Which muscle group is primarily responsible for flexing the shoulder joint?

The anterior deltoids and the biceps brachii are primarily responsible for flexing the shoulder joint due to their anatomical positions and functions.

The anterior deltoid is located at the front part of the shoulder and is one of the main muscles involved in shoulder flexion. When this muscle contracts, it raises the arm forward and upward, moving it away from the body.

The biceps brachii, although traditionally known for its role in elbow flexion, also assists in shoulder flexion. The biceps brachii has two heads that originate from different points on the shoulder girdle, which allows it to play a role in lifting the arm.

The combination of these two muscle groups effectively contributes to the flexion of the shoulder joint, making this answer the most accurate in the context of shoulder movement mechanics.