The primary function of the lower trapezius fibers is to depress the scapula (shoulder blade) and assist in its upward rotation.
Here's a more detailed breakdown:
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Depression of the Scapula: The lower trapezius actively pulls the scapula downwards. This is crucial for movements like reaching down or pulling something towards the floor.
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Upward Rotation of the Scapula (with assistance from upper trap): This rotation is essential for raising your arm overhead. The lower fibers work synergistically with the upper trapezius and serratus anterior muscles to achieve this. This coordinated action ensures the glenoid fossa (the socket of the shoulder joint) rotates upward, allowing for a full range of motion at the shoulder.
In essence, the lower trapezius plays a vital role in maintaining proper scapular mechanics, contributing to overall shoulder stability and efficient upper limb movement. Without proper function of the lower trap, individuals might experience scapular dyskinesis (abnormal scapular movement), potentially leading to shoulder pain or impingement.
