Final answer:
The claim that all rotator cuff muscles medially or laterally rotate the shoulder joint is false, as these muscles have various primary actions such as abduction and rotation, but their main function is to stabilize the glenohumeral joint. The correct option is false.
Step-by-step explanation:
The statement that all of the rotator cuff muscles either medially or laterally rotate the glenohumeral joint is false. While it is true that some of the rotator cuff muscles participate in medial or lateral rotation of the shoulder joint, they also have other primary actions. For example, the subscapularis muscle primarily medially rotates the arm. The supraspinatus muscle primarily aids in the abduction of the arm, while the infraspinatus and teres minor muscles mainly provide lateral rotation of the arm.
The primary support for the glenohumeral joint does not come only from the rotator cuff's ability to rotate the joint. The primary structural support for the glenohumeral, or shoulder, joint is provided by these muscles' constant adjustment of strength to keep the head of the humerus seated within the glenoid cavity. Additionally, these muscles are supported by other structures such as the subacromial bursa, which helps to prevent friction against the scapula.