When a baby is lying on its stomach, one of the first movements it makes is to lift its head. To do this, it tenses all three parts of its trapezius muscle. Tightening the fibers of its upper trapezius tips its head back and up. Tightening its middle trapezius pulls its shoulder blades together and stabilizes its arms so that they can support the weight of its body. Tightening the lower trapezius allows it to arch the entire length of the spine.
