or new medicine use. Your doctor may decide some simple blood tests are needed.