Final answer:
The least stable angle for shoulder abduction regarding the glenohumeral joint is 90°. This position increases the risk of dislocation or instability due to greater stress on the joint, as the rotator cuff muscles work harder to maintain stability.
Step-by-step explanation:
The angle of shoulder abduction that is considered least stable for the glenohumeral joint is when the shoulder is abducted at 90°. At this angle, the arm is aligned with the plane of the scapula, creating a lateral or sideways orientation that can place more stress on the joint. This position may increase the risk of dislocation or other instability issues more so than other degrees of abduction, such as 0°, 45°, or 120°. Shoulder stability is heavily dependent on the muscular and ligamentous structures around the shoulder, including the rotator cuff muscles and the glenoid labrum, which are the primary supporters of the joint. When the shoulder is abducted to 90°, the rotator cuff muscles have to work harder to maintain the stability of the joint as the head of the humerus is more likely to translate superiorly towards the acromion, potentially causing impingement or other forms of mechanical compromise to the structures of the shoulder joint.