From a seated position, with dumbbells to the sides, raising the dumbbells laterally to shoulder level with elbows slightly flexed, exercises primarily the _______:

The biceps brachii flexes the elbow and supinates the forearm, while the latissimus dorsi adducts and extends the shoulder - neither muscle acts as a prime mover during lateral shoulder abduction.

The posterior deltoid is recruited primarily during shoulder extension and horizontal abduction such as bent-over reverse flies, not during a standard lateral raise performed in the frontal plane.

The anterior deltoid drives shoulder flexion and internal rotation as in front raises, and the triceps brachii extends the elbow - neither is a prime mover during lateral abduction with slightly flexed elbows.

Raising the arm in the frontal plane through shoulder abduction is the primary mechanical action of the middle (lateral) head of the deltoid. With movement kept strictly lateral and elbows slightly flexed, the middle deltoid functions as the dominant prime mover and receives maximal mechanical loading throughout the full range of motion of this exercise.