Which muscle group is primarily involved in a hammer curl exercise?

In a hammer curl exercise, the primary focus is on the forearm flexors, specifically the brachialis and brachioradialis muscles. This exercise is performed with the palms facing each other (a neutral grip), which effectively targets these muscles due to their role in elbow flexion. As you lift the weight in a hammer curl, the brachialis, located beneath the biceps, plays a significant role in the movement, while the brachioradialis, located along the forearm, aids in flexing the elbow.

The other muscle groups listed do not play a central role in this particular movement. The triceps are primarily responsible for elbow extension and are not actively engaged during curls. The quadriceps are involved in knee extension and have no contribution to the motion performed in a hammer curl. The chest muscles are involved in pushing movements but are not activated in a curl exercise. Therefore, the forearm flexors are unequivocally the key players in the hammer curl, making this answer the most appropriate in the context of the exercise.