Running is a good way to work out the cardiovascular system and the muscles of the leg, but it is also very hard on your joints and bones. Each time your foot strikes the ground when running, it puts pressure equivalent to four to five times your body weight on your legs. Over time, that can cause damage to the bones, muscles, tendons and ligaments, especially if you have ill-fitting footwear or weak joints. Some lumps can be treated with self-care measurements, but there is a risk the lump could be malignant. Consult your doctor for a diagnosis.
Medial tibial stress syndrome, more commonly known as a shin splint, is characterized by pain on the tibia -- the bone on the inner part of the lower leg below the knee. Lumps caused by shin splints are felt along the inside of the tibia, and there may be associated swelling of the leg. The pain from shin splints often goes away once you start to run, but returns when you finish running. Shin splints occur most frequently in runners that run on an uphill or downhill grade, those that stop and start a lot and wear ill-fitting or worn-out shoes.
Hernias in the lower leg are caused by a weakness of the fascia -- the tissue that forms a sheath around the muscles and nerves. If the fascia is weak or defective, the muscles of the leg push through, forming a lump. The lump is not always painful, but it can be if the pressure exerted on the hernia is made worse by exercise. Lumps formed by muscle hernias usually occur on the outside of the lower leg.
Lumps can form on the lower leg for a variety of other reasons, and running may simply make them more noticeable or exacerbate the lumps. Trauma to the leg can cause swelling or a bump, as can an infection. Inflammatory diseases, such as rheumatoid arthritis and lupus can produce lumps, particularly around the knee. There is also the chance it could be a tumor of some kind, which in rare cases can be cancerous.
Shin splints can be treated by resting the leg and applying ice when the lump initially forms, then heat after it is reduced. Wearing shoes with arch supports or adding arch supports to an older shoe can help reduce the shock on the joints when running. Stretch the legs before and after running and wear a shin splint or calf brace to support you after you return to running. Muscle hernias, if not painful, should not cause a problem when running, but it's best to consult a doctor to be sure. All other possible causes will need to be diagnosed by a doctor. Even if the lump is not painful, there is the possibility it could be malignant.