In a shoulder shrug motion, which muscle acts as the prime mover?

In a shoulder shrug motion, the trapezius muscle is identified as the prime mover. This muscle plays a critical role in elevating the scapula, which is the primary motion involved in shrugging the shoulders. The trapezius has various segments that contribute to different movements of the shoulder girdle, including elevation, depression, and rotation of the scapula.

Additionally, when the trapezius contracts, it effectively lifts the shoulder blades towards the ears, providing the characteristic shrugging motion. This action is crucial not only during this specific movement but also in maintaining proper posture and supporting upper body movements.

The pectoralis major and infraspinatus do not function significantly in the shoulder shrug motion. The pectoralis major is primarily involved in shoulder flexion, adduction, and medial rotation, whereas the infraspinatus is mainly responsible for external rotation of the shoulder joint. The deltoid muscle, while involved in various shoulder movements, does not serve as the prime mover in shrugging the shoulders as its primary functions include arm abduction and flexion. Thus, the trapezius is the key muscle responsible for the action described.