Which movements of the upper extremity would be most painful with supraspinatus muscle tendinitis?

The correct answer is that abduction of the shoulder would be the most painful movement with supraspinatus muscle tendinitis. The supraspinatus muscle is one of the four rotator cuff muscles and is primarily responsible for initiating shoulder abduction, which means it helps lift the arm away from the body. When the tendon of the supraspinatus becomes inflamed due to tendinitis, activities that involve lifting the arm or moving it away from the body can cause significant pain and discomfort.

In contrast, the other movements listed may not evoke the same level of pain because they do not directly engage the supraspinatus muscle in the same manner. For example, flexion of the elbow primarily involves the muscles of the forearm and does not heavily recruit the shoulder muscles. Extension of the shoulder also tends to engage different muscle groups, and adduction of the shoulder involves bringing the arm closer to the body, which reduces strain on an inflamed supraspinatus tendon. Therefore, pain levels differ based on the specific muscle involvement and the biomechanics of shoulder movement, making abduction the most painful in this scenario.