CrossFlowX™: Upper Back Bonanza in 36. 36 minutes. When your upper back isn't strong and open, it allows your shoulders to round forward, and your traps and neck get tight. Then, when you move, your cervical and lumbar spine area have to work extra hard, which taxes both areas, and wreaks havoc on the rest of your body. This cardio flow will build strength and open your thoracic spine area, so that your body can feel its best and stay balanced. Two yoga blocks and a strap are recommended for this flow.