Lumps under the skin are usually harmless, but in some cases can be caused by something more serious, such as a tumor. If you only see the lump when you are exercising, it is often due to engorged veins, a cyst or a herniated muscle. If it's painful, the lump could be due to a knotted or cramped muscle. Most lumps can't be diagnosed just by looking at them, so have a doctor check the lump to make sure it is not malignant.
Veins carry de-oxygenated blood back to the lungs and heart and have valves in them to stop the blood from pooling and backing up. If the valves aren't working properly, blood can pool when the blood flow is more intense, which happens when you are exercising. Depending on the location of the veins in the body, you might see the veins visibly enlarged or you might just see a lump under the skin.
Cysts are sacs or pockets of tissue that are closed off and filled with air, fluid, pus or other material, according to Medline Plus. Cysts on the skin are caused by infection, clogged glands or as a protective measure against a foreign object embedded in the skin. You may only see the cyst when you are exercising because the movement involved shifts the position of the body, making the cyst more prominent.
Hernias are the result of a tear in the connective tissue that binds muscles, organs and intestines. The tear creates a hole and the muscles, intestines or other tissue poke through the hole, creating a lump. Although hernias happen most frequently in the abdomen, they can happen anywhere, particularly the legs. Hernias may not be noticeable until you exercise because of the added pressure on the body, which forces the tissue through, creating the lump.
If the lump is painful or knotted, you may have a torn muscle or ligament causing the problem. If the area has been previously injured, scar tissue could be causing the lump, which is made more noticeable during exercise because of increased blood flow. Many lumps are harmless, but in some cases such as an infected cyst or a hernia, surgery may be needed to fix the problem. Regardless of the possible cause, consult a doctor for a diagnosis. Although you may only notice the lump during exercise, it is still possible it's cancerous or otherwise harmful.