The empty can test refers to a position of the arm used to identify problems with the supraspinatus muscle. The supraspinatus is one of the four muscles/tendons that make up the shoulder rotator cuff.
The test was first described in the early 1980s by two very well-known orthopedic surgeons (Drs. Jobe and Moynes). The arm is raised out to the side with the elbow straight and the thumb pointing down towards the ground as if you were emptying out a can of soda pop.
The shoulder is fully rotated inward (internal rotation) during this test. Any weakness to resistance while in this position suggests a tear of the supraspinatus tendon.