As its name implies, the shoulder press targets the shoulders, specifically the deltoids. However, this classic exercise doesn't limit itself to one muscle group; it engages numerous secondary muscles as synergists and stabilizers. Because you can perform shoulder presses standing or sitting with barbells, dumbbells or a machine, some variations of this exercise work secondary muscle groups that other variations don't.
All common variations of the shoulder press – dumbbell, barbell and machine varieties, both seated and standing – target the anterior, posterior and medial deltoids. These muscles make up the bulk of the rounded portion of the shoulder. The rising motion of the shoulder press also contracts the middle and lower trapezius, using the upper traps to help stabilize the motion. Shoulder presses also work the rotator cuff and the small supraspinatus muscle of the shoulder blade as secondary muscles.
In all its common forms, the shoulder press engages the triceps brachii -- the rear muscle of the upper arm -- as a synergist, or a muscle that helps other muscles complete a movement. Specifically, shoulder presses work the long head of the three-headed triceps. This portion of the muscle lies on the side of the upper arm, spanning from just below the shoulder to the mid-bicep.
Shoulder press exercises engage the longer bottom muscles of the serratus anterior, the muscles of the upper ribcage, as synergists. Common standing and seated varieties of the shoulder press also employ the levator scapulae -- muscles that span the sides and back of the neck -- as stabilizers. Stabilizers contract during exercise to help the body maintain a certain posture.
While the shoulder press always caters to the shoulders, seated barbell shoulder presses and machine shoulder presses engage the upper-back rhomboids as secondary muscles. These variations also work the mid-back latissimus dorsi, which spans from the armpit to the spine. Standing varieties of the shoulder press help tone the obliques, the muscles located on either side of the torso, and work the erector spinae, a muscle that runs from just below the neck to the lower back. Barbell and dumbbell presses tend work the rectus and transverse abdominus -- or abs -- harder than machine shoulder presses.
- ExRx: Dumbbell Shoulder Press
- ExRx: Barbell Shoulder Press
- ExRx: Barbell Military Press
- American Council on Exercise: Standing Barbell Shoulder Press
- American Council on Exercise: Seated Barbell Shoulder Press
- American Council on Exercise: Seated Machine Shoulder Press
- American Council on Exercise: Seated Machine Close-Grip Shoulder Press
- Thinkstock/Comstock/Getty Images