The trapezius muscle (traps) is the flat triangular muscle the extends out and down from the neck down between the shoulder blades. The latissimus doris muscle (lats) is the large triangular muscle extending from under the shoulders down to the small of the back on both sides. The spinal erectors are several muscles in the lower back that guards the nerve channels and keep the spine erect. The width and thickness of a back is necessary in creating a quality bodybuilding physique. Strong back muscles are needed for carrying and lifting heavy weights. The best way to exercise the back are by rows and pulldowns.