The shoulders, arms, chest, abdomen, back, buttocks, thighs and calves contain eight skeletal muscle groups. These include deltoid and trapezius muscles in the back and shoulders; biceps and triceps in the arms; pectoralis major and abdominal muscles in the chest; erector spinae, latissimus dorsi and rhomboids in the back; gluteus maximus in the buttocks; quadriceps and hamstrings in the thighs; and gastrocnemius and soleus in the calves, according to AZCentral.
Skeletal muscles are the only ones under voluntary control, according to InnerBody. They are attached to two bones that are pulled closer together by shortening, or contracting, the length of the muscle. Some muscles get their names from the part of the anatomy where they are found, such as the rectus abdominis and transverse abdominis, which are both located in the abdomen. Muscles are also classified by shape; the deltoid muscles have a delta or triangular shape. Skeletal muscles rarely work in isolation but are paired as agonist (prime mover) and antagonist (opposing mover). A muscle can work as either agonist or antagonist, depending on a particular movement. When the biceps brachii flexes the arm, it is considered the agonist and the triceps brachii is the antagonist. When the triceps muscle extends the arm, it is the agonist and the biceps muscle is the antagonist.