The upper trapezius elevates the shoulder while the lower trapezius depresses the shoulder, making them opposing muscle groups. However, they both also upwardly rotate the scapula, making them synergists too. We do many things throughout the day that elevate our shoulders, such as hunching over a desk or carrying a heavy bag. This can lead to dominance in the upper trapezius. When hunched shoulders become a normal posture, this can lead to abnormal movement of the scapula (scapular dyskinesis).
Dan Pope from Fitness Pain Free gives a great description of how the upper and lower trapezius work together and how imbalance in strength between these two can lead to shoulder impingement:
“Patients with impingement had on average greater recruitment of the upper trapezius and less recruitment of the lower trapezius when raising their arms overhead… this upper trapezius dominance can cause hiking or shrugging of the shoulder during overhead movement and decrease the ability of the scapula to rotate normally. Taking a look at where the trapezius originates and inserts (attachment points to bone) you can see that the upper trapezius will be responsible for elevating the scapula and rotating it upward as you elevate them arms overhead. The lower trapezius will be responsible for keeping the shoulder blade stable and keeping it from excessively elevating. The lower trapezius counterbalances the upper trapezius and allows the scapula to rotate normally. Lastly, if the lower trapezius is not doing its job correctly then the upper trapezius will do more hiking/shrugging as opposed to rotating the scapula normally as you raise your arms overhead.”
Try this YTU Pose Shoulder Circles to help keep your trapezius balanced from top to bottom: