What is the anatomy of the neck? The neck is a complex anatomic region between the head and the body. In the front, the neck extends from the bottom part of the mandible (lower jaw bone) to the bones of the upper chest and shoulders (including the sternum and collar bones). The back of the neck is mostly comprised of muscles, as well as the spine. 3. Chin tucks. Chin tucks are another easy move to add to your stretching routine. "This stretches out the muscle at the back of your neck whilst working the muscle at the front of your neck," Dr. Causes of a Lump on Back of the Neck. 1. Injury to a muscle or tendon, from an accident or strenuous movement, can result in a lump on the back of the neck. The lump may be the muscle itself as it moves in place when damaged. Our muscles and tendons maintain our body shape. You may feel pain, neck stiffness, or have difficulty moving your neck.