Have you ever suffered a shoulder injury where you can lift your arm to the side with your elbow straight with no pain, but you do the same movement with your elbow bent and it's painful? Surely it shouldn't make a difference as it is the same shoulder movement right?
In fact, different muscles are stressed when you straighten or bend your elbow, and lift your arm to the side (abduction). When the elbow is straight (extended), the middle and anterior deltoid and supraspinatus muscles are the greatest contributor to shoulder abduction. In contrast, when the elbow is bent at 90 degrees, the anterior deltoid and subscapularis muscles are utilised most to perform abduction at the shoulder. This dependence of shoulder muscle function on elbow joint position is explained by the existence of dynamic coupling in multi-joint musculoskeletal systems.
To be able to diagnose an injury, a multi-faceted approach is often needed to collect data to either reinforce or negate potential diagnoses for the injury. Such a simple test could help in the diagnosis for shoulder injuries, such as muscle paralysis or full-thickness rotator cuff tears.
Yu, J. et al. (2011). Shoulder muscle function depends on elbow joint position: an illustration of dynamic coupling in the upper limb. National Library of Medicine. 44(10): 1859-68.