The pectoralis major is primarily responsible for adducting and medially rotating the arm. Its large size and position on the chest enable these movements effectively.

The biceps brachii is the primary muscle responsible for bending or flexing the elbow. It plays a significant role in daily lifting and pulling activities.

The deltoid muscle is mainly in charge of lifting the arm away from the body through shoulder abduction. Its structure allows for effective and controlled movement.

The trapezius muscle is essential for scapular movements such as elevation, retraction, and rotation. Its action contributes significantly to proper posture and upper body mechanics.

The triceps brachii is the major muscle that extends the elbow, allowing for effective pushing movements. It works in opposition to the biceps brachii to coordinate arm actions.

Latissimus dorsi is a large back muscle that helps in extending and adducting the shoulder. Its extensive origin and insertion provide the power needed for these movements.

The axillary nerve supplies motor function to the deltoid muscle. It is critical for shoulder abduction, and damage to this nerve can severely impair that movement.

The pectoralis minor lies beneath the pectoralis major and attaches to the scapula, playing a vital role in its stabilization. This muscle helps maintain proper scapular positioning during arm movements.

The biceps brachii spans both the shoulder and elbow joints, which enables it to participate in multiple movements including flexion and supination. This biarticular function makes it a versatile part of upper body motion.

The infraspinatus is a key member of the rotator cuff group, primarily responsible for externally rotating the shoulder. Its function is crucial for many activities that require precise shoulder movement.

The main function of the rhomboid muscles is to retract the scapula, pulling it towards the spine. This movement is essential for maintaining good posture and healthy shoulder mechanics.

The rotator cuff is comprised of the supraspinatus, infraspinatus, teres minor, and subscapularis muscles. Teres major, despite its similar name, is not part of the rotator cuff group.

The middle deltoid is the principal muscle responsible for abducting the arm at the shoulder. Its anatomical positioning makes it most effective for lateral arm lifts.

The upper trapezius is primarily responsible for elevating the scapula, such as when shrugging the shoulders. This region works distinctly from the middle and lower portions which have different actions.

The supraspinatus muscle initiates the abduction of the arm, lifting it away from the body. This initial movement is critical to beginning the abduction process before other muscles contribute.

Pectoralis major has a broad origin that includes the clavicle, sternum, and costal cartilages, reflecting its extensive role in upper body movements. In contrast, pectoralis minor originates from the upper ribs, which is key to its function in stabilizing the scapula.

The middle trapezius assists in scapular retraction, making it an ideal candidate to compensate when the rhomboids are weak. Its horizontal fiber alignment is optimized for pulling the scapula toward the spine during rowing motions.

The trapezius muscle, which plays a significant role in postural support, contains a high proportion of type I slow-twitch fibers. These fibers help the muscle resist fatigue during prolonged activities.

The subscapularis is a primary muscle responsible for internally rotating the humerus. Its anatomical position on the anterior surface of the scapula allows it to pull the arm inward effectively.

The biceps brachii crosses both the shoulder and elbow joints, contributing to both elbow flexion and forearm supination. In contrast, the brachialis is solely dedicated to elbow flexion, which makes its role very specific.