An arm lump can be caused by one of a variety of conditions, including inflammation, infection, trauma or tumor growth, according to Healthgrades. Lumps can be described as nodules, bumps, tumors, cysts or contusions and may be firm, painful, soft or painless.
Lumps that form in the arm muscle and subside after a while are typically caused by an infection or injury, according to the Healthgrades. However, lumps that don't resolve themselves should be examined by a doctor. The growths may be benign or cancerous.
Malignant or benign lumps in the arm are defined under several classifications, Healthgrades states. For example, lumps can be caused by nonmelanoma and melanoma cancers or may take the form of a fibroma, a benign tumor made of connective tissue.
Some lumps that appear in the arm muscle may be a soft tissue sarcoma, according to Cancer.net. The cancerous lump may present itself in the first stage of growth as a painless swelling. Fortunately, most soft-tissue lumps are defined as lipomas, which consist of fat cells and, therefore, are not cancerous.
Lumps in the arm muscles also may develop as the result of broken bones, rheumatoid arthritis or cellulitis, a skin infection under the skin, according to the Healthgrades site.