Which muscle rotates the upper arm at the shoulder?

Which muscle rotates the upper arm at the shoulder?

• A. Trapezius
• B. Latissimus dorsi
• C. Abdominals
• D. Quadriceps

Answer: (B)

The movement described is inward (medial) rotation of the upper arm at the shoulder, which is produced by muscles that cross the shoulder joint and pull the humerus toward the body. The latissimus dorsi fits this role perfectly: it attaches to the humerus and, when it contracts, pulls the upper arm inward, also assisting in extending and adducting the arm. This makes it the best choice for rotating the upper arm at the shoulder. The other muscles don’t primarily rotate the arm at the shoulder. The trapezius mainly moves and stabilizes the shoulder blades, helping with shoulder elevation rather than rotating the upper arm. The abdominals and quadriceps act on the trunk and knee, respectively, and aren’t involved in rotating the arm at the shoulder.

The movement described is inward (medial) rotation of the upper arm at the shoulder, which is produced by muscles that cross the shoulder joint and pull the humerus toward the body. The latissimus dorsi fits this role perfectly: it attaches to the humerus and, when it contracts, pulls the upper arm inward, also assisting in extending and adducting the arm. This makes it the best choice for rotating the upper arm at the shoulder.

The other muscles don’t primarily rotate the arm at the shoulder. The trapezius mainly moves and stabilizes the shoulder blades, helping with shoulder elevation rather than rotating the upper arm. The abdominals and quadriceps act on the trunk and knee, respectively, and aren’t involved in rotating the arm at the shoulder.