Which muscle might be shortened when active range of motion testing shows decreased internal rotation of the humerus?

Decreased internal rotation of the humerus during active range of motion testing is often indicative of muscle tightness or shortening in specific muscles that are responsible for this movement. In the context of shoulder motion, the infraspinatus muscle, which is part of the rotator cuff, is primarily responsible for external rotation.

When the infraspinatus is active, it assists with external rotation; if this muscle is shortened or tight, it may limit the ability of the shoulder to internally rotate due to its opposing action. As a result, decreased internal rotation could occur as the infraspinatus resists the motion.

Understanding muscle function in relation to joint movements is crucial for identifying muscular imbalances and designing appropriate rehabilitation or training programs. In contrast, the other muscles listed have different roles; for instance, the teres major and latissimus dorsi primarily aid in internal rotation and could actually contribute to the motion, while the subscapularis is also involved in internal rotation but is less likely to be involved in the scenario of decreased internal rotation when considering muscle shortening. Thus, the tightness of the infraspinatus can explain the observed limitation in internal rotation.