Final answer:
The infraspinatus muscle is responsible for external rotation of the shoulder during tubing rotator cuff exercises and is part of the rotator cuff group, which stabilizes and supports the shoulder joint.
Step-by-step explanation:
The muscle responsible for the external rotation of the shoulder during tubing rotator cuff exercises is the infraspinatus. This muscle is one of the four muscles comprising the rotator cuff, which also includes the supraspinatus, teres minor, and subscapularis. These muscles connect the scapula to the humerus, forming a cuff around the shoulder joint. They provide primary support to the joint and function as dynamic ligaments to maintain the position of the humerus within the glenoid cavity. The rotator cuff muscles are key for the stability and function of the shoulder, especially during movements that involve rotating the arm, as in tube exercises targeted at the external rotation of the cuff.