Which of the following is NOT a muscle of the upper body?

The gastrocnemius is the correct answer because it is a muscle located in the lower leg, specifically a major part of the calf. This muscle is primarily responsible for plantarflexion of the foot at the ankle joint and helps with activities such as walking, running, and jumping. It is not involved in the movements or functions of the upper body.

In contrast, the deltoid and pectoralis muscles are key muscles of the upper body. The deltoid covers the shoulder and facilitates arm movements in various directions, while the pectoralis major and minor muscles sit in the chest area and are important for arm adduction and flexion. The abdominals are also considered upper body muscles, as they are located in the trunk and play a central role in posture and core stability. Thus, the gastrocnemius distinguishes itself as a muscle relevant to the lower body rather than the upper body.